Article pubs.acs.org/JPCB

New Free Radicals to Measure Antiradical Capacity: A Theoretical Study Jorge Rafael León-Carmona,† Ana Martínez,*,† and Annia Galano‡ †

Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Circuito Ext. s/n, Ciudad Universitaria, P.O. Box 70-360, Coyoacán, C.P. 04510, México ‡ Departamento de Química. Universidad Autónoma Metropolitana-Iztapalapa, San Rafael Atlixco 186, Col. Vicentina, Iztapalapa, C.P. 09340, México S Supporting Information *

ABSTRACT: A new family of free radicals, that are soluble in water and stable at all pH values, were recently synthesized and used to assess the antiradical capacity of several polyphenols. In the present work, density functional calculations were used to investigate the single electron transfer reactions between these new free radicals and polyphenols in aqueous solution. The quantification of the antiradical capacity is a challenge, particularly for polyphenols, since they become unstable under experimental conditions. It was found that the electron transfer from polyphenols to the newly developed free radicals can be used to assess the efficiency of this kind of compound for preventing oxidative stress. Since one of the free radicals can be deprotonated under experimental conditions, this newly synthesized radical can help distinguish more clearly between different antiradical compounds with similar antioxidant capacity by modifying the pH in the experiments. The results reported here are in good agreement with the available experimental data and allowed making recommendations about possible experimental conditions in the design of antioxidant assays using the investigated radicals.



INTRODUCTION The oxidative stress is produced when there is a chemical imbalance between the production and consumption of free radicals.1 This has attracted much attention in the last decades due to its influence in the development of a large number of health disorders such as cancer,2−5 atherosclerosis,6−9 Alzheimer’s disease,10−14 and cardiovascular disorders.15−19 Since oxidative stress frequently involves reactions between free radicals and molecules of high biological importance such as DNA, lipids, and proteins, studying the ability of chemical compounds to efficiently scavenge free radicals (antiradical capacity) becomes an important area of research, with implications in the prevention of oxidative stress and the consequent cellular damage. Polyphenols are very efficient compounds as free radical scavengers.20−27 They are consumed in human diet in a wide variety of foods and beverages, such as fruits, vegetables, wine, coffee, and tea.28 They also have other important biological roles, including cardioprotective effects,29−32 anti-inflammatory,33,34 antimicrobial, and antiviral activities.35−38 In addition, they are used to prevent and treat cancer,39,40 osteoporosis,41 neurodegenerative diseases,42,43 and skin damage.44 There are several methodologies available to quantify how effective is the antiradical capacity of different substances.45−49 For polyphenols this is a challenge since they become unstable under some experimental conditions. Moreover, free radicals © 2014 American Chemical Society

used to analyze the antiradical capacity frequently have their own drawbacks, including low solubility in water and also instability. Consequently, a great effort has been devoted to improve the experimental conditions that allow quantifying and differentiate the antiradical activity of polyphenols. One of the ideas is to synthesize new, more stable, free radicals to determine the antiradical capacity.50 Some of these synthetic free radicals are the tripotassium salt of tris(2,3,5,6-tetrachloro4-hydroxysulphonylphenyl) methyl radical, (3K+[TSPTM3−]), the tris(2,4,6-trichloro-3,5-dinitrophenyl)methyl radical (HNTTM), and tris(2,3,5,6-tetrachloro-2-nitrophenyl)methyl radical (TNPTM; see Scheme 1). These free radicals were successfully used in the evaluation of the antiradical properties51−55 of several molecules, including polyphenols. Based on the structures of these radicals, which present a highly hindered radical center, it is assumed that they will react mainly by the electron transfer mechanism. To analyze this mechanism, it is important to determine the electron donor and acceptor properties of these free radicals and of the molecules they are reacting with. There is no information concerning the electron transfer capacity of these free radicals and neither about the kinetics of their reactions with polyphenols. For this Received: June 5, 2014 Revised: August 5, 2014 Published: August 6, 2014 10092

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Scheme 1. Schematic Representation of the Recently Synthesized Free Radicals50

Scheme 2. Polyphenols Studied in This Worka

a

The deprotonation sites are highlighted in red.

activity.56−58 With the results reported here it would be possible to establish whether these newly synthesized radicals can help distinguish between different antiradical compounds with similar antioxidant capacities.

reason, the main goal of this investigation is to study the electron transfer reaction between these free radicals and polyphenols in order to provide thermochemical and kinetic data that allows establishing reactivity trends. To this purpose we have investigated the reactions between 30 different polyphenols (Scheme 2) and the newly synthesized stablefree radicals shown in Scheme 1. Deprotonated polyphenols are also included since it is important to compare with the reported experimental data and because, under physiological conditions (pH = 7.4), their anionic forms can be crucial for the antiradical



COMPUTATIONAL DETAILS All the electronic calculations have been carried out with the package of programs Gaussian 09.59 Full geometry optimizations and frequency calculations were performed using the density functional B3LYP and the 6-31+G(d) basis set. The 10093

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(ΔG). The results show that the deviation between the different energy terms obtained with both approaches (B//A and all B) are very small. Actually they were found to be lower than 1.07 kcal/mol in all the studied cases, which is very close to the currently accepted chemical accuracy (1 kcal/mol). This case corresponds to reaction ii, which involves the phenoxide anion. Thus, this is in line with the geometrical features abovediscussed. For the other three reaction the largest deviation is 0.56 kcal/mol. These results also validate using the B//A double level approach for the system investigated in the present work. In addition to the fully optimized structures that are used to obtain the Gibbs energies of the studied reaction, single point calculations were also performed for the cation and the anion, using the fully optimized ground state structure of the neutral molecules to obtain vertical energies. With these values, vertical ionization energies (IE) and electron affinities (EA) were calculated as

energy values were improved by single point calculations with the 6-311++G(d,p) basis set. This two level approach is commonly referred as B//A. Full geometry optimizations, without any symmetry constraints, and frequency calculations were performed for all the species in gas phase (vacuum). Local minima were identified by the absence of imaginary frequencies. Solvent effects were included a posteriori by single point calculations using SMD model at B3LYP/6-311++G(d,p) level of theory. Thermodynamic corrections at 298.15 K were included in the calculation of relative energies. The B//A approach is very often applied, but optimized structures in gas and water might be significantly different, particularly for charged species. For this reason, it is necessary to verify that the optimized structures obtained using both levels of theory are similar. To this purpose, model molecules similar in nature to those studied in the present work were analyzed. To represent the phenolic compounds, we have used phenol and phenoxide anion. To test the other reactant, that is, the studied radicals, we have used the simplified models shown in Scheme 3. The modeled reactions then are HPh + MR1• → HPh•+ + MR1−

(i)

HPh + MR2• → HPh•+ + MR2−

(ii)

Ph− + MR1• → Ph• + MR1−

(iii)









Ph + MR2 → Ph + MR1

IE = EN − 1 − EN

(1)

EA = EN − EN + 1

(2)

where EN is the energy of the N-electron system and EN−1 and EN+1 are the energies of the (N − 1) and (N + 1) electron systems. E stands for electronic energy. This information, IE and EA, was used to locate the molecules at the full electron donor−acceptor map (FEDAM, Figure 1), a useful tool to classify any substance regarding its

(iv)

Scheme 3. Simplified Models Used to Test the Reliability of the B//A Approach

First, the geometries of all the species involved, optimized at levels A and B, were analyzed (Table S1). The largest deviations were found for the phenoxide anion. The mean unsigned error, expressed in percent, was found to be 0.88% in this case, while the maximum absolute error, also expressed in percent, is 2.82%. This means that the effect of increasing the basis set are negligible on the geometries. Moreover, it means that the geometries of the studied species are quite similar in vacuum and in water solution. Thus, it is expected that performing geometry optimizations and frequency calculations at the B level, for the species studied in this work, would drastically increase the computational cost without significantly improving the quality of the results. These findings clearly indicate that the geometries obtained with both levels of theory are quite similar, which validates using the B//A double level approach for the system investigated in the present work. In addition, since the main data proposed in this investigation comprise energy values, we have performed an additional test. This consist on comparing the energetic data obtained using the B//A approach with that arising from pure B level calculations (Table S2). The energy changes associated with the four model chemical reactions i−iv have been analyzed in terms of total energy without (ΔE) and with zero point energy corrections (ΔEZPE), enthalpies, and Gibbs free energies

Figure 1. Full electron donator acceptor map (FEDAM).

electron-donating and electron-accepting properties.60,61 The compounds with low IE values are the best electron donors and, as a result, they represent the best antiradicals in terms of the electron donating capability. Substances with high and positive EA values have a greater capacity for accepting electrons, and thus, they represent the most efficient antiradicals, in terms of the electron accepting capability. The rate constants (k) were calculated using the conventional transition state theory (TST)62−64 and 1 M standard state. The barriers of reaction were estimated using the Marcus theory.65−67 Some of the calculated rate constants (k) are close to the diffusion limit. Accordingly, the apparent rate constant (kapp) cannot be directly obtained from TST calculations. In the present work, the Collins-Kimball theory is used to that purpose:68 kapp =

kDkact kD + kact

(3)

where kact is the thermal rate constant, obtained from TST calculations, and kD is the steady-state Smoluchowski69 rate 10094

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constant for an irreversible bimolecular diffusion-controlled reaction: kD = 4πRDABNA

(4)

where R denotes the reaction distance, NA is the Avogadro number, and DAB is the mutual diffusion coefficient of the reactants A (free radical) and B (polyphenol). DAB was calculated from DA and DB according to ref 70. DA and DB were estimated from the Stokes−Einstein approach.71,72

D=

kBT 6πηa

(5)

where kB is the Boltzmann constant, T is the temperature, η denotes the viscosity of the solvent, in our case, water (η = 8.91 × 10−4 Pa s), and a is the radius of the solute.



RESULTS AND DISCUSSION In all the studied free radicals, there is a central carbon atom bonded to three aromatic rings containing chlorine, nitro, or sulfonate groups as substituents. The bond lengths are larger when the substituent is the sulfonate group. The bonds between radical carbon and the ring, the chlorine and the ring, and the nitro group are equivalent for all the radicals, and the distances remain equal. To prevent steric hindrance, the benzene rings are not parallel, and the optimized structures are not planar. The optimized structures of the three deprotonated molecules of TSPTM are similar to the neutrals, but the orientation of the of the sulfonate moiety is different after deprotonation (with the dihedral angle changing from 52° in the protonated form to 62° when is deprotonated). In addition, the C−S bond length increases when the free radical is deprotonated from 1.83 to 1.91 Å. Polyphenols shown in Scheme 2 present different electron donor and electron acceptor capabilities, which can be useful to assess if the free radicals shown in Scheme 1 can be used to differentiate their relative antiradical activity.73 Since both, the studied polyphenols and free radicals are susceptible to deprotonation, the corresponding anions, based in the deprotonation sites suggested by other authors for polyphenols74−77 were considered in this study. Polyphenols (neutral and anionic) were fully optimized. The optimized structures are available upon request. To classify the electron donating and accepting capabilities of polyphenols and free radicals, they have been simultaneously included in a FEDAM (Figure 2). The free radicals are located up to the right of the FEDAM. Neutral polyphenols are placed close to the free radicals. The electron donor capability of neutral polyphenols is similar to that of the free radicals but they are worse electron acceptors since their EA is smaller. Monoanionic polyphenols (deprotonated polyhenols) are situated down to the left of the FEDAM. As expected due to the negative charge, they are good electron donors and bad electron acceptors. In fact, their electron affinity is negative, indicating that the electron acceptance will be energetically unfavorable. According to the FEDAM, deprotonation increases the antiradical character of polyphenols when the scavenging mechanism is the electron transfer from the deprotonated polyphenols to the neutral free radicals. For TSPTM the three possible deprotonated species (Scheme 1) have been included in the analysis. As expected, the electron donor−acceptor capability is reduced in the series monoanion, dianion, trianion (Figure 2). However, there is not

Figure 2. FEDAM of polyphenols and free radicals (aqueous phase).

much difference since the ions are stabilized in water. Therefore, neutral and deprotonated TSPTM will accept electrons from deprotonated polyphenols. This is in agreement with previous experimental results. Mesa et al.50 suggested that the electron transfer reaction takes place in such a way that TSPTM−3 accepts one electron from the polyphenols, yielding TSPTM−4. According to the FEDAM, this is a viable process, because the relative location of TSPTM−3 reveals that it is a better electron-acceptor than polyphenols and deprotonated polyphenols. While the relative position of chemical species in the FEDAM allows a qualitative assessment of their ability as electron donors or acceptors, quantitative information is still needed to predict if the electron transfer reactions are thermodynamically feasible. Therefore, the Gibbs free energies of reaction for the possible electron transfer processes were used to this purpose. For all the reactions, the free radicals are considered the electron acceptors based on their position in the FEDAM, relative to polyphenols. The results are reported as Supporting Information. The reactions between the neutral free radicals and the neutral polyphenols are reported in Table S3 and are all endergonic. This is in agreement with the position of these molecules in the FEDAM, since they are located very close. Table S4 reports the values for the reactions between the neutral free radicals and deprotonated polyphenols (anions), considering the free radicals as electron acceptors. Most of these reactions are exergonic, in line with their relative position in the FEDAM. The Gibbs free energies for the reactions between TSPTM−2 and TSPTM−3 and polyphenols (neutral and deprotonated) are reported in Tables S5 and S6, respectively. The values are all positive, indicating that the reactions are endergonic and, therefore, thermodynamically unviable. The reaction of TSPTM−1 with some deprotonated polyphenols is exergonic. Accordingly, with these results it is possible to conclude that TSPTM−1 could be useful for distinguishing the antiradical properties of different polyphenols. In addition, we have also compared the energies of the HOMOs of the neutral and anionic polyphenols, with the SOMOs of the studied radicals (Table S7). This gap has been previously proposed by Markovic et al.78 to predict the spontaneity of electron transfer reactions. It was found that this criterion is adequate for the studied reactions. The energies of 10095

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Table 1. Calculated Rate Constants for the SET Reactions at 298.15 K, in Aqueous Solution, for the Reaction between New Free Radicals and Polyphenols neutral polyphenols TSPTM luteolin apigenin kaempferide quercetin kaempferol myricetin fisetin isorhamnetin pachypodol rhamnazin hesperetin naringenin eriodictyol homoeriodictyol taxifolin aromadedrin genistein daidzein glycitein catechin gallocatechin resveratrol laricitrin syringetin piceatannol aesculetin galangin morin azaleatin gossypetin

1.41 5.35 3.91 9.25 1.39 1.76 9.03 1.57 1.59 2.68 1.68 2.66 1.32 2.93 1.18 1.09 2.09 5.47 6.73 1.41 1.08 7.44 1.64 5.28 6.00 1.20 2.68 3.49 8.49 1.57

× × × × × × × × × × × × × × × × × × × × × × × × × × × × × ×

10−06 10−10 1001 10−02 10−02 1002 10−02 1002 1002 10−12 10−01 10−11 10−03 10−01 10−06 10−14 10−03 10−03 10−02 10−05 10−03 1005 1002 10−02 1005 1000 10−06 10−03 1001 1003

HNTTM 1.89 3.86 1.79 5.86 8.40 5.38 1.29 5.88 2.91 5.44 3.54 4.22 1.19 1.31 2.68 1.07 5.70 5.23 1.81 3.07 1.37 1.15 6.83 2.24 5.24 2.51 7.39 2.50 3.37 1.29

× × × × × × × × × × × × × × × × × × × × × × × × × × × × × ×

10−08 10−13 1002 10−02 10−03 1003 10−01 1002 1003 10−02 1000 10−14 10−03 1000 10−08 10−19 10−04 10−03 10−01 10−07 10−04 1007 1002 10−02 1006 1000 10−08 10−03 1002 1004

deprotonated polyphenols TNPTM 3.25 3.07 3.14 1.82 2.59 1.44 2.96 1.09 5.31 9.38 7.11 1.95 3.07 2.27 3.43 9.91 2.40 1.34 3.45 4.24 8.22 1.90 1.24 8.17 8.78 5.22 8.67 7.08 6.13 2.13

× × × × × × × × × × × × × × × × × × × × × × × × × × × × × ×

10−11 10−15 10−02 10−05 10−06 1000 10−05 10−01 10−01 10−06 10−04 10−16 10−07 10−04 10−11 10−21 10−07 10−06 10−05 10−10 10−08 1003 10−01 10−06 1002 10−04 10−11 10−07 10−02 1000

TSPTM 7.52 7.44 6.12 7.54 7.60 7.50 7.54 7.54 7.48 7.52 7.51 7.55 7.52 7.48 7.51 7.47 7.50 7.56 7.58 7.51 7.48 7.48 7.49 7.48 7.56 7.68 3.47 7.43 7.51 7.55

× × × × × × × × × × × × × × × × × × × × × × × × × × × × × ×

1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009

HNTTM 7.49 7.48 7.43 7.48 7.54 7.46 7.47 7.42 7.44 7.42 7.45 7.50 7.41 7.37 6.85 7.45 7.00 6.90 6.95 7.46 1.54 6.90 7.34 7.36 1.54 7.62 7.27 7.48 7.34 7.51

× × × × × × × × × × × × × × × × × × × × × × × × × × × × × ×

1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009

TNPTM 7.39 6.86 1.85 7.46 7.49 7.43 7.46 7.47 7.42 7.45 7.45 7.46 7.46 7.43 7.45 7.37 7.44 7.49 7.50 7.44 7.44 7.47 7.44 7.43 7.49 7.53 2.99 7.04 7.45 7.44

× × × × × × × × × × × × × × × × × × × × × × × × × × × × × ×

1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1008 1009 1009 1009

reaction distances for electron transfers are expected to be longer than those for other reaction mechanisms such as H transfer or radical adduct formation. Thus, the formed products are far enough from each other to allow interactions with other molecules in their environment, which can lead to an irreversible process. Accordingly, for the particular case of the SET reactions, it has been recommend to include them in the kinetic calculations also when their ΔG values are positive, but small (ΔG ≤ 10 kcal/mol), and their rate constants are high.46 For the reactions between neutral polyphenols and neutral radicals (Table 1), it was found that most of the studied reactions are very slow, with rate constants lower than 1. Regardless of the free radical, the polyphenols are reacting with TSPTM, HNTTM, or TNPTM. The only exceptions to this trend are the reactions of TSPTM with resveratrol, piceatannol, and gossypetin (with rate constants equal to 7.44 × 105, 6.00 × 105, and 1.57 × 103 M−1 s−1); the reactions of HNTTM with resveratrol, piceatannol, gossypetin, myricetin, and pachypodol (1.15 × 107, 5.24 × 106, 1.29 × 104, 5.38 × 103, and 2.91 × 103 M−1 s−1); and the reaction of TNPTM with resveratrol (1.90 × 103 M−1 s−1). Accordingly, it can be stated that, under experimental conditions, corresponding to pH values acidic enough to guarantee that both polyphenols and the tested free radicals are mainly in their neutral forms, an assay based on the reactions with TSPTM, HNTTM, or TNPTM would only identify the most reactive polyphenols as antiradical protectors. This means that such an assay might be used to recognize the

the SOMOs of the studied free radicals are very close to the energies of the HOMO of the neutral polyphenols, which is in agreement with the positive ΔG values previously described for the reactions between these species. On the other hand, the energies of the HOMO of the anionic polyphenols are systematically higher than the SOMOs of the free radicals, which is in agreement with our findings that such reactions are exergonic. To gain further insights on the studied reactions, kinetic calculations were also performed. Particular attention is paid to the direction of the electron transfer reaction, and to the acid− base forms of polyphenols and free radicals. The rate constants for the SET reactions were calculated in aqueous solution to mimic the biological systems. Even though usually only exergonic reaction paths are included in kinetic calculations, this simplified approach is based on the assumption that even if the endergonic paths take place at a significant rate, they would be reversible and therefore the formed products will not be observed. However, such paths might still be significant if their products rapidly react further. This would be particularly important if these latter stages are sufficiently exergonic to provide a driving force, and if their barriers of reactions are low. Thus, in such cases the kinetic study must be performed. The electron transfer mechanism is an example of that situation. Moreover, for this kind of reactions, positive low values of ΔG frequently correspond to high values of rate constants and the ionic intermediates are likely to react fast. In addition, the 10096

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most potent antioxidants. In addition, under such conditions, HNTTM is the only studied radical able to rapidly react with a large subset of polyphenols via one electron transfer from the later. The trends obtained here are in agreement with that reported for the antiradical activity of polyphenols in gas phase,73 being piceatannol, resveratrol, and gossypetin, the polyphenols with higher activity against these radicals. For the reactions involving deprotonated polyphenols and neutral radicals (Table 1), all the rate constants were found to be within, or very close to, the diffusion limited regime. This means that under experimental conditions favoring the formation of these acid−base forms, any assay based on the reactivity of polyphenols toward TSPTM, HNTTM or TNPTM would indicate that at least all the studied polyphenols are excellent antioxidants. Therefore, it can be concluded that such conditions are not favorable for differentiating the efficiency of polyphenols as free radical scavengers. These findings also indicate that experimental conditions promoting deprotonation of polyphenols would increase their protective effects against oxidative stress. For the reactions involving the anionic forms of TSPTM (TSPTM−1 , TSPTM −2 , and TSPTM −3 ) based on the thermochemical considerations above-discussed, the only feasible processes would be those involving the deprotonated polyphenols. Accordingly, the kinetic data reported here corresponds only to those reactions (Table 2). In this case it was found that the rate constants of the reactions with TSPTM−1 are all within, or very close to, the diffusion limit, except those involving kaempferide and galangin (9.77 × 106 and 4.13 × 105 M−1 s−1). Accordingly, it can be stated that the monoanion of TSPTM is not very selective toward polyphenols, and thus, it is proposed that experimental assays using this radical are designed under conditions that do not promote the formation of TSPTM−1. The selectivity of this compound toward the studied polyphenols increases after the second deprotonation takes place. For this species the rate constants are spanned to a wide range. Even though most of the polyphenols will still donate one electron to TSPTM−2 at very high rates, the reactions with luteolin, apigenin, kaempferide, kaempferol, aromadedrin, galangin, morin, and gossypetin are order of magnitude slower. The case of TSPTM−3 deserves special attention. Among all the reactions investigated in this work, those between this radical and the deprotonated polyphenols are the most promising for accomplishing the goal of differentiating among the antiradical activity of polyphenols (Table 2). In this case the separation of the rate constants is maximum, which indicates that this radical is the most selective of the six studied here. Consequently, it is proposed as the optimum free radical for experimental assays in aqueous solution designed to differentiate polyphenols according to their antiradical activity. Logically that implies rather basic media, and the viability of such an assay would depend on the stability of the investigated compounds under such conditions. Based on these findings, it was possible to assess the relative antiradical activity of the studied polyphenols, via electron transfer. Taking the rate constants as the quantitative criterion, their efficiency for such a task is proposed to follow the order resveratrol > piceatannol > gallocatechin > homoeriodictyol > eriodictyol > hesperetin > laricitrin > syringetin > catechin > glycitein > azaleatin > daidzein > rhamnazin > isohamnetin > genistein > pachypodol > morin > myricetin > naringenin >

Table 2. Calculated Rate Constants for the SET Reactions at 298.15 K, in Aqueous Solution, for the Reaction between the Deprotonated Polyphenols and the First, Second, and Third Deprotonated TSPTM luteolin (−) apigenin (−) kaempferide (−) quercetin (−) kaempferol (−) myricetin (−) fisetin (−) isorhamnetin (−) pachypodol (−) rhamnazin (−) hesperetin (−) naringenin (−) eriodictyol (−) homoeriodictyol (−) taxifolin (−) aromadedrin (−) genistein (−) daidzein (−) glycitein (−) catechin (−) gallocatechin (−) resveratrol (−) laricitrin (−) syringetin (−) piceatannol (−) aesculetin (−) galangin (−) morin (−) azaleatin (−) gossypetin (−)

TSPTM−1

TSPTM−2

× × × × × × × × × × × × × × × × × × × × × × × × × × × × × ×

× × × × × × × × × × × × × × × × × × × × × × × × × × × × × ×

6.79 1.23 9.77 7.49 7.27 7.43 7.51 7.55 7.42 7.52 7.52 7.42 7.54 7.49 7.52 6.97 7.51 7.58 7.60 7.49 7.51 7.58 7.50 7.48 7.60 7.33 4.13 5.04 7.52 7.16

09

10 1009 1006 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1009 1005 1009 1009 1009

1.73 1.73 2.17 6.20 8.72 8.33 9.64 5.94 1.17 5.83 6.56 4.21 6.99 7.27 3.89 6.60 6.34 7.00 7.15 5.17 7.43 7.52 6.89 6.77 7.50 1.64 1.10 4.28 6.70 4.49

07

10 1003 1000 1008 1006 1008 1008 1009 1009 1009 1009 1008 1009 1009 1009 1007 1009 1009 1009 1009 1009 1009 1009 1009 1009 1008 10−02 1007 1009 1007

TSPTM−3 1.47 2.72 3.48 8.28 2.02 4.67 8.28 4.12 8.02 4.70 1.35 1.89 1.41 1.67 4.90 2.48 8.42 7.66 2.13 2.39 4.27 1.01 4.17 3.85 5.99 1.31 3.51 4.87 1.67 3.96

× × × × × × × × × × × × × × × × × × × × × × × × × × × × × ×

1001 10−06 10−09 1002 10−01 1003 1002 1005 1003 1005 1007 1003 1007 1008 1002 1002 1004 1005 1006 1006 1008 1009 1006 1006 1008 1003 10−12 1003 1006 1001

aesculetin > fisetin > quercetin > taxifolin > aromadedrin > gossypetin > luteolin > kaempferol > apigenin > kaempferide > galangin. Accordingly, even though the specific structure of each polyphenol has its own influence on the reactivity toward TSPTM−3, via electron transfer, some generalizations can be made. Stilbenes were identified as the most reactive among the studied polyphenols, via SET, followed by flavanones, flavanols, isoflavones, and coumarins; while the weakest electron donors were identified to be flavones and flavanonols.



CONCLUSIONS The reactivity of a set of three recently developed free radicals toward polyphenols has been investigated, taking into account the acid−base species, when possible. One characteristic of these radicals is that they react exclusively by a single electron transfer mechanism due to steric hindrance. Thus, this is the kind of reaction that has been investigated. The reactions between the neutral free radicals and deprotonated polyphenols, considering the free radicals as electron acceptors are all exergonic, and predicted to take place at rates within, or very close to, the diffusion limit. These newly designed free radicals were found to be good chemical sensors for the radical scavenging activity of polyphenols in aqueous media, in particular, TSPTM−3. The water solubility of this new radical species and its stability at all pH values, from acid to basic solutions, facilitates the study of 10097

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(9) Panasenko, O. M.; Nova, T. V.; Azizova, O. A.; Vladimirov, Y. A. Free Radical Modification of Lipoproteins and Cholesterol Accumulation in Cells upon Atherosclerosis. Free Radicals Biol. Med. 1991, 10, 137−148. (10) Rosini, M.; Simoni, E.; Milelli, A.; Minarini, A.; Melchiorre, C. Oxidative Stress in Alzheimer’s Disease: Are we Connecting the Dots? J. Med. Chem. 2014, 57, 2821−2831. (11) Bayrakdar, E. T.; Uyanikgil, Y.; Kanit, L.; Koylu, E.; Yalcin, A. Nicotinamide Treatment Reduces the Levels of Oxidative Stress, Apoptosis, and PARP-1 Activity in Aβ(1−42)-Induced Rat Model of Alzheimer’s Disease. Free Radical Res. 2014, 48, 146−158. (12) Fay, D. S.; Fluet, A.; Johnson, C. J.; Link, C. D. In Vivo Aggregation of β-Amyloid Peptide Variants. J. Neurochem. 1998, 71, 1616−1625. (13) Butterfield, D. A. β-Amyloid-Associated Free Radical Oxidative Stress and Neurotoxicity: Implications for Alzheimer’s Disease. Chem. Res. Toxicol. 1997, 10, 495−506. (14) Hensley, K.; Carney, J. M.; Mattson, M. P.; Aksenova, M.; Harris, M.; Wu, J. F.; Floyd, R. A.; Butterfield, D. A. A Model for βAmyloid Aggregation and Neurotoxicity Based on Free Radical Generation by the Peptide: Relevance to Alzheimer Disease. Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 3270−3274. (15) Matsuda, M.; Shimomura, I. Roles of Adiponectin and Oxidative Stress in Obesity-Associated Metabolic and Cardiovascular Diseases. Rev. Endocr. Metab. Dis. 2014, 15, 1−10. (16) Eren, E.; Ellidag, H. Y.; Cekin, Y.; Ayoglu, R. U.; Sekercioglu, A. O.; Yilmaz, N. Heart Valve Disease: The role of Calcidiol Deficiency, Elevated Parathyroid Hormone Levels and Oxidative Stress in Mitral and Aortic Valve Insufficiency. Redox Rep. 2014, 19, 34−39. (17) Stephens, N. G.; Parsons, A.; Schofield, P. M.; Kelly, F.; Cheesman, K.; Mitchisnon, M. J.; Brown, M. J. Randomised Controlled Trial of Vitamin E in Patients with Coronary Disease: Cambridge Heart Antioxidant Study (CHAOS). Lancet 1996, 347, 781−786. (18) Street, D. A.; Comstock, G.; Salkeldy, R.; Klag, M. Serum Antioxidants and Myocardial Infarction. Are Low Levels of Carotenoids and alpha-Tocopherol Risk Factors for Myocardial Infarction? Circulation 1994, 90, 1154−1161. (19) Salonen, J. T.; Nyyssöner, K.; Korpela, H.; Tuomilehto, J.; Seppanen, R.; Salonen, R. High Stored Iron Levels are Associated with Excess Risk of Myocardial Infarction in Eastern Finnish Men. Circulation 1992, 86, 803−811. (20) Rice-Evans, C. A.; Miller, N. J.; Bolwell, P. G.; Bramley, P. M.; Pridham, J. B. The Relative Antioxidant Activities of Plant-Derived Polyphenolic Flavonoids. Free Radical Res. 1995, 22, 375−383. (21) Di Meo, F.; Lemaur, V.; Cornil, J.; Lazzaroni, R.; Duroux, J.-L.; Olivier, Y.; Trouillas, P. Free Radical Scavenging by Natural Polyphenols: Atom versus Electron Transfer. J. Phys. Chem. A 2013, 117, 2082−2092. (22) Royer, M.; Diouf, P. N.; Stevanovic, T. Polyphenol Contents and Radical Scavenging Capacities of Red Maple (Acer Rubrum L.) Extracts. Food Chem. Toxicol. 2011, 49, 2180−2188. (23) Cao, G.; Sofic, E.; Prior, R. L. Antioxidant and Prooxidant Behavior of Flavonoids: Structure-Activity Relationships. Free Radicals Biol. Med. 1997, 22, 749−760. (24) Silvia, V.; Baldisserotto, A.; Scalambra, E.; Malisardi, G.; Durini, E.; Manfredini, S. Novel Molecular Combination Deriving from Natural Aminoacids and Polyphenols: Design, Synthesis and FreeRadical Scavenging Activities. Eur. J. Med. Chem. 2012, 50, 383−392. (25) Lemanska, K.; Szymusiak, H.; Tyrakowsla, B.; Zielinski, R.; Soffers, A. E. M.; Rietens, I. M. C. M. The Influence of pH on Antioxidant Properties and the Mechanism of Antioxidant Action of Hydroxyflavones. Free Radicals Biol. Med. 2001, 31, 869−881. (26) Materska, M.; Perucka, I. Antioxidant Activity of the Main Phenolic Compounds Isolated from Hot Pepper Fruit (Capsicum annuum L.). J. Agric. Food Chem. 2005, 53, 1750−1756. (27) Fernandez-Panchon, M. S.; Villano, D.; Troncoso, A. M.; Garcia-Parrilla, M. C. Antioxidant Activity of Phenolic Compounds:

the influence of the pH of the medium on the activity of polyphenols. Therefore, we have estimated the radicalscavenging activity of 30 polyphenols, both in neutral and deprotonated forms, which are expected present at physiological pH value (7.4), using these radicals. It is predicted that TSPTM−3 would allow the best differentiation among polyphenols, regarding their antioxidant activity via electron transfer. Thus, this is the radical recommended for experimental antioxidant assays, provided that under the necessary conditions to promote the formation of TSPTM−3, there is no stability issues for the investigated compounds.



ASSOCIATED CONTENT

S Supporting Information *

Geometry optimization data, total energies, Gibbs free energies, and energies of the frontier orbitals are included. This material is available free of charge via the Internet at http://pubs.acs.org.



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (52) 55 5622 4596. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This study was funded by DGAPA-PAPIIT, Consejo Nacional de Ciencia y Tecnologiá (CONACyT), and resources provided by the Instituto de Investigaciones en Materiales (IIM). This work was partially supported by the Project SEP-CONACyT 167491. This work was carried out using a NES supercomputer, ́ de provided by Dirección General de Cómputo y Tecnologias Información y Comunicación (DGTIC), the Laboratorio de Visualización y Cómputo Paralelo at Universidad Autónoma Metropolitana-Iztapalapa. J.R.L.-C. acknowledges the economic support of the Program of Postdoctoral Scholarships from DGAPA (UNAM).



REFERENCES

(1) Sayre, L. M.; Perry, G.; Smith, M. A. Oxidative Stress and Neurotoxicity. Chem. Res. Toxicol. 2008, 21, 172−188. (2) Wang, J.; Li, J.-Z.; Lu, A.-X.; Zhang, K.-F.; Li, B.-J. Anticancer Effect of Salidroside on A549 Lung Cancer Cells through Inhibition of Oxidative Stress and Phospho-P38 Expression. Oncol. Lett. 2014, 7, 1159−1164. (3) Granados-Principal, S.; El-azem, N.; Pamplona, R.; RamirezTortosa, C.; Pulido-Moran, M.; Vera-Ramirez, L.; Quiles, J. L.; Sanchez-Rovira, P.; Naudí, A.; Portero-Otin, M.; et al. Hydroxytyrosol Ameliorates Oxidative Stress and Mitochondrial Dysfunction in Doxorubicin-Induced Cardiotoxicity in Rats with Breast Cancer. Biochem. Pharmacol. 2014, 90, 25−33. (4) Tekiner-Gulbas, B.; Westwell, A. D.; Suzen, S. Oxidative Stress in Carcinogenesis: New Synthetic Compounds with Dual Effects upon Free Radicals and Cancer. Curr. Med. Chem. 2013, 20, 4451−4459. (5) Knekt, P.; et al. Body Iron Stores and Risk of Cancer. Int. J. Cancer 1994, 56, 379−382. (6) Al-Aubaidy, H. A.; Jelinek, H. F. Oxidative Stress and Triglycerides as Predictors of Subclinical Atherosclerosis in Prediabetes. Redox Rep. 2014, 19, 87−91. (7) Şerban, C.; Dragan, S. The Relationship between Inflammatory and Oxidative Stress Biomarkers, Atherosclerosis and Rheumatic Diseases. Curr. Pharm. Des. 2014, 20, 585−600. (8) Zampetaki, A.; Dudek, K.; Mayr, M. Oxidative Stress in Atherosclerosis: The Role of MicroRNAs in Arterial Remodeling. Free Radicals Biol. Med. 2013, 64, 69−77. 10098

dx.doi.org/10.1021/jp505586k | J. Phys. Chem. B 2014, 118, 10092−10100

The Journal of Physical Chemistry B

Article

From In Vitro Results to In Vivo Evidence. Crit. Rev. Food Sci. 2008, 48, 649−671. (28) Perron, N. R.; Brumaghim, J. L. A Review of the Antioxidant Mechanisms of Polyphenol Compounds Related to Iron Binding. Cell. Biochem. Biophys. 2009, 53, 75−100. (29) Tresserra-Rimbau, A.; Rimm, E. B.; Medina-Remón, A.; Martínez-González, M. A.; de la Torre, R.; Corella, D.; SalasSalvadó, J.; Gómez-Gracia, E.; Lapetra, J.; Arós, F.; et al. Inverse Association between Habitual Polyphenol Intake and Incidence of Cardiovascular Events in the PREDIMED Study. Nutr. Metab. Cardiovasc. Dis. 2014, 24, 639−647. (30) Kishimoto, Y.; Tani, M.; Kondo, K. Pleiotropic Preventive Effects of Dietary Polyphenols in Cardiovascular Diseases. Eur. J. Clin. Nutr. 2013, 67, 532−535. (31) Corder, R.; Mullen, W.; Khan, K. Q.; Marks, S. C.; Wood, E. G.; Carrier, M. J.; Crozier, A. Oenology: Red Wine Procyanidins and Vascular Health. Nature 2006, 444, 566. (32) Manach, C.; Mazur, A.; Scalbert, A. Polyphenols and Prevention of Cardiovascular Diseases. Curr. Opin. Lipidol. 2005, 16, 77−84. (33) Palaska, I.; Papathanasiou, E.; Theoharides, T. C. Use of Polyphenols in Periodontal Inflammation. Eur. J. Pharmacol. 2013, 720, 77−83. (34) Ha, J.-H.; Shil, P. K.; Zhu, P.; Gu, L.; Li, Q.; Chung, S. Ocular Inflammation and Endoplasmic Reticulum Stress are Attenuated by Supplementation with Grape Polyphenols in Human Retinal Pigmented Epithelium Cells and in C57BL/6 Mice. J. Nutr. 2014, 144, 799−806. (35) Rizzo, A.; Carratelli, C. R.; Losacco, A.; Iovene, M. R. Antimicrobial Effect of Natural Polyphenols with or without Antibiotics on Chlamydia Pneumoniae Infection in Vitro. Microb. Drug Resist. 2014, 20, 1−10. (36) Kolodziejczyk, K.; Sojka, M.; Abadias, M.; Viñas, I.; Guyot, S.; Baron, A. Polyphenol Composition, Antioxidant Capacity, and Antimicrobial Activity of the Extracts Obtained from Industrial Sour Cherry Pomace. Ind. Crop Prod. 2013, 51, 279−288. (37) Dhiman, R. K. The Green Tea Polyphenol, Epigallocatechin-3Gallate (EGCG)-One Step Forward in Antiviral Therapy against Hepatitis C Virus. J. Clin. Exp. Hepatol. 2011, 1, 159−160. (38) Queffelec, C.; Bailly, F.; Mbemba, G.; Mouscadet, J.-F.; Hayes, S.; Debyser, Z.; Witvrouw, M.; Cotelle, P. Synthesis and Antiviral Properties of Some Polyphenols Related to Salvia Genus. Bioorg. Med. Chem. Lett. 2008, 18, 4736−4740. (39) Khan, N.; Bharali, D. J.; Adhami, V. M.; Siddiqui, I. A.; Cui, H.; Shabana, S. M.; Mousa, S. A.; Mukhtar, H. Oral Administration of Naturally Occurring Chitosan-Based Nanoformulated Green Tea Polyphenol EGCG Effectively Inhibits Prostate Cancer Cell Growth in a Xenograft Model. Carcinogenesis 2014, 35, 415−423. (40) Vizzotto, M.; Porter, W.; Byrne, D.; Cisneros-Zevallos, L. Polyphenols of Selected Peach and Plum Genotypes Reduce Cell Viability and Inhibit Proliferation of Breast Cancer Cells while not Affecting Normal Cells. Food Chem. 2014, 164, 363−370. (41) Cornwell, T.; Cohick, W.; Raskin, I. Dietary Phytoestrogens and Health. Phytochemistry 2004, 65, 995−1016. (42) Pacifico, S.; Gallicchio, M.; Lorenz, P.; Duckstein, S. M.; Potenza, N.; Galasso, S.; Marciano, S.; Fiorentino, A.; Stintzing, F. C.; Monaco, P. Neuroprotective Potential of Laurus nobilis Antioxidant Polyphenol-Enriched Leaf Extracts. Chem. Res. Toxicol. 2014, 27, 611− 626. (43) Costa, L. G.; Tait, L.; de Laat, R.; Dao, K.; Giordano, G.; Pellacani, C.; Cole, T. B.; Furlong, C. E. Modulation of Paraoxonase 2 (PON2) in Mouse Brain by the Polyphenol Quercetin: A Mechanism of Neuroprotection? Neurochem. Res. 2013, 38, 1809−1818. (44) Lephart, E. D. Protective Effects of Equol and their Polyphenolic Isomers Against Dermal Aging: Microarray/Protein Evidence with Clinical Implications and Unique Delivery into Human Skin. Pharm. Biol. 2013, 51, 1393−1400. (45) Alberto, M. E.; Russo, N.; Grand, A.; Galano, A. A Physicochemical Examination of the Free Radical Scavenging Activity

of Trolox: Mechanism, Kinetics and Influence of the Environment. Phys. Chem. Chem. Phys. 2013, 15, 4642−4650. (46) Galano, A.; Alvarez-Idaboy, J. R. A Computational Methodology for Accurate Predictions of Rate Constants in Solution: Application to the Assessment of Primary Antioxidant Activity. J. Comput. Chem. 2013, 34, 2430−2445. (47) Prior, R. L.; Wu, X.; Schaich, K. Standardized Methods for the Determination of Antioxidant Capacity and Phenolics in Foods and Dietary Supplements. J. Agric. Food Chem. 2005, 53, 4290−4302. (48) Molyneux, P. The Use of the Stable Free Radical Diphenylpicrylhydrazyl (DPPH) for Estimating Antioxidant Activity. Songklanakarin J. Sci. Technol. 2004, 26, 211−219. (49) Brand-Williams, W.; Cuvelier, M. E.; Berset, C. Use of a Free Radical Method to Evaluate Antioxidant Activity. LWT−Food Sci. Technol. 1995, 28, 25−30. (50) Mesa, J. A.; Torres, J. L.; Juliá, L. Selective Control of the Radical-Scavenging Activity of Poly(phenols) in Aqueous Media in Terms of Their Electron-Donor Properties, Using a Stable Organic Radical as Chemical Sensor. Talanta 2012, 101, 141−147. (51) Mesa, J. A.; Velázquez-Palenzuela, A.; Brillas, E.; Torres, J. Ll.; Juliá, L. Synthesis of a New Stable and Water-Soluble Tris(4hydroxysulfonyltetrachlorophenyl)methyl Radical with Selective Oxidative Capacity. Tetrahedron 2011, 67, 3119−3123. (52) Jiménez, A.; Selga, A.; Torres, J. Ll.; Juliá, L. Reducing Activity of Polyphenols with Stable Radicals of the TTM Series. Electron Transfer versus H-Abstraction Reactions in Flavan-3-ols. Org. Lett. 2004, 6, 4583−4586. (53) Torres, J. Ll.; Varela, B.; Brillas, E.; Juliá, L. Tris(2,4,6-trichloro3,5-dinitrophenyl)methyl Radical: A New Stable Coloured Magnetic Species as a Chemosensor for Natural Polyphenols. Chem. Commun. 2003, 74−75. (54) Touriño, S.; Selga, A.; Jiménez, A.; Juliá, L.; Lozano, C.; Lizárraga, D.; Cascante, M.; Torres, J. Ll. Procyanidin Fractions from Pine (Pinus pinaster) Bark: Radical Scavenging Power in Solution, Antioxidant Activity in Emulsion, and Antiproliferative Effect in Melanoma Cells. J. Agric. Food Chem. 2005, 53, 4728−4735. (55) Yang, J.; Liu, G.-Y.; Lu, D.-L.; Dai, F.; Qian, Y.-P.; Jin, X.-L.; Zhou, B. Hybrid-Increased Radical-Scavenging Activity of Resveratrol Derivatives by Incorporating a Chroman Moiety of Vitamin E. Chem.Eur. J. 2010, 16, 12808−12813. (56) Medina, M. E.; Galano, A.; Alvarez-Idaboy, J. R. Theoretical Study on the Peroxyl Radicals Scavenging Activity of Esculetin and Its Regeneration in Aqueous Solution. Phys. Chem. Chem. Phys. 2014, 16, 1197−1207. (57) Galano, A.; Pérez-González, A. On the Free Radical Scavenging Mechanism of Protocatechuic Acid, Regeneration of the Catechol Group in Aqueous Solution. Theor. Chem. Acc. 2012, 131, 1−13. (58) León-Carmona, J. R.; Alvarez-Idaboy, J. R.; Galano, A. On the Peroxyl Scavenging Activity of Hydroxycinnamic Acid Derivatives. Mechanisms, Kinetics, and Importance of the Acid/Base Equilibria. Phys. Chem. Chem. Phys. 2012, 14, 12534−12543. (59) Frisch, M. J. et al. Gaussian 09, Revision A.08; Gaussian, Inc.: Wallingford, CT, 2009. (60) Martínez, A.; Rodríguez-Gironés, M. A.; Barbosa, A.; Costas, M. Donator Acceptor Map for Carotenoids, Melatonin and Vitamins. J. Phys. Chem. A 2008, 112, 9037−9042. (61) Galano, A. Relative Antioxidant Efficiency of a Large Series of Carotenoids in Terms of One Electron Transfer Reactions. J. Phys. Chem. B 2007, 111, 12898−12908. (62) Eyring, H. The Activated Complex in Chemical Reactions. J. Chem. Phys. 1935, 3, 107−115. (63) Evans, M. G.; Polanyi, M. Some Applications of the Transition State Method to the Calculation of Reaction Velocities, Especially in Solution. Trans. Faraday Soc. 1935, 31, 875−894. (64) Truhlar, D. G.; Hase, W. L.; Hynes, J. T. Current Status of Transition-State Theory. J. Phys. Chem. 1983, 87, 2664−2682. (65) Marcus, R. A. Chemical and Electrochemical Electron-Transfer Theory. Annu. Rev. Phys. Chem. 1964, 16, 155−196. 10099

dx.doi.org/10.1021/jp505586k | J. Phys. Chem. B 2014, 118, 10092−10100

The Journal of Physical Chemistry B

Article

(66) Marcus, R. A. Electron Transfer Reactions in Chemistry. Theory and Experiment. Rev. Mod. Phys. 1993, 65, 599−610. (67) Marcus, R. A. Transfer Reactions in Chemistry. Theory and Experiment. Pure Appl. Chem. 1997, 69, 13−30. (68) Collins, F. C.; Kimball, G. E. Diffusion-Controlled Reaction Rates. J. Colloid Sci. 1949, 4, 425−437. (69) von Smoluchowski, M. Versuch einer Mathematischen Theorie der Koagulationskinetik Kolloider Lösungen. Z. Phys. Chem. 1917, 92, 129−168. (70) Truhlar, D. G. Nearly Encounter-Controlled Reactions: The Equivalence of the Steady-State and Diffusional Viewpoints. J. Chem. Educ. 1985, 62, 104−106. (71) Einstein, A. On the Movement of Small Particles Suspended in Stationary Liquids Required by the Molecular-Kinetic Theory of Heat. Ann. Phys. (Leipzig) 1905, 17, 549−560. (72) Stokes, G. G. Mathematical and Physical Papers; Cambridge University Press: Cambridge, U.K., 1903; Vol. 3 (esp. section IV). (73) Perez-Gonzalez, A.; Rebollar-Zepeda, A. M.; Leon-Carmona, J. R.; Galano, A. Reactivity Indexes and OH Bond Dissociation Energies of a Large Series of Polyphenols: Implications for Their Free Radical Scavenging Activity. J. Mex. Chem. Soc. 2012, 56, 241−249. (74) Toth, J.; Remko, M.; Nagy, M. Structural Study of Flavonoids and their Protonated Forms. Z. Naturforsch. C 1996, 51, 784−790. (75) Leopoldini, M.; Russo, N.; Toscano, M. Gas and Liquid Phase Acidity of Natural Antioxidants. J. Agric. Food Chem. 2006, 54, 3078− 3085. (76) Leopoldini, M.; Russo, N.; Toscano, M. The Molecular Basis of Working Mechanism of Natural Polyphenolic Antioxidants. Food Chem. 2011, 125, 288−306. (77) Zhang, J.; Brodbelt, J. S. Gas-Phase Hydrogen/Deuterium Exchange and Conformations of Deprotonated Flavonoids and GasPhase Acidities of Flavonoids. J. Am. Chem. Soc. 2004, 126, 5906− 5919. (78) Markovic, Z.; Amic, D.; Milenkovic, D.; Dimitric-Markovic, J. M.; Markovic, S. Examination of the Chemical Behavior of the Quercetin Radical Cation towards Some Bases. Phys. Chem. Chem. Phys. 2013, 15, 7370−7378.

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dx.doi.org/10.1021/jp505586k | J. Phys. Chem. B 2014, 118, 10092−10100

New free radicals to measure antiradical capacity: a theoretical study.

A new family of free radicals, that are soluble in water and stable at all pH values, were recently synthesized and used to assess the antiradical cap...
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