Non-planar conformational distortions have recently been implicated in the biological activity of porphyrins and related tetrapyrroles in proteins, and several studies have used highly substituted porphyrins to model these nonplanar conformational distortions. Several aspects of non-planarity in the highly substituted metalloporphyrins are discussed, focusing on resonance Raman spectroscopy as a technique for investigating these structural issues. First, different non-planar distortions cause characteristic changes in the Raman spectrum. Specifically, the decreases in frequency of several Raman lines when compared with planar porphyrin analogs are shown to be similar for several classes of non-planar highly substituted porphyrins. Second, the effect of the central metal ion [ M = Ni(II), Co(II), Cu(II), Zn(II), Co(III), Fe(II1)I on the conformation of the sterically constrained non-planar porphyrin octaethyltetraphenylporphyrin is considered. Responding to the conformational adjustments resulting from different metal size, the frequency of the structure-sensitive Raman line v2 correlates with several structural parameters obtained from either mechanics calculations or crystallographic studies. The parameters include C,--C, bond length, core size and C,-N-Ca angle.. Finally, an effect of electron-withdrawing substituents on the Raman frequencies is evident for the different classes of highly substituted porphyrins.
INTRODUCTIONNon-planar conformational distortions may play a role in modifying the biological properties of tetrapyrroles in proteins as diverse as the photosynthetic reaction center of Rhodopseudomonas viridis,' the photosynthetic antenna system of Prosthecochloris aestuarii,' methylred~ctase,~ vitamin B,, and B,,-dependent enzymes4 and heme protein^.^ In this study, the highly substituted porphyrins (Fig. l), which are generally non-planar as a result of steric crowding of the peripheral substituents, have been used as model compounds to investigate the consequences of non-planar distortions on the resonance Raman spectra. Our goal is to develop resonance Raman spectroscopy as a technique that can serve as a measure of the type and degree of distortion. A better understanding of the relationships between the frequencies of Raman lines and the conformation of the porphyrin macrocycle will also improve the utility and reliability of Raman spectroscopy as a probe of tetrapyrrole-containing proteins.There is now a considerable body of information describing the structural and spectroscopic properties of highly substituted non-planar p~rphyrins.~-'~ For example, the crystal structures of a series of metal complexes of OETPP (lb) show that the porphyrin macrocycle exists in a highly non-planar saddle conformation (the ruflled and saddle conformations are those defined by Scheidt and Lee13. In a saddle conformation alternate pyrrole rings are tilted up and down with respect to a least-squares plane through the 24 atoms of the porphyrin core: in a ruffled conformation alternate pyrrole rings are twisted clock...
