No abstract

No abstract

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...

No abstract

No abstract

No abstract

No abstract

The reaction of certain heme-containing proteins with phenylhydrazine results in TV-phenylprotoporphyrin formation. N-substituted porphyrins are known to inhibit ferrochelatase and to be formed in the inactivation of hepatic cytochrome P450 by various agents. These molecules therefore play an important role in understanding the biological function of these enzymes. We have used molecular modeling in conjunction with resonance Raman and UV-visible absorption spectroscopies to investigate the structure of the four distal regioisomers of TV-phenylprotoporphyrin IX dimethyl ester (NPhPPDME) metal derivatives. Regardless of which pyrrole ring (A, B, C, or D) bears the TV-phenyl substituent, a novel distortion from planarity of the protoporphyrin macrocycle results from addition of the phenyl group. The molecular mechanics calculations for various isomers and conformers of copper(II) TV-phenylprotoporphyrin IX predict structures similar to the structure reported for zinc TV-phenyltetraphenylporphyrin (Kuila, D.; et al. J. Am. Chem Soc. 1984,106, 448), with the major distortion from planarity occurring for the N-substituted pyrrole ring. Although the four isomers of CuNPhPPDME are all similar when contrasted with CuPPDME, closer examination of the vibrational structure allowed the isomers to be separated into two groups. Differences among pairs of isomers are apparent primarily in the vinyl vibrational modes and vinyl-sensitive porphyrin modes. Even more subtle spectral differences distinguish all of the isomers. We present arguments suggesting that the spectral characteristics are the result of electronic rather than kinematic effects.