ACID-BASE AND METAL ION-BINDING PROPERTIES OF 2'-DEOXYCYTIDINE 5'-MONOPHOSPHATE (DCMP(2-)) ALONE AND COORDINATED TO CIS-DIAMMINE-PLATINUM(II) - FORMATION OF MIXED-METAL ION NUCLEOTIDE COMPLEXES

The acidity constants of diprotonated 2'-deoxycytidine 5'-monophosphat e, i.e. H-2(dCMP)(+/-), were determined by potentiometric pH titration in aqueous solution (25 degrees C; I = 0.1 M, NaNO3) and compared wit h the previously determined (S.S. Massoud and H. Sigel, Inorg. Chem., 27 (1988) 1447-1453) corresponding constants of diprotonated cytidine 5'-monophosphate, i.e. H-2(CMP)(+/-). The absence of the 2'-hydroxy gr oup makes dCMP(2-) slightly more basic, compared with CMP(2-). The sta bility constants of the M(H . dCMP)(+) and M(dCMP) complexes of Mg2+, Cu2+ and Zn2+ were determined and those for the corresponding CMP comp lexes reevaluated. It is concluded that in the M(H . CMP)(+) and M(H . CMP)(+) species the metal ion is mainly located at N-3 and the proton at the phosphate group. On the basis of recent measurements with simp le phosphate monoesters and phosphonate derivatives, i.e. R-PO32- with R being a non-coordinating residue (H. Sigel et al., Helv. Chim. Acta , 75 (1992) 2634-2656), it is shown that the stability of all the M(dC MP) and M(CMP) complexes is solely determined by the basicity of the p hosphate group. Coordination of two H(dCMP)(-) ions via N-3 to cis-(NH 3Pt2+ gives H-2[cis-(NH3)(2)Pt(dCMP)(2)],, abbreviated as H-2(Pt(dC)(2 )), the synthesis of which is described and the acidity constants of w hich were determined. Pt2+ bound to the N-3 sites apparently has only a small effect on the basicity of the two phosphate groups in Pt(dC)(2 )(2-). In addition, also via potentiometric pH titrations, the stabili ty constants of the M(H . Pt(dC)(2))(+) and M(Pt(dC)(2)) complexes wit h Mg2+ Cu2+ and Zn2+ were determined. Based on the previously determin ed (see the above Ref.) linear log K-M(R-PO3(M)) versus pK(H(R-PO3)(H) ) relationships it is shown that the metal ion-binding properties of t he phosphate groups in the mentioned platinum(II) complex are still re markable, allowing thus the formation of mixed metal ion complexes. In fact, the effect of Pt2+ at the N-3 sites on the binding properties o f the phosphate groups is relatively small; to a first approximation, though there are some minor additional effects, one may conclude that also in these cases the complex stabilities are mainly determined by t he basicity of the phosphate groups.