Binding mode of porphyrins to poly[d(A-T)₂] and poly[d(G-C)₂]

We examined die binding geometry of Co-meso-tetrakis (N-methyl pyridinium-4-yl)porphyrin, Co-meso-tetrakis (N-n-butyl pyridinium-4-yl)porphyrin and their metal-free ligands to poly[d(A-T)₂] and poly[d(G-C)₂] by optical spectroscopic methods including absorption, circular and linear dichroism spectroscopy, and fluorescence energy transfer technique. Signs of an induced CD spectrum in the Sorer band depend only on the 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)₂]. Close analysis of the linear dichroism result reveals that all porphyrins exhibit outside binding when complexed with poly[d(A-T)₂], regardless of the existence of a central metal and side chain. However, in the case of poly[d(G-C)₂], we observed intercalative binding mode for two nonmetalloporphyrins and an outside binding mode for metalloporphyrins. The nature of the outside binding modes of the porphyrins, when complexed 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-bases to porphyrins can occur for metalloporphyrins.