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Tuning Polarity of Polyphenylene Dendrimers by Patched Surface Amphiphilicity—Precise Control over Size, Shape, and Polarity René Stangenberg, Irfan Saeed, Seah Ling Kuan, Martin Baumgarten, Tanja Weil, Markus Klapper, Klaus Müllen* In the ideal case, a precise synthesis yields molecules with a constitutional as well as a conformational perfectness. Such a case of precision is demonstrated by the synthesis of semi-rigid amphiphilic polyphenylene dendrimers (PPDs). Polar sulfonate groups are precisely placed on their periphery in such a manner that patches of polar and non-polar regions are created. Key structural features are the semi-rigid framework and shape-persistent nature of PPDs since the limited flexibility introduces a nano-phase-separated amphiphilic rim of the dendrimer. This results in both attractive and repulsive interactions with a given solvent. Frustrated solvent structures then lead to a remarkable solubility in solvents of different polarity such as toluene, methanol, and water or their mixtures. Water solubility combined with defined surface structuring and variable hydrophobicity of PPDs that resemble the delicate surface textures of proteins are important prerequisites for their biological and medical applications based upon cellular internalization.
1. Introduction “Precisely controlled polymer architectures” define one of the hottest topics in polymer synthesis.[1] This line of research was boosted by the discovery of controlled radical polymerization methods such as reversible additionfragmentation chain transfer (RAFT),[2] nitroxide-mediated radical polymerization (NMRP),[3] and atom transfer radical polymerization (ATRP)[4] providing access to polymers with narrow polydispersity and to new polymer architectures such as block, stars, or comb-type polymers. Further, R. Stangenberg, I. Saeed, Dr. M. Baumgarten, Dr. M. Klapper, Prof. K. Müllen Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany E-mail: [email protected] Dr. S. L. Kuan, Prof. T. Weil University of Ulm, Institute for Organic Chemistry III/Macromolecular Chemistry, Albert-Einstein-Allee 11, 89081 Ulm, Germany
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the range of applicable monomers was dramatically broadened.[5] Controlling the formation of precise polymer architectures also comprises the exact positioning of functional groups along a polymer chain. A prominent example is ADMET (acyclic diene metathesis), which polymerizes functionalized terminal dienes to linear polyenes.[6] Functional groups such as carboxylic acid groups can be attached to a backbone at defined distances whereas a copolymerization of functionalized and unfunctionalized monomers would only afford a statistical distribution. Although both controlled radical polymerization and ADMET allow for a particular control of molecular weight, composition, and structure, the “precision” is still limited by the kinetics of the polymerization yielding molecular weight distributions.[7] Monodispersity can be accomplished by the synthesis of dendrimers.[8] Dendrimers consist of a core, a scaffold with multiple branching points, and a shell. The divergent, which is generation-by-generation, growth of a dendrimer utilizes branching reagents of the ABn-type
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DOI: 10.1002/marc.201300671
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Tuning Polarity of Polyphenylene Dendrimers by Patched Surface Amphiphilicity
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and needs protection–deprotection protocols to avoid premature reactions of A and B functions.[9] While polymerizations are, in principle, possible, polycondensations and polyadditions are most commonly applied. By these approaches, molecular weights up to 1.9 MDa can be obtained, which is by far exceeding the range of many polymeric materials synthesized by the above-mentioned controlled polymerization methods.[10] Most dendrimers contain ether, amide, or ester linkages.[11] Their formation is far from quantitative and side reactions can occur. Any defect thus introduced cannot be healed, but is instead amplified upon further growth. As a result, defect-free macromolecules are hardly accessible and molecular distributions, even if narrow, are unavoidable.[12] It should be noted that structural definition of polymers does not only demand constitutional perfection but also conformational control in solution and in the solid state.[13] If the conformation of flexible macromolecules is not easy to adjust, the position and chemical environment of functional groups cannot reliably be predicted.[14] Both linear polymers and the dendrimers described above contain conformational flexible moieties, which give rise to structural and functional “uncertainty” under the prevailing experimental conditions (temperature, solvent, etc). We have offered a solution to constitutional and conformational perfection by the synthesis of semi-rigid polyphenylene dendrimers (PPDs).[15] They are composed of twisted, tightly packed interlocked benzene rings. Their synthesis is based on the Diels–Alder cycloaddition of ethinyl-substituted benzenes and tetraphenylcyclopentadienones (see below), which proceeds with a much higher perfection than the dendrimer-forming reactions mentioned above.[15a] These dendrimers are mainly based on para-connected polyaromatic buildings blocks. Therefore, PPDs are unique within the dendrimer field, because the rigid polyphenylene dendron arms do not allow their backbending and thus exclude the reorientation of functional groups.[16] Size and shape become exactly defined and are essentially independent of the solvent.[17] By using differently functionalized building blocks in the sequential synthesis, numerous functional groups can be exactly placed in the core, the scaffold or the shell.[15c] Two examples should be mentioned: Introducing chromophores into the nano-environment of PPDs affords light-harvesting systems with vectorial energy transfer.[18] Likewise, since the PPDs comprise defined voids they serve as receptors for guest molecules.[19] Their uptake can be analyzed by microbalance measurements or isothermal titration calorimetry (ITC) and leads to sensing devices for explosives.[20] Here, we consider another challenge for precision polymer synthesis: the control of amphiphilicity as an opportunity to mimic the delicate polar- and non-polar
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surface textures of proteins.[21] Proteins are compact macromolecules with precisely defined 3D architectures where each surface functionality is located at a distinct location. Some proteins such as serum albumins form defined lipophilic pockets that accommodate a wide variety of drug molecules thus serving as efficient natural transporter molecules.[22] The functionalization of rigid dendrimers can be used to introduce precisely at certain areas hydrophobic and hydrophilic parts. Such a control is rather common in nature but hard to achieve with synthetic structures. Synthetic amphiphiles such as dodecylsulfonates or the pluronics, which are widely used as surfactants[23] or emulsifiers,[24] are mostly very flexible molecules comprising one hydrophilic and one hydrophobic part. Natural amphiphiles such as lecithin, one of the main components of cell membranes or proteins, are structurally more complex since they possess several hydrophilic and lipophilic parts at defined areas either on the surface or inside. The control over the amphiphilicity plays a vital role in biology[25] as amphiphilicity is a decisive driving force for cell penetration processes with for example peptides. Thereby, the internalization of molecules into cells requires a precise control over size and shape as well as an exact positioning of hydrophilic and hydrophobic areas. Amphiphilic block or statistical copolymers are far from having the required structural and conformational precision due to their conformational mobility. This gap between synthetic and natural amphiphiles should be bridged by amphiphilic PPDs. Amphiphilicity in dendrimers[26] can be realized in: i) core–shell-(A, B)[27] or, ii) Janus-type-systems (C),[28] and iii) by the alternate placement of polar and nonpolar groups on the surface (D) (Figure 1). To the best of our knowledge, only few examples of type D dendrimers that resemble the patchy structures of proteins have been reported so far,[29] but in contrast to proteins, they have a flexible framework, which is not matching the defined positioning of the polarity observed for natural amphiphiles. We decided to decorate the periphery of stiff PPDs with an alternating arrangement of polar (sulfonic acid) and non-polar (propyl) groups on a sub-nanometer length scale (≈6 Å) to create patterned hydrophilic and hydrophobic surface domains as shown in Figure 3 and 5. Based on site-specific covalent attachment, each group can be placed at the desired position, which stands in strong contrast to coated gold nanoparticles, where such
Figure 1. Schematic illustration of different types of amphiphilicity.
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Figure 2. a) Synthetic steps for protected benzene sulfonates 2 and 4c; b) General synthetic procedure of hydrophilic, hydrophobic, and amphiphilic cyclopentadienone (CP) building units 8; c) Alternative synthetic route towards stable hydrophilic cyclopentadienone building unit 10; d) Tetraphenylmethane (Td) and perylenediimide (PDI) cores together with the cyclopentadienone building blocks (AB2 and AB4).
an arrangement is driven by self-assembly of the appropriate composition of the ligand mixture.[30] Moreover, the amount, order, location, and distance of the functional groups can be monitored by synthetic modification of the organic building units (Figure 2).[31] Thus, we create a system, which cannot cluster the functional groups of same polarity by conformational changes. Classical amphiphilic surfaces accumulate polar and non-polar solvent molecules around the appropriate regions. Due to the complex surface structure and molecular motion assembly of solvent molecules around type D PPDs, which is not as well defined as for “standard” amphiphiles; no homogeneous solvation shell can be formed and periodic attractive and repulsive forces on a very small scale arise against the environment. Not only the solvation shell is affected by the patterned amphiphilic surface but also interactions with membranes occur.[32] The amphiphilic surface should allow for selective interactions between both hydrophilic and lipophilic areas of the dendrimer and lipid membranes, therefore promoting membrane permeability. Thus PPDs with a patterned amphiphilic periphery can be envisaged a potential candidate for bio-medicinal applications since membrane uptake is independent of cationic groups, which are often responsible for the increased cytotoxicity of polycationic macromolecules.[33] In this work, we outline the synthetic approaches to modulate fundamental properties such as solubility and the lipophilicity of amphiphilic PPDs at the sub-nanometer level. These features have great impact on the interaction of PPDs with cellular membranes thus directly influencing membrane
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uptake, cellular toxicity as well as the integrity of biological barriers, i.e., the blood brain barrier. PPDs with precisely defined amphiphilic surfaces patches could contribute to an improved understanding of surface textures of proteins, which could pave the way to improved, noncationic drug delivery vehicles that traffic via alternative uptake mechanisms.
2. Results and Discussion 2.1. Synthesis Our purpose is to introduce alternate polar and non-polar groups on the PPD periphery to create a kind of amphiphilicity, which affects the solvent shell offering unusual solubility trends. All dendrimers are composed of a core, scaffold, and a periphery. Different dendrimer cores determine the geometry of the dendrons and thus predefine the overall shape of the PPD.[34] Here, we discuss PPDs that were grown on either a tetraphenylmethane (Td: 11a–18a)[35] or a perylenediimide (PDI: 11b–18b)[36] core. Td leads to tetrahedral symmetry while dendronized PDI appear dumbbell shaped. Tetraphenylcyclopentadienones (CP, as well known as tetracyclones) AB2[37] and AB4[15a] (Figure 2d) were used as branching units to grow PPDs. Pre-functionalized CPs serve as end-capping units for defined creation of the final patterned peripheral topology of PPDs. To achieve the required appropriate asymmetric building block, it is necessary to unify two aromatic units substituted with different polar end groups to a
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Figure 3. Synthesized lipophilic, amphiphilic, and hydrophilic polyphenylene dendrimers in different generations built on Td or PDI cores (11–18).
bi-functionalized tetraphenylcyclopentadienone. Sulfonic acid has been chosen to form polar areas due to its strong hydrophilicity[38] and a propyl chain has been selected as the non-polar moiety in addition to the phenylenes of the dendrimer. The reaction sequence for the divergent synthesis of monodisperse PPDs 11–18 (Figure 3) is based on a [4+2] Diels–Alder cycloaddition of a cyclopentadienone branching unit (AB2 or AB4) to an ethinyl-substituted core (Td or PDI) or dendrimer, followed by removal of the silyl groups, which activates the molecule for further growth, as previously reported.[37] The repetitive cycloaddition is finalized by conversion of free acetylenes with an end capping unit (8). To elucidate the significance of the patterned surfaces, we also synthesized completely hydrophobic (only substituted with alkyl chains—peralkylated) and hydrophilic (fully sulfonated) PPDs to compare differences in solubility, polarity, and lipophilicity. The initial step in the synthesis of the Td-core was improved in this work. We were able to synthesize this important key chemical in a less toxic way and under more gentle conditions in higher yields as described[26] using dichloromethane at room temperature. While the iodination of tetraphenylmethane in tetrachloromethane afforded only 42–71% yield at 50–80 °C within 1 d, we
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obtained 70% yield in chloroform at 50 °C and 80% in methylene chloride at room temperature overnight. We assume that the higher dipole moment of the solvent promotes the iodination because the quantity of the reaction is increased from tetrachloromethane to chloroform to dichloromethane (see Supporting Information). The CPs 8, used as end-capping agents, were synthesized in good yields by the Knoevenagel-condensation[37] of diphenylacetone 7 with the benzil derivatives 6. Compound 6 was obtained by the oxidation of the corresponding diphenylacetylene derivatives 5. In case of 5a, oxidation was accomplished by usage of iodine in DMSO at 155 °C,[39] while these drastic conditions were not suitable for the oxidation of 5b and 5c, possibly due to the partial decomposition of the neopentyl ester. Thus a Wacker-type oxidation[40] was suitable for 5b and 5c under oxygen atmosphere at milder condition to afford 6b and 6c in high yields. The diphenylacetylenes (5) are easily accessible by Sonogashira-Hagihara-coupling of the appropriate 4-ethynylbenzene (1) and the bromobenzene derivatives (2) (Figure 2b). A free sulfonic acid cannot be introduced directly from the beginning of the divergent dendrimer growth as the free acid group was observed to cause considerable isolation problems caused by the polar character, which impedes washing and purification via column
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chromatography during several synthetic steps. “A posteriori” functionalization was disregarded concerning possible incomplete conversion. Therefore, the sulfonic acid must be introduced in a protected form, i.e., as sulfonic ester and should be deprotected in the last step of dendrimer synthesis. Neopentyl ester was chosen as a protecting group due to its high chemical and thermal stability, especially under Diels–Alder reaction conditions for the synthesis of PPDs. Therefore, the p-bromo benzene neopentyl sulfonate (2) was synthesized by the reaction of the corresponding sulfonyl chloride 1 with neopentyl alcohol in the presence of pyridine (Figure 2a).[41] Although the Knoevenagel-condensation afforded CP 8c, as proven by TLC and FD-mass spectrometry, it could not be isolated due to its rapid decomposition. The instability is most likely caused by the presence of two strong electron-withdrawing sulfonate groups rendering the CP ring highly electron-deficient, and thus vulnerable to nucleophilic attack. A phenylene spacer was introduced between the CP and the sulfonate groups to improve the stability, and CP 10 was accessible by the Suzuki-coupling of 2 with 9 (Figure 2c), which was synthesized according to literature.[42] To achieve the target amphiphilic dendrimers, the above-described CPs 8 were reacted with the dendrimer cores (Td or PDI). In the present case, the Diels–Alder cycloaddition was performed at milder conditions than the typical reaction conditions for PPD synthesis[37] because the neopentyl sulfonates undergo de-esterification at temperatures higher than 145 °C. Consequently, the reaction time in the present systems was increased. It was figured out that neopentylester protected sulfonic acids on the dendrimer periphery can be quantitatively converted to free acids by thermal decomposition at 180 °C over 7 d in DMF without the addition of any reagent. p-Toluene sulfonic acid is valid to catalyze this thermal deprotection as described for polymers containing neopentyl sulfonates.[43] Although all dendrimers were prepared on rather low quantities the building kit principle of PPDs and the large-scale availability of all precursors allows up-scaling, which is necessary for planned biological and medical applications. 2.2. Characterization of the Dendrimers The characterization and the proof of purity of all compounds relied on NMR-spectroscopy performed in deuterated methylene chloride or deuterated methanol. The signals of the aromatic parts and the aliphatic substituents were well-separated and assignable. The intensity ratios and the multiplicity of the signals agree with the expected values (see Supporting Information). For all non-symmetric-substituted dendrimers (13, 14, 17a, and 18a), we obtained regioisomers due to
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the orientation of the dienophile to the diene in the [4+2]-Diels–Alder cycloaddition.[28a] A slight splitting of 0.05 ppm of the 1H NMR resonances was observed for the peripheral substituents depending on the position A or B (see Figure SI1, Supporting Information). For 13 an isomer ratio of 60:40 and for 14 a ratio of 57:43 was determined, while both isomers were equally favored for 18 (50:50). The different isomers cannot be separated due to their similar physical and chemical properties. Therefore, the presence of isomers does not affect our principle of amphiphilicity, because the alternate patterning is retained. The de-esterification was monitored by 1H NMR in DMF-d6 until the proton signals of the ester group disappeared. The molecular ion peaks of the protected PPDs were observed in MALDI-TOF mass spectrometry in reflector mode and for the deprotected dendrimers by linear mode as a negative charged species. The presence of a single molecular ion peak indicates the formation of the monodisperse macromolecule (see Figure SI2, Supporting Information). 2.3. Solubility Studies To the best of our knowledge, only a single publication is describing the quantitative solubility of 1,3,5-phenylenebased dendrimers in toluene.[44] The solubility of the dendrimers was quantitatively determined from their UV-absorbance values in methanol after calibration.[45] In contrast to peralkylated (11 and 12, shown in red) or fully sulfonated (15 and 16, illustrated in blue) dendrimers, amphiphilic PPDs (13, 14, 17, and 18, depicted in green) have unusual solubility as presented in Figure 4. Peralkylated PPDs show very high solubility in nonpolar solvents as cyclohexane up to ethyl acetate, with maximal solubility in dichloromethane, chloroform, and THF, however a weaker solubility in acetone (compare Figure 4A,B,D,E). In general, all PPDs built from Td (Figure 4A,B) possessing, both amphiphilic and fully sulfonated surfaces show moderate or good solubility in aromatic solvents like benzene or toluene. The solubility in alkanes such as hexane or cyclohexane was below the detection limit. However, the solubility of fully sulfonated PPDs and the third-generation amphiphile 18 was promoted in acetone. The highest solubility was obtained in methanol for amphiphilic and fully sulfonated dendrimers, which represents an 80% increase compared to other solvents. The excellent solubility in methanol is noteworthy as the majority of the PPDs are purified by precipitation in methanol.[37] The second-generation of the amphiphiles (14a) is not well soluble in methanol, caused by the large hydrophobic cavities and the low ratio of sulfonates to hydrophobic moieties. However, 14b offers higher solubility in methanol, which results from a different core and
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meet fundamental solubility requirements for biological experiments and as cell uptake studies. This means that one sulfonic acid per end-capping unit is sufficient to enable water solubility of hydrophobic PPDs. Concomitantly the impact of the amount and alignment of polar and non-polar peripheral groups on the lipophilicity and on the membrane permeation is enormous as recently evidenced.[33] With increasing generation the solubility increases dramatically for the aromatic solvents, acetone, and methanol whereas the water solubility stays constant. The number of peripheral groups doubles per generation[46] and thereby the amount of surface patches increases, leading to densely packed surface functionalization. The generation dependence and thus the change of the dendrimer size and the amount of polar patches on solubility for the different dendrimer generations G1 up to G3 (13a, 14a, 17 and 18) are shown in Figure 4C. 2.4. Quantifying Hydrophilicity and Hydrophobicity Membrane permeation primarily depends on the molecular size and on Figure 4. Quantitative solubilities of the synthesized PPDs in first generation (A) and [47] the lipophilicity of a compound. Parin second generation (B) containing tetraphenylmethane as a core in various solvents. Generation effect on solubility of the Td-cored PPDs in first, second, highly branched tition coefficients (log P) were used to second and third generation (C). Quantitative solubilities of the synthesized PPDs in first classify a compound as hydrophilic or generation (D) and in second generation (E) containing PDI as a core in various solvents. hydrophobic.[48] In medical practice, partition coefficients are a valuable tool to thus a changed geometry of the particle. The same explaestimate the distribution of drugs or their carrier within nation is valid for the fully sulfonated dendrimers (15 and the body, and picture the pharmacokinetic property of a 16), because the additional benzene ring in the scaffold material, which has to pass through membranes. An effiincreases also the hydrophobicity. cient transport is given by a particle, which is hydrophobic enough to penetrate into the lipid bilayer, but is still sufWhile the amphiphilic Td-based dendrimers (13a, ficiently hydrophilic, so that the molecule permeates the 14a, 17a, 18a) showed solubility only in aromatic solmembrane.[49] Hydrophilic particles (log P < 0) are prefervents, acetone, and methanol or water, amphiphilic and fully sulfonated dendrimers on PDI cores (13b, 14b, 15b, entially distributed to hydrophilic areas such as the cytosol 16b, Figure 4D,E) exhibited higher solubility in methwhile hydrophobic particles (log P > 0) are preferentially anol than the Td cored dendrimers, and solubilities of found in hydrophobic parts like the lipid bilayer. 0.5 to 3 mg mL−1 were detected over a huge range of solThe shake-flask or tube method[50] was used for log P vents with different polarities from toluene and methdetermination, by dissolving the PPD in 2 mL of 1-octanol ylene chloride to alcoholic solvents and water. Finally and water in equal shares at equilibrium, then measuring and most importantly, the observed solubility of about the concentration and distribution of the solute in each 1 mg mL−1 in water, and even more in different water solvent using UV–VIS spectroscopy as described above. All dendrimers containing sulfonic acids offer log P values of mixtures as 1% DMSO/H2O of all amphiphilic and fully less than 2, which is referred to a moderate lipophilicity, sulfonated dendrimers (13ab, 14ab, 15b, 16b, 17a, 18a)
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it could give an idea whether hydrophilic groups could co-localize at the PPD surface. As expected, PPDs 11a and 12a without sulfonic acid groups at the surface are lipophilic and display no positive surface areas (Figure 5). With increasing PPD generation, e.g., from 13a>14a>18 or from 15a to 16a, the number of sulfonate groups also increases. However, the% PSA decreases for higher PPD generations, which is also in line with the increasing log P values indicating more lipophilic surface areas and therefore less solubility in polar solvents. Considering the 3D structures in Figure 5, the “inner” lipophilic polyphenylene scaffold of these PPDs seems to be solvent accessible and it increases with the PPD generation. Within one PPD generation, increased branching of the PPD scaffold leads to higher% PSA, e.g., for the second-generation PPDs 14a (7.64%) and 17 (9.13%). This finding is also in agreement with the measured log P values suggesting that higher branching leads to higher solubility in polar solvents. This at first Figure 5. Representation of the different PPDs in this study with varying hydrophilic glance contradictory finding could and lipophilic surface patches and visualization of their electrostatic surface maps. Red color denotes negatively charged regions, white color indicates hydrophobic (neutral) indicate that 17 adopts a dense archiand blue color represents positively charged regions clearly indicating the formation tecture with a less accessible “inner” of patched surfaces. Partition coefficients of the PPDs in 1-octanol-water and% polar polyphenylene scaffold, which reduces surface area (PSA). the accessible nonpolar polyphenylene surface areas. Thus, the combination of log P measurements and% PSA calculations offers an except PPD 13a and 17, which are slightly hydrophilic improved understanding of the amphiphilic nano-envi(Figure 5). In general, the amphiphilic dendrimers (13a, ronment of different PPDs. By varying the dendrimer 14a, and 17) can be assigned as true amphiphiles based generation, the polyphenylene branching density as well on their log P close to zero, which reflects the solubility as the number of polar surface groups, the ratio and disin organic and aqueous media. Dendrimers offering log P tribution of hydrophilicity versus hydrophobicity surface values close to neutral (14a, 15a, and 16a) should migrate patterns can be balanced with a high degree of spatial faster through membranes than other highly hydrophilic definition, which will have an impact on interactions PPDs at the expense of solubility in the aqueous media. with membranes or on cellular toxicity. For comparison, efficient dermal penetration of amphiphilic compounds was predicted for amphiphiles with log P values from −0.31 up to 1.17.[51] These values are within 3. Conclusions the log P range, which we can adapt precisely with our amphiphilic dendrimers (13a–18). Nano-sized monodisperse PPDs possessing a patched In addition, the relative polar surface area (% PSA) peripheral amphiphilicity have been synthesized and of the PPDs synthesized here was calculated from the investigated offering an inherently stable 3D architecture. 3D structures of the PPDs after minimization applying A defined structure leads to precise setting of hydrophilic the MMFF94x force field by fixing the RMS gradient at (sulfonic acids) and hydrophobic (propyl chains and phe0.05 kcal mol−1 Å (Figure 5). It refers to the percentage of nylenes) areas on the periphery in an alternating fashion. the polar surface area versus the total van der Waals area. This arrangement causes a unique solubility depending This value indicates the presence of the surface areas of on the number of polar patches, the PPD size, and the core the polar patches within a lipophilic surrounding and
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geometry. Td-cored PPDs are soluble in toluene and methanol, as well as water or aqueous mixtures and in the case of using PDI, solubility in a wide range of solvents with different polarities was observed. Balancing lipophilicity and water solubility is important for biological investigations, e.g., cellular internalization by membrane penetration or cytotoxicity due to increased membrane leakiness. Systematic variation of the peripheral amphiphilicity is thus an enormous challenge for precision polymer synthesis and allows studying the impact of patched structures on physical, chemical, and biological properties, i.e., polarity, solubility, and membrane permeation. Such a constitutional perfection has to be accompanied by a conformational control. Only a combination of both yielding optimized surface patterns, could pave the way to efficient and non-cytotoxic PPD drug transporters and help to improve our fundamental understanding of comparable surface textures found in proteins. Polymer synthesis in its methods is very limited as flexible structures with a polydispersity in size and in the degree of functionalization are obtained. A. Constitutional and conformational perfection, which has been defined as a key target of precision polymer synthesis above and demonstrated for PPDs, can thus be brought to a further sophistication by controlling surface amphiphilicity of 3D-macromolecules and by furnishing surrogates of proteins. Ongoing and future work addresses the dendrimer scaffold, which can be modified to adjust the cavities, functionalized to bind drugs, and tailored to introduce stimuli-responsive (i.e., heat, pH, or reductive cleavage of disulfide bonds etc.) moieties in order to design highly efficient drug-delivery systems.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements: For financial support of this research we thank gratefully the Deutsche Forschungsgemeinschaft (SFB625). We thank Dr. Brenton Hammer for carefully reading of the manuscript. Received: August 30, 2013; Revised: October 7, 2013; Published online: November 24, 2013; DOI: 10.1002/marc.201300671 Keywords: amphiphiles; biomimetic; dendrimers; Diels–Alder polymers; solution properties
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Communication
Tuning Polarity of Polyphenylene Dendrimers by Patched Surface Amphiphilicity—Precise Control over Size, Shape, and Polarity René Stangenberg, Irfan Saeed, Seah Ling Kuan, Martin Baumgarten, Tanja Weil, Markus Klapper, Klaus Müllen* In the ideal case, a precise synthesis yields molecules with a constitutional as well as a conformational perfectness. Such a case of precision is demonstrated by the synthesis of semi-rigid amphiphilic polyphenylene dendrimers (PPDs). Polar sulfonate groups are precisely placed on their periphery in such a manner that patches of polar and non-polar regions are created. Key structural features are the semi-rigid framework and shape-persistent nature of PPDs since the limited flexibility introduces a nano-phase-separated amphiphilic rim of the dendrimer. This results in both attractive and repulsive interactions with a given solvent. Frustrated solvent structures then lead to a remarkable solubility in solvents of different polarity such as toluene, methanol, and water or their mixtures. Water solubility combined with defined surface structuring and variable hydrophobicity of PPDs that resemble the delicate surface textures of proteins are important prerequisites for their biological and medical applications based upon cellular internalization.
1. Introduction “Precisely controlled polymer architectures” define one of the hottest topics in polymer synthesis.[1] This line of research was boosted by the discovery of controlled radical polymerization methods such as reversible additionfragmentation chain transfer (RAFT),[2] nitroxide-mediated radical polymerization (NMRP),[3] and atom transfer radical polymerization (ATRP)[4] providing access to polymers with narrow polydispersity and to new polymer architectures such as block, stars, or comb-type polymers. Further, R. Stangenberg, I. Saeed, Dr. M. Baumgarten, Dr. M. Klapper, Prof. K. Müllen Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany E-mail: [email protected] Dr. S. L. Kuan, Prof. T. Weil University of Ulm, Institute for Organic Chemistry III/Macromolecular Chemistry, Albert-Einstein-Allee 11, 89081 Ulm, Germany
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the range of applicable monomers was dramatically broadened.[5] Controlling the formation of precise polymer architectures also comprises the exact positioning of functional groups along a polymer chain. A prominent example is ADMET (acyclic diene metathesis), which polymerizes functionalized terminal dienes to linear polyenes.[6] Functional groups such as carboxylic acid groups can be attached to a backbone at defined distances whereas a copolymerization of functionalized and unfunctionalized monomers would only afford a statistical distribution. Although both controlled radical polymerization and ADMET allow for a particular control of molecular weight, composition, and structure, the “precision” is still limited by the kinetics of the polymerization yielding molecular weight distributions.[7] Monodispersity can be accomplished by the synthesis of dendrimers.[8] Dendrimers consist of a core, a scaffold with multiple branching points, and a shell. The divergent, which is generation-by-generation, growth of a dendrimer utilizes branching reagents of the ABn-type
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DOI: 10.1002/marc.201300671
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and needs protection–deprotection protocols to avoid premature reactions of A and B functions.[9] While polymerizations are, in principle, possible, polycondensations and polyadditions are most commonly applied. By these approaches, molecular weights up to 1.9 MDa can be obtained, which is by far exceeding the range of many polymeric materials synthesized by the above-mentioned controlled polymerization methods.[10] Most dendrimers contain ether, amide, or ester linkages.[11] Their formation is far from quantitative and side reactions can occur. Any defect thus introduced cannot be healed, but is instead amplified upon further growth. As a result, defect-free macromolecules are hardly accessible and molecular distributions, even if narrow, are unavoidable.[12] It should be noted that structural definition of polymers does not only demand constitutional perfection but also conformational control in solution and in the solid state.[13] If the conformation of flexible macromolecules is not easy to adjust, the position and chemical environment of functional groups cannot reliably be predicted.[14] Both linear polymers and the dendrimers described above contain conformational flexible moieties, which give rise to structural and functional “uncertainty” under the prevailing experimental conditions (temperature, solvent, etc). We have offered a solution to constitutional and conformational perfection by the synthesis of semi-rigid polyphenylene dendrimers (PPDs).[15] They are composed of twisted, tightly packed interlocked benzene rings. Their synthesis is based on the Diels–Alder cycloaddition of ethinyl-substituted benzenes and tetraphenylcyclopentadienones (see below), which proceeds with a much higher perfection than the dendrimer-forming reactions mentioned above.[15a] These dendrimers are mainly based on para-connected polyaromatic buildings blocks. Therefore, PPDs are unique within the dendrimer field, because the rigid polyphenylene dendron arms do not allow their backbending and thus exclude the reorientation of functional groups.[16] Size and shape become exactly defined and are essentially independent of the solvent.[17] By using differently functionalized building blocks in the sequential synthesis, numerous functional groups can be exactly placed in the core, the scaffold or the shell.[15c] Two examples should be mentioned: Introducing chromophores into the nano-environment of PPDs affords light-harvesting systems with vectorial energy transfer.[18] Likewise, since the PPDs comprise defined voids they serve as receptors for guest molecules.[19] Their uptake can be analyzed by microbalance measurements or isothermal titration calorimetry (ITC) and leads to sensing devices for explosives.[20] Here, we consider another challenge for precision polymer synthesis: the control of amphiphilicity as an opportunity to mimic the delicate polar- and non-polar
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surface textures of proteins.[21] Proteins are compact macromolecules with precisely defined 3D architectures where each surface functionality is located at a distinct location. Some proteins such as serum albumins form defined lipophilic pockets that accommodate a wide variety of drug molecules thus serving as efficient natural transporter molecules.[22] The functionalization of rigid dendrimers can be used to introduce precisely at certain areas hydrophobic and hydrophilic parts. Such a control is rather common in nature but hard to achieve with synthetic structures. Synthetic amphiphiles such as dodecylsulfonates or the pluronics, which are widely used as surfactants[23] or emulsifiers,[24] are mostly very flexible molecules comprising one hydrophilic and one hydrophobic part. Natural amphiphiles such as lecithin, one of the main components of cell membranes or proteins, are structurally more complex since they possess several hydrophilic and lipophilic parts at defined areas either on the surface or inside. The control over the amphiphilicity plays a vital role in biology[25] as amphiphilicity is a decisive driving force for cell penetration processes with for example peptides. Thereby, the internalization of molecules into cells requires a precise control over size and shape as well as an exact positioning of hydrophilic and hydrophobic areas. Amphiphilic block or statistical copolymers are far from having the required structural and conformational precision due to their conformational mobility. This gap between synthetic and natural amphiphiles should be bridged by amphiphilic PPDs. Amphiphilicity in dendrimers[26] can be realized in: i) core–shell-(A, B)[27] or, ii) Janus-type-systems (C),[28] and iii) by the alternate placement of polar and nonpolar groups on the surface (D) (Figure 1). To the best of our knowledge, only few examples of type D dendrimers that resemble the patchy structures of proteins have been reported so far,[29] but in contrast to proteins, they have a flexible framework, which is not matching the defined positioning of the polarity observed for natural amphiphiles. We decided to decorate the periphery of stiff PPDs with an alternating arrangement of polar (sulfonic acid) and non-polar (propyl) groups on a sub-nanometer length scale (≈6 Å) to create patterned hydrophilic and hydrophobic surface domains as shown in Figure 3 and 5. Based on site-specific covalent attachment, each group can be placed at the desired position, which stands in strong contrast to coated gold nanoparticles, where such
Figure 1. Schematic illustration of different types of amphiphilicity.
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Figure 2. a) Synthetic steps for protected benzene sulfonates 2 and 4c; b) General synthetic procedure of hydrophilic, hydrophobic, and amphiphilic cyclopentadienone (CP) building units 8; c) Alternative synthetic route towards stable hydrophilic cyclopentadienone building unit 10; d) Tetraphenylmethane (Td) and perylenediimide (PDI) cores together with the cyclopentadienone building blocks (AB2 and AB4).
an arrangement is driven by self-assembly of the appropriate composition of the ligand mixture.[30] Moreover, the amount, order, location, and distance of the functional groups can be monitored by synthetic modification of the organic building units (Figure 2).[31] Thus, we create a system, which cannot cluster the functional groups of same polarity by conformational changes. Classical amphiphilic surfaces accumulate polar and non-polar solvent molecules around the appropriate regions. Due to the complex surface structure and molecular motion assembly of solvent molecules around type D PPDs, which is not as well defined as for “standard” amphiphiles; no homogeneous solvation shell can be formed and periodic attractive and repulsive forces on a very small scale arise against the environment. Not only the solvation shell is affected by the patterned amphiphilic surface but also interactions with membranes occur.[32] The amphiphilic surface should allow for selective interactions between both hydrophilic and lipophilic areas of the dendrimer and lipid membranes, therefore promoting membrane permeability. Thus PPDs with a patterned amphiphilic periphery can be envisaged a potential candidate for bio-medicinal applications since membrane uptake is independent of cationic groups, which are often responsible for the increased cytotoxicity of polycationic macromolecules.[33] In this work, we outline the synthetic approaches to modulate fundamental properties such as solubility and the lipophilicity of amphiphilic PPDs at the sub-nanometer level. These features have great impact on the interaction of PPDs with cellular membranes thus directly influencing membrane
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uptake, cellular toxicity as well as the integrity of biological barriers, i.e., the blood brain barrier. PPDs with precisely defined amphiphilic surfaces patches could contribute to an improved understanding of surface textures of proteins, which could pave the way to improved, noncationic drug delivery vehicles that traffic via alternative uptake mechanisms.
2. Results and Discussion 2.1. Synthesis Our purpose is to introduce alternate polar and non-polar groups on the PPD periphery to create a kind of amphiphilicity, which affects the solvent shell offering unusual solubility trends. All dendrimers are composed of a core, scaffold, and a periphery. Different dendrimer cores determine the geometry of the dendrons and thus predefine the overall shape of the PPD.[34] Here, we discuss PPDs that were grown on either a tetraphenylmethane (Td: 11a–18a)[35] or a perylenediimide (PDI: 11b–18b)[36] core. Td leads to tetrahedral symmetry while dendronized PDI appear dumbbell shaped. Tetraphenylcyclopentadienones (CP, as well known as tetracyclones) AB2[37] and AB4[15a] (Figure 2d) were used as branching units to grow PPDs. Pre-functionalized CPs serve as end-capping units for defined creation of the final patterned peripheral topology of PPDs. To achieve the required appropriate asymmetric building block, it is necessary to unify two aromatic units substituted with different polar end groups to a
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Figure 3. Synthesized lipophilic, amphiphilic, and hydrophilic polyphenylene dendrimers in different generations built on Td or PDI cores (11–18).
bi-functionalized tetraphenylcyclopentadienone. Sulfonic acid has been chosen to form polar areas due to its strong hydrophilicity[38] and a propyl chain has been selected as the non-polar moiety in addition to the phenylenes of the dendrimer. The reaction sequence for the divergent synthesis of monodisperse PPDs 11–18 (Figure 3) is based on a [4+2] Diels–Alder cycloaddition of a cyclopentadienone branching unit (AB2 or AB4) to an ethinyl-substituted core (Td or PDI) or dendrimer, followed by removal of the silyl groups, which activates the molecule for further growth, as previously reported.[37] The repetitive cycloaddition is finalized by conversion of free acetylenes with an end capping unit (8). To elucidate the significance of the patterned surfaces, we also synthesized completely hydrophobic (only substituted with alkyl chains—peralkylated) and hydrophilic (fully sulfonated) PPDs to compare differences in solubility, polarity, and lipophilicity. The initial step in the synthesis of the Td-core was improved in this work. We were able to synthesize this important key chemical in a less toxic way and under more gentle conditions in higher yields as described[26] using dichloromethane at room temperature. While the iodination of tetraphenylmethane in tetrachloromethane afforded only 42–71% yield at 50–80 °C within 1 d, we
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obtained 70% yield in chloroform at 50 °C and 80% in methylene chloride at room temperature overnight. We assume that the higher dipole moment of the solvent promotes the iodination because the quantity of the reaction is increased from tetrachloromethane to chloroform to dichloromethane (see Supporting Information). The CPs 8, used as end-capping agents, were synthesized in good yields by the Knoevenagel-condensation[37] of diphenylacetone 7 with the benzil derivatives 6. Compound 6 was obtained by the oxidation of the corresponding diphenylacetylene derivatives 5. In case of 5a, oxidation was accomplished by usage of iodine in DMSO at 155 °C,[39] while these drastic conditions were not suitable for the oxidation of 5b and 5c, possibly due to the partial decomposition of the neopentyl ester. Thus a Wacker-type oxidation[40] was suitable for 5b and 5c under oxygen atmosphere at milder condition to afford 6b and 6c in high yields. The diphenylacetylenes (5) are easily accessible by Sonogashira-Hagihara-coupling of the appropriate 4-ethynylbenzene (1) and the bromobenzene derivatives (2) (Figure 2b). A free sulfonic acid cannot be introduced directly from the beginning of the divergent dendrimer growth as the free acid group was observed to cause considerable isolation problems caused by the polar character, which impedes washing and purification via column
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chromatography during several synthetic steps. “A posteriori” functionalization was disregarded concerning possible incomplete conversion. Therefore, the sulfonic acid must be introduced in a protected form, i.e., as sulfonic ester and should be deprotected in the last step of dendrimer synthesis. Neopentyl ester was chosen as a protecting group due to its high chemical and thermal stability, especially under Diels–Alder reaction conditions for the synthesis of PPDs. Therefore, the p-bromo benzene neopentyl sulfonate (2) was synthesized by the reaction of the corresponding sulfonyl chloride 1 with neopentyl alcohol in the presence of pyridine (Figure 2a).[41] Although the Knoevenagel-condensation afforded CP 8c, as proven by TLC and FD-mass spectrometry, it could not be isolated due to its rapid decomposition. The instability is most likely caused by the presence of two strong electron-withdrawing sulfonate groups rendering the CP ring highly electron-deficient, and thus vulnerable to nucleophilic attack. A phenylene spacer was introduced between the CP and the sulfonate groups to improve the stability, and CP 10 was accessible by the Suzuki-coupling of 2 with 9 (Figure 2c), which was synthesized according to literature.[42] To achieve the target amphiphilic dendrimers, the above-described CPs 8 were reacted with the dendrimer cores (Td or PDI). In the present case, the Diels–Alder cycloaddition was performed at milder conditions than the typical reaction conditions for PPD synthesis[37] because the neopentyl sulfonates undergo de-esterification at temperatures higher than 145 °C. Consequently, the reaction time in the present systems was increased. It was figured out that neopentylester protected sulfonic acids on the dendrimer periphery can be quantitatively converted to free acids by thermal decomposition at 180 °C over 7 d in DMF without the addition of any reagent. p-Toluene sulfonic acid is valid to catalyze this thermal deprotection as described for polymers containing neopentyl sulfonates.[43] Although all dendrimers were prepared on rather low quantities the building kit principle of PPDs and the large-scale availability of all precursors allows up-scaling, which is necessary for planned biological and medical applications. 2.2. Characterization of the Dendrimers The characterization and the proof of purity of all compounds relied on NMR-spectroscopy performed in deuterated methylene chloride or deuterated methanol. The signals of the aromatic parts and the aliphatic substituents were well-separated and assignable. The intensity ratios and the multiplicity of the signals agree with the expected values (see Supporting Information). For all non-symmetric-substituted dendrimers (13, 14, 17a, and 18a), we obtained regioisomers due to
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the orientation of the dienophile to the diene in the [4+2]-Diels–Alder cycloaddition.[28a] A slight splitting of 0.05 ppm of the 1H NMR resonances was observed for the peripheral substituents depending on the position A or B (see Figure SI1, Supporting Information). For 13 an isomer ratio of 60:40 and for 14 a ratio of 57:43 was determined, while both isomers were equally favored for 18 (50:50). The different isomers cannot be separated due to their similar physical and chemical properties. Therefore, the presence of isomers does not affect our principle of amphiphilicity, because the alternate patterning is retained. The de-esterification was monitored by 1H NMR in DMF-d6 until the proton signals of the ester group disappeared. The molecular ion peaks of the protected PPDs were observed in MALDI-TOF mass spectrometry in reflector mode and for the deprotected dendrimers by linear mode as a negative charged species. The presence of a single molecular ion peak indicates the formation of the monodisperse macromolecule (see Figure SI2, Supporting Information). 2.3. Solubility Studies To the best of our knowledge, only a single publication is describing the quantitative solubility of 1,3,5-phenylenebased dendrimers in toluene.[44] The solubility of the dendrimers was quantitatively determined from their UV-absorbance values in methanol after calibration.[45] In contrast to peralkylated (11 and 12, shown in red) or fully sulfonated (15 and 16, illustrated in blue) dendrimers, amphiphilic PPDs (13, 14, 17, and 18, depicted in green) have unusual solubility as presented in Figure 4. Peralkylated PPDs show very high solubility in nonpolar solvents as cyclohexane up to ethyl acetate, with maximal solubility in dichloromethane, chloroform, and THF, however a weaker solubility in acetone (compare Figure 4A,B,D,E). In general, all PPDs built from Td (Figure 4A,B) possessing, both amphiphilic and fully sulfonated surfaces show moderate or good solubility in aromatic solvents like benzene or toluene. The solubility in alkanes such as hexane or cyclohexane was below the detection limit. However, the solubility of fully sulfonated PPDs and the third-generation amphiphile 18 was promoted in acetone. The highest solubility was obtained in methanol for amphiphilic and fully sulfonated dendrimers, which represents an 80% increase compared to other solvents. The excellent solubility in methanol is noteworthy as the majority of the PPDs are purified by precipitation in methanol.[37] The second-generation of the amphiphiles (14a) is not well soluble in methanol, caused by the large hydrophobic cavities and the low ratio of sulfonates to hydrophobic moieties. However, 14b offers higher solubility in methanol, which results from a different core and
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meet fundamental solubility requirements for biological experiments and as cell uptake studies. This means that one sulfonic acid per end-capping unit is sufficient to enable water solubility of hydrophobic PPDs. Concomitantly the impact of the amount and alignment of polar and non-polar peripheral groups on the lipophilicity and on the membrane permeation is enormous as recently evidenced.[33] With increasing generation the solubility increases dramatically for the aromatic solvents, acetone, and methanol whereas the water solubility stays constant. The number of peripheral groups doubles per generation[46] and thereby the amount of surface patches increases, leading to densely packed surface functionalization. The generation dependence and thus the change of the dendrimer size and the amount of polar patches on solubility for the different dendrimer generations G1 up to G3 (13a, 14a, 17 and 18) are shown in Figure 4C. 2.4. Quantifying Hydrophilicity and Hydrophobicity Membrane permeation primarily depends on the molecular size and on Figure 4. Quantitative solubilities of the synthesized PPDs in first generation (A) and [47] the lipophilicity of a compound. Parin second generation (B) containing tetraphenylmethane as a core in various solvents. Generation effect on solubility of the Td-cored PPDs in first, second, highly branched tition coefficients (log P) were used to second and third generation (C). Quantitative solubilities of the synthesized PPDs in first classify a compound as hydrophilic or generation (D) and in second generation (E) containing PDI as a core in various solvents. hydrophobic.[48] In medical practice, partition coefficients are a valuable tool to thus a changed geometry of the particle. The same explaestimate the distribution of drugs or their carrier within nation is valid for the fully sulfonated dendrimers (15 and the body, and picture the pharmacokinetic property of a 16), because the additional benzene ring in the scaffold material, which has to pass through membranes. An effiincreases also the hydrophobicity. cient transport is given by a particle, which is hydrophobic enough to penetrate into the lipid bilayer, but is still sufWhile the amphiphilic Td-based dendrimers (13a, ficiently hydrophilic, so that the molecule permeates the 14a, 17a, 18a) showed solubility only in aromatic solmembrane.[49] Hydrophilic particles (log P < 0) are prefervents, acetone, and methanol or water, amphiphilic and fully sulfonated dendrimers on PDI cores (13b, 14b, 15b, entially distributed to hydrophilic areas such as the cytosol 16b, Figure 4D,E) exhibited higher solubility in methwhile hydrophobic particles (log P > 0) are preferentially anol than the Td cored dendrimers, and solubilities of found in hydrophobic parts like the lipid bilayer. 0.5 to 3 mg mL−1 were detected over a huge range of solThe shake-flask or tube method[50] was used for log P vents with different polarities from toluene and methdetermination, by dissolving the PPD in 2 mL of 1-octanol ylene chloride to alcoholic solvents and water. Finally and water in equal shares at equilibrium, then measuring and most importantly, the observed solubility of about the concentration and distribution of the solute in each 1 mg mL−1 in water, and even more in different water solvent using UV–VIS spectroscopy as described above. All dendrimers containing sulfonic acids offer log P values of mixtures as 1% DMSO/H2O of all amphiphilic and fully less than 2, which is referred to a moderate lipophilicity, sulfonated dendrimers (13ab, 14ab, 15b, 16b, 17a, 18a)
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it could give an idea whether hydrophilic groups could co-localize at the PPD surface. As expected, PPDs 11a and 12a without sulfonic acid groups at the surface are lipophilic and display no positive surface areas (Figure 5). With increasing PPD generation, e.g., from 13a>14a>18 or from 15a to 16a, the number of sulfonate groups also increases. However, the% PSA decreases for higher PPD generations, which is also in line with the increasing log P values indicating more lipophilic surface areas and therefore less solubility in polar solvents. Considering the 3D structures in Figure 5, the “inner” lipophilic polyphenylene scaffold of these PPDs seems to be solvent accessible and it increases with the PPD generation. Within one PPD generation, increased branching of the PPD scaffold leads to higher% PSA, e.g., for the second-generation PPDs 14a (7.64%) and 17 (9.13%). This finding is also in agreement with the measured log P values suggesting that higher branching leads to higher solubility in polar solvents. This at first Figure 5. Representation of the different PPDs in this study with varying hydrophilic glance contradictory finding could and lipophilic surface patches and visualization of their electrostatic surface maps. Red color denotes negatively charged regions, white color indicates hydrophobic (neutral) indicate that 17 adopts a dense archiand blue color represents positively charged regions clearly indicating the formation tecture with a less accessible “inner” of patched surfaces. Partition coefficients of the PPDs in 1-octanol-water and% polar polyphenylene scaffold, which reduces surface area (PSA). the accessible nonpolar polyphenylene surface areas. Thus, the combination of log P measurements and% PSA calculations offers an except PPD 13a and 17, which are slightly hydrophilic improved understanding of the amphiphilic nano-envi(Figure 5). In general, the amphiphilic dendrimers (13a, ronment of different PPDs. By varying the dendrimer 14a, and 17) can be assigned as true amphiphiles based generation, the polyphenylene branching density as well on their log P close to zero, which reflects the solubility as the number of polar surface groups, the ratio and disin organic and aqueous media. Dendrimers offering log P tribution of hydrophilicity versus hydrophobicity surface values close to neutral (14a, 15a, and 16a) should migrate patterns can be balanced with a high degree of spatial faster through membranes than other highly hydrophilic definition, which will have an impact on interactions PPDs at the expense of solubility in the aqueous media. with membranes or on cellular toxicity. For comparison, efficient dermal penetration of amphiphilic compounds was predicted for amphiphiles with log P values from −0.31 up to 1.17.[51] These values are within 3. Conclusions the log P range, which we can adapt precisely with our amphiphilic dendrimers (13a–18). Nano-sized monodisperse PPDs possessing a patched In addition, the relative polar surface area (% PSA) peripheral amphiphilicity have been synthesized and of the PPDs synthesized here was calculated from the investigated offering an inherently stable 3D architecture. 3D structures of the PPDs after minimization applying A defined structure leads to precise setting of hydrophilic the MMFF94x force field by fixing the RMS gradient at (sulfonic acids) and hydrophobic (propyl chains and phe0.05 kcal mol−1 Å (Figure 5). It refers to the percentage of nylenes) areas on the periphery in an alternating fashion. the polar surface area versus the total van der Waals area. This arrangement causes a unique solubility depending This value indicates the presence of the surface areas of on the number of polar patches, the PPD size, and the core the polar patches within a lipophilic surrounding and
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geometry. Td-cored PPDs are soluble in toluene and methanol, as well as water or aqueous mixtures and in the case of using PDI, solubility in a wide range of solvents with different polarities was observed. Balancing lipophilicity and water solubility is important for biological investigations, e.g., cellular internalization by membrane penetration or cytotoxicity due to increased membrane leakiness. Systematic variation of the peripheral amphiphilicity is thus an enormous challenge for precision polymer synthesis and allows studying the impact of patched structures on physical, chemical, and biological properties, i.e., polarity, solubility, and membrane permeation. Such a constitutional perfection has to be accompanied by a conformational control. Only a combination of both yielding optimized surface patterns, could pave the way to efficient and non-cytotoxic PPD drug transporters and help to improve our fundamental understanding of comparable surface textures found in proteins. Polymer synthesis in its methods is very limited as flexible structures with a polydispersity in size and in the degree of functionalization are obtained. A. Constitutional and conformational perfection, which has been defined as a key target of precision polymer synthesis above and demonstrated for PPDs, can thus be brought to a further sophistication by controlling surface amphiphilicity of 3D-macromolecules and by furnishing surrogates of proteins. Ongoing and future work addresses the dendrimer scaffold, which can be modified to adjust the cavities, functionalized to bind drugs, and tailored to introduce stimuli-responsive (i.e., heat, pH, or reductive cleavage of disulfide bonds etc.) moieties in order to design highly efficient drug-delivery systems.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements: For financial support of this research we thank gratefully the Deutsche Forschungsgemeinschaft (SFB625). We thank Dr. Brenton Hammer for carefully reading of the manuscript. Received: August 30, 2013; Revised: October 7, 2013; Published online: November 24, 2013; DOI: 10.1002/marc.201300671 Keywords: amphiphiles; biomimetic; dendrimers; Diels–Alder polymers; solution properties
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