Properties of the magnesium(II) and calcium(II) complexes of 5- and 6-uracilmethylphosphonate (5Umpa(2-) and 6Umpa(2-)) aqueous solution

The stability constants of the 1:1 complexes formed between Mg2+ or Ca2+ an d 5 Umpa(2-) or 6 Umpa(2-) were determined by potentiometric pH titrations in aqueous solution (25 degreesC. I = 0.1 M, NaNO3). Based on previously es tablished log K-M(R-PO3)(M) versus pK(H(R-PO3))(H) straight-line plots (M2 = Mg2+ or Ca2+; R-PO32- = simple phosphate monoester or phosphonate ligand s where R is a non-interacting residue), it is shown that the Mg(5 Umpa), C a(5 Umpa), Mg(6 Umpa) and Ca(6 Umpa) complexes have the stability expected on the basis of the basicity of the phosphonate group in 5 Umpa(2-) and 6 U mpa(2-). This means, these Ligands may be considered as simple analogues of nucleotides, e.g. of uridine 5 ' -monophosphate. In the higher pH range de protonation of the uracil residue in the M(5 Umpa) and M(6 Umpa) complexes occurs and this leads to the negatively charged M(5 Umpa-H)(-) and M(6 Umpa -H)(-) species. Based on the comparison of various acidity constants it is shown that the M(5 Umpa) complexes are especially acidic; or to say it diff erently, the M(5 Umpa-H)(-) species are especially stable. This increased s tability is attributed to the formation of a seven-membered chelate involvi ng next to the phosphonate group also the carbonyl oxygen atom at C4 (after deprotonation of the (N3)H site). The formation degree of this chelated is omer reaches about 45% for the Mg(5 Umpa-H)(-) and Ca(5 Umpa-H)(-) species. No indication for chelate formation was observed for the M(6 Umpa-H)(-) co mplexes.