Stabilities of complexes formed between lead(II) and simple phosphonate or phosphate monoester ligands including some...

The stability constants of the 1:1 complexes formed between Pb2+ and several simple phosphate monoesters (4-nitrophenyl phosphate, phenyl phosphate, D-ribose 5-monophosphate, n-butyl phosphate) or phosphonate ligands (methylphosphonate, ethylphosphonate) (R-PO3(2-)) were determined by potentiometric pH titrations in aqueous solution (25 degrees C; I = 0.1 M, NaNO3). The construction of a logK(Pb)Pb(R-PO3) versus pK(H)H(R-PO3) plot for the mentioned ligand systems results in a straight line on which the data pairs (the corresponding equilibrium constants were also measured) for uridine 5'-monophosphate (UMP2-) and thymidine 5'-monophosphate (dTMP2-) also fall; this result shows that in the Pb2+ complexes of UMP2-and dTMP2- 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 determined by the basicity of the phosph(on)ate group. The mentioned straight-line correlation (as defined by the least-squares procedure) allowed us to demonstrate (via constants determined now) that the stability of the Pb2+ complex of cytidine 5'-monophosphate (CMP2-) is also solely determined by the basicity of its phosphate group. A similar evaluation, based on literature data, for the Pb(HPO4) complex reveals that its stability corresponds closely 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 agreement with previous observations of other M(HPO4) complexes]; clearly, more such comparisons are possible with the reference line given now. Based on the stability constants of the monoprotonated Pb(H;CMP)+ complex and the Pb(cytidine)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 solution, it is further concluded that the affinity of Pb2+ towards the negatively mono-charged phosphate unit, -O--P(O)2- -O-, of a nucleic acid backbone is comparable to that of the cytosine moiety, the affinity towards other nucleobase residues being smaller. This information may prove helpful regarding the properties of lead ribozymes.