BINDING MODE OF PORPHYRINS TO POLY[D(A-T)(2)] AND POLY[D(G-C)(2)]

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.