ON THE METAL-ION BINDING-PROPERTIES OF OROTIDINE

The stability constants of the 1:1 complexes formed between orotidinate (Or-) and Cu2+ or Zn2+ were determined by potentiometric pH titrations in solvent mixtures (vol./vol.) consisting of 70% 1,4-dioxane and 30% water at 25-degrees-C and I = 0.1 M (NaNO3). Orotidine is first deprotonated at the carboxylic acid residue in position 6 (pK(H(Or))H = 2.36) and next at the H(N-3) site (pK(Or)H = 10.67). The latter site plays no role in the physiological pH range, i.e. the obvious site for metal ion coordination is the carboxylate group at position 6. For comparison we have studied therefore also the stabilities of the 1:1 complexes formed between Cu2+ or Zn2+ and the simple carboxylates (CA-): HCOO-, CH3COO-, CH2ClCOO-, CHCl2COO- and CHF2COO-. Plots of log K(M(CA))M versus pK(H(CA))H indicated that the stability of the M(Or)+ complexes is largely governed by the basicity of the corresponding carboxylate groups, but there are also indications that in the mentioned solvent mixture some (macro)chelate formation may occur, i.e. the carboxylate-coordinated metal ion is possibly interacting with a further site of the Or- ligand (probably the 'ether' oxygen of the ribosyl residue which is accessible in the less favored anti conformation). The upper limit for the formation degree of such a chelate is 45%. It should be pointed out that in aqueous solution (I = 0.1 M, NaNO3; 25-degrees-C) the basicity of the carboxylate group is strongly reduced (pK(H(Or))H = 0.5 +/- 0.3) and consequently the metal ion affinity of this group is also expected to be considerably smaller; indeed, estimations of the stability of the orotidinate complexes of Cu2+, Zn2+ or Mg2+ confirm this expectation. Some relevant points following from the present results regarding biological systems are indicated.