The interaction of copper tetrakis(4-N-methylpyridyl) porphine with polynucleotides studied by ultraviolet resonance Raman spectroscopy.

Resonance Raman spectra are reported for poly[dA.dT]2, poly[dA].poly[dT], poly[dG.dC]2, poly[dG].poly[dC], poly[dA.dC].poly[dG.dT] and Calf Thymus DNA in interaction with the porphyrin Cu(TMpy-P4). The spectra were obtained under conditions of low salinity and a ratio of porphyrin molecules to nucleotide base pairs of r = 0.05, with an excitation wavelength of 257 nm. The differences between the spectra in interaction with Cu(TMpy-P4) and those of the unmodified nucleic acids are interpreted in terms of the structural changes imposed by the porphyrin. Two different binding modes have been reported in previous studies, a 5'CG-3' specific intercalative mode and an A-T specific outside binding mode. Our results suggest that this latter mode may show a preference for A-A or T-T sites. Furthermore a transition in structure, while remaining within the B-form family of structures, is indicated for both poly[dA].poly[dT] and poly[dG.dC]2 upon addition of Cu(TMpy-P4).