Computational studies of peripheral ring twisting in meso‐N‐methyl pyridyl‐substituted porphyrins

Semiempirical molecular orbital calculations for the porphyrins tetrakis(4‐N‐methyl pyridyl)porphine (H2TMpyP‐4) and tetrakis(2‐N‐methyl pyridyl)porphine using the MNDO and AM‐1 Hamiltonians suggest that twisting one or more of the pyridinium rings results, at considerable energy expense, in highly nonplanar macrocycle configurations as the exocyclic ring(s) approach coplanarity. The results imply that the mechanism of intercalation of H2TMpyP‐4 into DNA cannot require twisting the exocyclic rings anywhere close to coplanarity with the central porphine core, but involves, instead, the inherent flexibility of DNA itself.