Resonance Raman and absorption characterization of cationic Co(II)-porphyrin in its complexes with nucleic acids: binding modes, nucleic base specificity and role of water in Co(II) oxidation processes

Abstract

A comparative study of Co(II) and Co(III) complexes of the water-soluble cationic tetrakis(N-methyl-4-pyridiniumyl)porphyrin [CoII- and CoIIITMpyP(4)] with calf thymus DNA (CT DNA) and synthetic double-stranded (ds) and single-stranded (ss) polynucleotides, namely, poly(dA-dT)2, poly(dG-dC)2, [poly(dA)·poly(dT)], poly(A), poly(dT), poly(G) and poly(C) was carried out by resonance Raman (RR) as well as stationary absorption spectroscopy. It is shown that free CoIITMpyP(4) in aqueous solution is easily oxidized to CoIIITMpyP(4) by dissolved molecular oxygen, but Co(II) to Co(III) oxidation ability fundamentally changes when CoIITMpyP(4) is bound to nucleic acids (NA). In double helical DNA and AT-containing polynucleotides, where the porphyrin is assumed to be outside bound mainly in the minor groove near AT sites, Co(II) is stable for weeks and even months. It is suggested that Co(II) axial ligation by immobilized water (i.e. water molecule that is hydrogen bound to O or N atoms from DNA bases) results in such CoIITMpyP(4) stabilization. In contrast, CoIITMpyP(4) binding with ds and ss guanine-containing polynucleotides results in instantaneous Co(II) to Co(III) transformation. This can be explained by Co ion ligation by the guanine N7 atoms, which are known to be strong electron-donating ligands. It is assumed that partial intercalation occurs when CoIITMpyP(4) is bound with poly(dG-dC)2, in particular, at low ionic strength values, where cobalt ion is Co(III) in character. Copyright © 2005 John Wiley & Sons, Ltd.