Application of polarography to the study of complexation and stacking interactions of bases in the B–Z helical transition of poly[d(G–C)] and polyld(G-m 5 C)] induced by Ni(II) and Mn(II)

Abstract

The complexation of polynucleotides, poly[d(G–C)] and poly [d(G-m 5 C)], by transition metal ions, Ni(II) and Mn(II), was studied by polarographic methods (dc and DP) at the conditions of the B–Z helical transition of these polymers. In the case of Ni(II), the appearance of two peaks or waves corresponding to the reduction of (Ni 2+-base) aq complex and Ni aq 2+, respectively, allows the monitoring of the complexation of the bases indepently from the complexation of phosphate groups of polynucleotides. The changes in the stacking interactions of nucleic acid bases in nucleosides and dinucleotides in the presence of Ni(II) and Mn(II) were studied by ac polarography, as changes in “pits” or “wells” on double layer capacitance curves due to adsorption and association of bases on the electrode. Simultaneously, changes in complexation of these metal ions with the addition of nucleic acid components were monitored by DPP. The results were used to predict the stacking interactions in the B and Z form of poly[d(G-C)] and poly[d(G-m 5 C)]. It was found that there exists a good correlation between the stacking pattern predicted from our ac polarographic data and that known from crystal structure data of these polynucleotides. The nature of the B to Z transition was explained as a change in stacking interactions between nucleic acid bases due to their complexation by transition metal ion. The proposed mechanism of the B to Z transition of polynucleotides resembles Calladine and Drew's explanation of the B to A transition based on the analysis of appropriate crystal structures.