Stabilities of complexes formed between lead(II) and simple phosphonate orphosphate monoester ligands including some pyrimidine-nucleoside 5 '-monophosphates (CMP2-, UMP2-, dTMP(2-))

The stability constants of the 1:1 complexes formed between Pb2+ and severa l simple phosphate monoesters (4-nitrophenyl phosphate, phenyl phosphate, D -ribose 5-monophosphate, n-butyl phosphate) or phosphonate ligands (methylp hosphonate, ethylphosphonate) (R-PO32-) were determined by potentiometric p H titrations in aqueous solution (25 degrees C; I=0.1 M, NaNO3). The constr uction of a log(pb)((R-PO3))(pb) versus pK(HH(R-PO3)) plot for the mentione d ligand systems results in a straight line on which the data pairs (the co rresponding equilibrium constants were also measured) for uridine 5'-monoph osphate (UMP2-) and thymidine 5'-monophosphate (dTMP(2-)) also fall; this r esult shows that in the Pb2+ complexes of UMP2- and dTMP(2-) the nucleobase residues do not interfere, in neither a positive nor a negative way, with the binding of Pb2+ and that the stability of all these complexes is determ ined by the basicity of the phosph(on)ate group. The mentioned straight-lin e correlation las defined by the least-squares procedure) allowed us to dem onstrate (via constants determined now) that the stability of the Pb2+ comp lex of cytidine 5'-monophosphate (CMP2-) is also solely determined by the b asicity of its phosphate group. A similar evaluation, based on literature d ata, for the Pb(HPO4) complex reveals that its stability corresponds closel y to the expectations based on the Pb(R-PO3) data, though there is a slight hint that Pb(HPO4) may be somewhat more stable [which would be in agreemen t with previous observations of other M(HPO4) complexes]; clearly, more suc h comparisons are possible with the reference line given now. Based on the stability constants of the monoprotonated Pb(H;CMP)(+) complex and the Pb(c ytidine)(2+) species (which was also measured now), it is concluded that in Pb(H;CMP)(+) the proton is located at the phosphate group and Pb2+ mainly at the N3/(C2)O site of the cytosine residue. Regarding nucleic acids in so lution, it is further concluded that the affinity of Pb2+ towards the negat ively mono-charged phosphate unit, -O-P(O)(2)(-)-O-, of a nucleic acid back bone is comparable to that of the cytosine moiety, the affinity towards oth er nucleobase residues being smaller. This information may prove helpful re garding the properties of lead ribozymes.