COMPUTATIONAL STUDIES OF PERIPHERAL RING TWISTING IN MESO-N-METHYL PYRIDYL-SUBSTITUTED PORPHYRINS

Semiempirical molecular orbital calculations for the porphyrins tetrak is(4-N-methyl pyridyl)porphine (H-2TMpyP-4) and tetrakis(2-N-methyl py ridyl)porphine using the MNDO and AM-1 Hamiltonians suggest that twist ing one or more of the pyridinium rings results, at considerable energ y expense, in highly nonplanar macrocycle configurations as the exocyc lic ring(s) approach coplanarity. The results imply that the mechanism of intercalation of H-2TMpyP-4 into DNA cannot require twisting the e xocyclic rings anywhere close to coplanarity with the central porphine core, but involves, instead, the inherent flexibility of DNA itself.