Annu. Rev. Phys. Chern. 1992. 43: 465-96 Copyright © 1992 by Annual Reviews Inc. All rights reserved


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VIBR"ATIONAL CHARACTERISTICS OF Annu. Rev. Phys. Chem. 1992.43:465-496. Downloaded from by University of Sydney on 08/05/13. For personal use only.

TETltAPYRROLIC MAC�ROCYCLES Alexander D. Procyk and David F. Bocian

Department of Chemistry, University of California, Riverside, California 9252 1 -0403 KEY WORDS:

porphyrins, hydroporphyrins, normal coordinates, Raman spec­ troscopy, infrared spectroscopy


Tetrapyrrolic macrocycles serve as the functional groups in a wide variety of biological systems. These macrocycles are involved in such activities as oxygen binding and transport, electron transfer, catalysis, and light harvesting ( 1-2b). The porphyrins are probably the most familiar and most widely studied of these ring systems (Figure 1 ). Iron is generally the central ion in biologically active porphyrins, which are substituted at the /3positions of the pyrrole rings (3). The nature and location of the peripheral groups vary and range from saturated moieties that cannot conjugate into the n-electron system of the macrocycle to unsaturated groups (vinyls, formyls, acetyls) that can potentially participate in the n network. Prosthetic groups that are variants of the basic porphyrinic structure are also widespread in nature. The photosynthetic pigments (bacterio)­ chlorophyll and (bacterio)pheophytin are reduced-pyrrole (/3-hydro­ porphyrin) macrocycles that are further modified by the addition of a fifth isocyclic ring (Figure 1) (4). In the former pigments, magnesium is the central metal ion, whereas the latter are metal free. Chlorophyll and pheophytin contain a single reduced-pyrrole ring (chlorin); bac­ teriochlorophyll and bacteriopheophytin contain two opposite reduced­ pyrrole rings (bacteriochlorin). /3-hydroporphyrins in which iron is the 465 0066-426X/92/1101--0465$02.00




Annu. Rev. Phys. Chem. 1992.43:465-496. Downloaded from by University of Sydney on 08/05/13. For personal use only.






chlorin Chlorophyllll




Bacteriochiorophyllll caOH HOOC




COOH Siroheme




Cofactor F430 Coenzyme B!2

Figure 1 Structures of model tetrapyrrolic ring systems (right) and related naturally occur­ ring systems (middle and left).


central metal i on (Figure 1 ) also serve as the prosthetic groups in m any plant and bacterial proteins (5-34). The p-tetrahydroporphyrins in this group contain adjacent reduced-pyrrole rings (isobacteriochlorins), rather than the opposite-ring structure found in photosynthetic pigments. [Interestingly a recent report indicates that a metal-free isobacteriochlorin may be the active form in certain bacterial reductases (35).] M acroc:ycles that are more highly reduced than (iso)bacteriochlorins are also found in nature. Cofactor F430 of methylreductase contains four reduced-pyrrole rings and two reduced-methine bridges (Figure I) (3640). The ring system of this moiety exhibits structural features of both hydroporphyrins and corrins. The l atter ring system is found in vitamin B 12 (41, 42). Both cofactor F 430 and vitamin B 12 contain central metal ions other than iron or magnesium; the former contains nickel, whereas the latter contains cobalt. Both of these cofactors, as well as other naturally occurring hydroporphyrins, are richly substituted at the ring periphery. The diversity of biological activity mediated by tetrapyrrolic macro­ cycles is indicative of the extraordinary versatility of this basic structural unit, which can accommodate different types of central metal ions and a variety of peripheral substituent groups. This structure is also amenable to such chemical modifications as pyrrole-ring reduction and the addition of isocyclic rings. All of these features influence the macrocycle's structural and electronic properties (43-65), which, in turn, must have some influence on the specific biological activity of a particular ring system. Indeed, it is interesting to speculate on which structural features might have been tailored for more efficacious bi ologi cal activity and which are artifacts of the biosynthetic pathways. The biological importance of tetrapyrrolic macrocydes is the primary driving force for the wide variety of studies of model systems, natural pigments, and proteins. Several excellent volumes discuss these studies (66-68). This review focuses on the information derived from a single technique-vibrational spectroscopy. There have been many relatively recent reviews on the vibrational characteristics of tetrapyrrolic macro­ cycles (69·-76); consequently, we limit our discussion to a subset of recent developments in this area. In particular, we focus on the nature of the normal modes of vibration and the factors that influence their charac­ teristics. The review is also relatively general in nature, rather than focused on specific details of the vibrational spectra. One might well ask, "What is to be gained by a detailed characterization of the molecular vibrations of these large, complicated macrocydes?" The answer is that the vibrational frequencies and normal modes of vibration are highly sensitive indicators of the structural and electronic properties of the molecules (69-81). Changes in these properties that occur upon ligand binding, oxidation/ ,

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reduction, or electronic excitation are reflected in the vibrational charac­ teristics. The vibrational frequencies of certain modes have long been used as indicators of the structure of protein prosthetic groups (72, 80). Knowledge of the vibrational eigenvectors allows a quantitative assess­ ment of the structural and electronic properties of the ring.

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For nearly two decades, porphyrins have been the subj ect of numerous vibrational studies (69-74, 77-80). The vibrational spectra and normal modes of vibration of neutral rings are now well characterized. Kitagawa & Ozaki (70) and Spiro et al (74) have provided excellent accounts of this subject. The vibrational spectra of n-cation and n-anion radicals have also been reported (82- 1 02 ). At this time, the vibrational features of porphyrin n-cation radicals are reasonably well characterized (85a,b, 87-89), although no detailed vibrational analyses have been reported. Fewer vibrational studies have been reported for porphyrin n-anion radicals, and the vibrational characteristics of these species are o nly now being elucidated (99a-IOOb). We discuss the vibrational features of neutral, cationic, and anionic porphyrins in more detail below. Neutral Molecules

As noted above, the vibrational features of neutral porphyrins have been discussed in many recent reviews. Consequently, we present only an over­ view of this subject. This information is included both for the sake of completeness and, more importantly, because the vibrational charac­ teristics of metalloporphyrins serve as the benchmarks for delineating these features of related ring systems. IN-PLANE VIBRATIONS The in-plane vibrations of porphyrins are the best characterized modes of the macrocycle. Resonance Raman (RR) and infrared (IR) spectra have been reported for metalated complexes of the unsubstituted ring (porphine) ( 1 03-108), fJ-substituted rings (72-74, 7880, 87, 1 09-1 1 8 ), and meso-substituted rings ( 1 08, 1 1 9-1 22). The free base of porphine (P) has also been studied in detail ( 1 23-125). Normal coordinate calculations have been reported for both metalated P ( 1 03, 1 25-1 27) and free-base P ( 1 23, 1 24, 1 28 , 129). Calculations havc also bcen reported for metalated fJ-substituted rings (l1 8, 1 26, 1 27, 1 30-1 33) and meso-substituted rings (99b, 1 08, 1 3 4). In most calculations, metallo­ octaethylporphyrins (MOEP) and metallotetraphenylporphyrins (MTPP) have served as the prototypical fJ- and meso-substituted complexes (99b, 108, 1 1 8, 1 30, 1 32, 1 34). All of these calculations have been facilitated by



the availability of vibrational data for numerous isotopomers, including and P-fJ,meso-dI2; OEP-meso-d4, OEP-C 5N-pyrrole)4, and OEP-nleso-d4-C 5N-pyrrole)4; and TPP-fJ-d8, TPP-C 5N-pyrrole), and TPP-(meso-13C)4. I n most early calculations on substituted rings, the sub­ stituent groups were treated as point masses. However, Lee et al ( 1 33) examined several protoporphyrin models containing deuterated vinyl groups and explicitly included these groups in normal coordinate cal­ culations. In recent normal coordinate calculations on MOEP and MTPP complexes, the ethyl and phenyl groups have also been explicitly included (99b, 1 08, 1 1 8, 1 32). These calculations were facilitated by the availability of vibrational data for methylene predeuterated MOEP and phenyl per­ deuterated MTPP. The in-plane vibrations of the basic 37-atom metalloporphyrin skeleton can be classified under D4h symmetry as

Annu. Rev. Phys. Chem. 1992.43:465-496. Downloaded from by University of Sydney on 08/05/13. For personal use only.

P-fJ-d8, P-meso-d4,



9Alg+8A2g+9Blg+9B2g+ 18Eu.

All the gerade modes are RR active, whereas the Eu modes are IR active. Vibrational bands corresponding to most of these modes have been identi­ fied (72, 74, 80, 108, 112-122). The most complete vibrational charac­ terization of metalloporphyrins has been reported by Spiro and coworkers ( l 08, 1 1 8), who performed a systematic study o n the Ni(II) complexes of P, OEP, and TPP. Based on this investigation, these workers developed valence force fields that successfully predict the frequencies and isotope shifts for all three ring systems. The vibrational data for the three rings could be successfully accounted for with minor changes in certain force constants and with interaction constants kept to a minimum. The diagonal stretching force constants were also constrained to values commensurate with the bond lengths. In their analysis of the spectra of the Ni(II) complexes of P, OEP, and TPP, Spiro and coworkers also developed a classification scheme in which the normal modes of vibration of the macrocycle are characterized in terms of the appropriate symmetry combinations of local motions of pyrrole ring and-methine bonds that comprise one quarter of the symmetrical ring. In D4h symmetry, a particular local mode on each of the four symmetry equivalent pyrrole-methine units necessarily makes an equal contribution to a given normal mode. Figure 2 illustrates the different types of local motions of the pyrrole-methine unit. This classification scheme is extremely convenient and amenable to extension to other types of ring systems. Li & Zgierski (123) recently used this classification method in a detailed normal coordinate analysis of free-base P. This molecule has effective D2h symmetry. Consequently, there are two symmetry-inequivalent pairs of pyrrole-me:thine units, and these units do not necessarily contribute equally



� tf N


Annu. Rev. Phys. Chem. 1992.43:465-496. Downloaded from by University of Sydney on 08/05/13. For personal use only.

V(Cb" Y).ym

V(Cb" Y)..ym

w V(Cm" X)

~ ~


V(Ca" CnJaym

V(C a" Cm)••ym

Vibrational characteristics of tetrapyrrolic macrocycles.

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