STABILITY AND STRUCTURE OF BINARY AND TERNARY METAL-ION COMPLEXES OF OROTIDINATE 5'-MONOPHOSPHATE (OMP(3-)) IN AQUEOUS-SOLUTION

The stability constants of the 1:1 complexes formed between Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+or Cd2+and orotidinate 5’-monophosphate (OMP3-) were determined by potentiometric pH titrations in aqueous solution (I =0.1 M, NaNO3; 253C). In addition to the stability constants of these M(OMP)- complexes, for several cases also the corresponding acidity constants for the release of the proton from the H(N-3) site were calculated; i.e., the formation of M(OMP-H)2- complexes was quantified. On the basis of recent measurements for simple phosphate monoesters [R-MP2-; R is a noncoordinating residue; S.S. Massoud and H. Sigel, Inorg, Chem., 27, 1447–1453 (1988)], evidence is provided that the somewhat increased stability of all the mentioned M(OMP)− complexes 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 is different for the M(OMP-H)2-complexes of Co2+, Ni2+ and Cd2+: in these cases significant amounts of macrochelates form; i.e., the metal ion is not only coordinated to the phosphate group but also (in part) to the ionized -(N-3) site, which is placed in the neighbourhood of the phosphate residue in the dominating syn conformation of this nucleotide. For the metal ions Mg2+, Ca2+, Sr2+, Ba2+and Mn2+, which have in general a rather low affinity for N binding sites, no evidence for the formation of macrochelates is detected. In addition, the stability constants of the ternary Cu(Arm)(OMP)- complexes, where Arm = 2, 2’-bipyridyl or 1, 10-phenanthroline, were determined by potentiometric pH titrations. Evaluation of the stability data shows that an equilibrium betweeen an ‘open’ isomer and a Cu(Arm)(OMP)~ species with an intramolecular stack exists; the formation degree of these aromatic ring stacks reaches about 40 percent. Overall it is quite evident that OMP3-is a versatile ligand with remarkable properties which may be utilized by nature in recognition reactions during the intricate metabolic processes in which this nucleotide is involved. © 1991, Taylor & Francis Group, LLC. All rights reserved.