We examined the binding geometry of Co-meso-tetrakis (N-methyl pyridin
ium-4-yl)porphyrin, Co-meso-tetrakis (N-n-butyl pyridinium-4-yl)porphy
rin and their metal-free ligands to poly[d(A-T)(2)] and poly[d(G-C),]
by optical spectroscopic methods including absorption, circular and li
near dichroism spectroscopy, and fluorescence energy transfer techniqu
e. Signs of an induced CD spectrum in the Soret band depend only on th
e nature of the DNA sequence; all porphyrins exhibit negative CD when
bound to poly[d(G-C)(2)] and positive when bound to poly[d(A-T),]. Clo
se analysis of the linear dichroism result reveals that all porphyrins
exhibit outside binding when complexed with poly[d(A-T),], regardless
of the existence of a central metal and side chain. However, in the c
ase of poly[d(G-C)(2)], we observed intercalative binding mode for two
nonmetalloporphyrins and an outside binding mode for metalloporphyrin
s, The nature of the outside binding modes of the porphyrins, when com
plexed with poly[d(A-T)(2)] and poly[d(G-C)(2)], are quite different,
We also demonstrate that an energy transfer from the excited nucleo-ba
ses to porphyrins can occur for metalloporphyrins. (C) 1998 Elsevier S
cience B.V.