Rr. Monaco et M. Zhao, COMPUTATIONAL STUDIES OF PERIPHERAL RING TWISTING IN MESO-N-METHYL PYRIDYL-SUBSTITUTED PORPHYRINS, International journal of quantum chemistry, 46(6), 1993, pp. 701-709
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.