STABILITY AND STRUCTURE OF THE MG2+, CA2+ AND CU2+ COMPLEXES OF OROTIDINATE 5'-MONOPHOSPHATE (OMP)3- IN VARIOUS AQUEOUS 1,4-DIOXANE MIXTURES

The stability constant of the 1:1 complexes formed between Mg2+, Ca2+ or Cu2+ and orotidinate 5'-monophosphate (OMP3-) were determined by potentiometric pH titrations in water containing 30 or 50% (vol./vol.) 1,4-dioxane (I = 0.1 M, NaNO3; 25-degrees-C). In addition to the stability constants of these M(OMP)- complexes, for Mg(OMP)- and Ca(OMP)- the acidity constants for the release of the proton from the H(N-3) site were also calculated, i.e. the formation of the corresponding M(OMP-H)2- complexes was quantified. All the corresponding equilibrium constants for aqueous solution are known from a previous study (M. Bastian and H. Sigel, J. Coord. Chem., 23 (1991) 137) and in the discussions these data are also taken into account. On the basis of recent measurements in aqueous dioxane mixtures with simple phosphate monoesters (M. C. F. Magalhaes and H. Sigel, J. Indian Chem. Soc., in press), evidence is provided that the somewhat increased stability of the Cu(OMP)- complex in the various solvents is mainly the result of a charge effect of the carboxylate group (in position 6 of OMP3-) and not of a direct participation in complex formation, i.e. there are no indications for the formation of significant amounts of macrochelates involving the phosphate and the carboxylate groups. This result is in agreement with the dominating syn conformation of OMP3- in which the 5'-phosphate and 6-carboxylate groups are pointing away from each other. However, for the Cu2+ complex of salicyl phosphate (SaP3-) evidence is given by evaluating previously published equilibrium constants (R. W. Hay, A. K. Basak, M. P. Pujari and A. Perotti, J. Chem. Soc., Dalton Trans., (1986) 2029) that a simultaneous coordination of a phosphate and a carboxylate group to the same metal ion is possible; it is estimated that the eight-membered chelate of Cu(SaP)- reaches a formation degree of about 65 (+/- 25)%.