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 macr ocycle that perturb the chemical and photochemical properties of these important protein cofactors. Further, the tertiary structure of the s urrounding protein can manipulate these distortions as a means of regu lating biological function. For cytochromes c, for example, an energet ically unfavorable, conserved nonplanar distortion of the heme exists and likely plays a role in its electron-transfer function. The heme di stortion is primarily of the ruffling (run type (corresponding to the lowest frequency B-1u-symmetry normal mode) in which the pyrroles are twisted about the metal-N-pyrrole bond. This B-1u-symmetry nonplanar d istortion is commonly observed in metalloporphyrin crystal structures, as are the saddling (sad) B-2n-symmetry distortion, waving (wav) E(g) -symmetry distortions, and doming (dom) A(2u)-symmetry distortion. Eac h of these nonplanar distortions is expected to result in unique alter ations of the chemical and physical properties of the nominally planar porphyrin macrocycle. Symmetrical porphyrin substitution with tetrahe drally bonded atoms at the four meso bridging carbons generally result s in the B-1u ruffling distortion; therefore, we investigated a series of meso-tetrasubstituted porphyrins for which the substituents vary i n size (methyl, ethyl, propyl, pentyl, isopropyl, cyclopropyl derivati ve 11a, cyclohexyl, apopinenyl (10), tert-butyl, adamantyl), increasin g the steric crowding at the periphery. Molecular mechanics calculatio ns show increasing degree of ruffling (CalphaNNCalpha angle for opposi te pyrroles varies from 0 to 57 degrees) 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 decre ase nonlinearly with increasing ruffling angle. The localization of th e B-1u nonplanar distortion in only the C(a)lpha-C-m bond torsion (not the case for the B-2u sad distortion) suggests a means by which the B -1u distortion might be distinguished from other types of nonplanar di stortion by using resonance Raman spectroscopy. Also, the size of the red shifts in the pi-->pi absorption bands depends on C(a)lpha-C-m to rsion angle in a nonlinear fashion and the shift is accurately predict ed by INDO/s molecular orbital calculations when the nonplanar structu res obtained from molecular mechanics are used.