Assessing the sequence specificity in the binding of Co(III) to DNA via a thermodynamic approach

The interaction specificities of Co(III) with DNA were investigated via consideration of thermodynamic characteristics of the duplex to single strand transition for DNA oligomers incubated in the presence of [Co(NH3)5(OH2)](ClO4)3. It has previously been demonstrated that incubation of the DNA oligomer [(5medC-dG)4]2 with this cobalt complex leads to coordination of the cobalt center to the DNA, presumably at N7 of guanine bases [D. C. Calderone, E. J. Mantilla, M. Hicks, D. H. Huchital, W. R. Murphy, Jr. and R. D. Sheardy, (1995) Biochemistry 34, 13841]. In this report, DNA oligomers of different sequence were incubated with [Co(NH3)5(OH2)](ClO4)3 via protocols developed previously and the treated oligomers were subjected to thermal denaturation for comparison to the untreated oligomers. The DNA oligomers were designed in order to investigate the sequence specificity, if any, in the reaction of the cobalt complex with DNA. The values of Tm, delta HvH, and delta n (the differential ion binding term) obtained from the thermal denaturations were used to assess the sequence specificity of the interaction. For all oligomers, treated or untreated, Tm and delta HvH vary linearly with log [Na+] and hence the value of delta n is a function of the Na+ concentration. The results indicate no significant reaction between the cobalt complex and oligomers possessing isolated -GA- or -CG- sites; however, the thermodynamic characteristics of DNA oligomers possessing either an isolated -GG- site or an isolated -GC- site were altered by the treatment. Atomic absorption studies of the treated oligomers demonstrate that only the DNA oligomers possessing isolated -GG- or -GC- sites bind cobalt. Hence, the changes in the thermodynamic properties of these oligomers are a result of cobalt binding with a remarkable sequence specificity.