Extent of the Acidification by N7-Coordinated cis-Diammine-Platinum(II) on the Acidic Sites of Guanine Derivatives

Abstract

Coordination of two monoprotonated 2'-deoxyguanosine 5'-monophosphate species, H(dGMP)−, via N7 to cis-(NH3)2Pt2+ gives the complex cis-(NH3)2Pt(H·dGMP)2 which is a four-protonic acid. The corresponding acidity constants were measured by potentiometric pH titrations (25℃; I = 0.1 M, NaNO3). The first two protons are released from the two -P(O)2(OH)− groups (PKa/1 = 5.57; PKa/2 = 6.29) and the next two protons are from the H(N1) sites of the guanine residues (PKa/3 = 8.73; PKa/4 = 9.48). The micro acidity constants of the various sites are also evaluated. Comparison of these data with those determined for the three-protonic H2(dGMP)± (PKa/1 = 2.69 for the H+(N7) site; PKa/2 = 6.29 for -P(O)2(OH)− ;PKa/3 = 9.56 for H(N1)) shows that on average the N-7-coordinated Pt2+ acidifies the phosphate protons by Δ pKa = 0.36 and the H(N1) sites by Δ pKa = 0.46. These results are further compared with those obtained previously for cis-(NH3)2Pt(L)2, where L = 9-ethylguanine or monoprotonated 2'-deoxycytidine 5'-monophosphate. Conclusions regarding platinated DNA are also presented.