Raman spectroscopic characterization of isomers of metal N-phenyl protoporphyrin IX dimethylesters

The reaction of heme proteins with phenylhydrazine results in N-phenylprotoporphyrin formation. The authors are using resonance Raman and UV-visible adsorption spectroscopies to investigate the structure of N-phenylprotoporphyrin dimethylesters (N[Phi]PPDME) metal derivatives. Distortion from planarity of the protoporphyrin macrocycle results from addition of the phenyl group. Porphyrin planarity is thought to influence chemical and electron-transfer properties of tetrapyrroles contained in proteins such as methylreductase and the photosynthetic reaction center. Specifically, the authors investigated the conformation of regioisomers obtained by substitution at the A, B, C, and D pyrrole rings on the distal side. Isomers A and B of Cu- and ZnN[Phi]PPDME show spectroscopic similarities and isomers C and D are similar; A and B are different from isomers C and D. This difference is due mainly to the location of the vinyl substituents at the periphery of the porphyrin. In the Raman spectra, the differences show up primarily in the vinyl vibrational modes and vinyl-sensitive porphyrin modes. Molecular mechanics calculations also are used to interpret the spectroscopic results. The calculations were carried out for various isomers and conformers of Fe(III) N-phenylprotoporphyrin chloride and Cu(II) N-phenylprotoporphyrin. Predicted structures are similar to reported N-substituted tetraphenylporphyrins, with the major distortion from planarity occurring formore » the N-substituted pyrrole ring. The energies are found to be affected by orientations of the vinyl substituent groups.« less