Ruffling in a Series of Nickel(II) meso-Tetrasubstituted Porphyrins as a Model for the Conserved Ruffling of the Heme of Cytochromes c

Metalloporphyrins undergo remarkable nonplanar distortions of the macrocycle that perturb the chemical and photochemical properties of these important protein cofactors. Further, the tertiary structure of the surrounding protein can manipulate these distortions as a means of regulating biological function. Symmetrical porphyrin substitution with tetrahedrally bonded atoms at the four meso bridging carbons generally results in the B{sub 1u} ruffling distortion; therefore, we investigated a series of meso-tetrasubstituted porphyrins for which the substituents vary in size (methyl, ethyl, propyl, pentyl, isopropyl, cyclopropyl derivative 11a, cyclohexyl, apopinenyl (10), tert-butyl, adamantyl), increasing the steric crowding at the periphery. Molecular mechanics calculations show increasing degree of ruffling (C{sub {alpha}} NNC{sub {alpha}} angle for opposite pyrroles varies from 0 to 57{degree}) for this series of porphyrins, generally agreeing with the X-ray structures that are available. In addition, the frequencies of the structure-sensitive Raman lines decrease nonlinearly with increasing ruffling angle. The localization of the B{sub 1u} nonplanar distortion in only the C{sub {alpha}}-C{sub m} bond torsion (not the case for the B{sub 2u} sad distortion) suggests a means by which the B{sub 1u} distortion might be distinguished from other types of nonplanar distortion by using resonance Raman spectroscopy. 41 refs., 12 figs., 4 tabs.