Intercalative and nonintercalative binding of large cationic porphyrin ligands to calf thymus DNA

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Abstract
The large meso-substituted porphine, meso-tetra(4-N-methylpyridyl)porph1ne has been Identified as a DNA-interactive ligand with a capacity for intercalation (1,2). Subsequently, the 2-N-methyl, 3-N-methyl and N-trimethylaniliniun analogues of this porphyrin intercalator have been obtained for physico-chemical analyses (absorption spectroscopy, viscometry, circular dichroism, unwinding of supercoiled DNA). In this paper we discuss the factors affecting the character of porphyrin binding (intercalative, as is the case for the 4-N-methyl and 3-N-methyl porphines, versus non-intercalat1ve, as is the case for the 2-N-methyl and N-trimethylanilin1ium porphines) and the impact that porphyr ins' binding has upon the structure of DMA. The molecular conformation of the porphyrin Ugand varies slightly within this series so that the ability of a given porphyrin to intercalate nay be correlated with the arrangement of charg ed groups, the planarlty of the porphine ring and the effective width of the individual molecules. The results from these studies indicate that sequence selective binding occurs within a small aperture of solution conditions.