RAMAN-SPECTROSCOPIC CHARACTERIZATION OF ISOMERS OF COPPER AND ZINC N-PHENYLPROTOPORPHYRIN IX DIMETHYL ESTER

The reaction of certain heme-containing proteins with phenylhydrazine results in N-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 N-phenylprotoporphyrin IX dimethyl ester (NPhPPDME) metal derivatives. Regardless of which pyrrole ring (A, B, C, or D) bears the N-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)N-phenylprotoporphyrin IX predict structures similar to the structure reported for zinc N-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.