Complexation between 3-methylorotic (3-methyluracil-6-carboxylic) acid dianion and divalent metal cations. The slow acid dissociation of transition metal complexes and its possible relevance to orotic acid phosphoribosylation

Abstract

3-Methylorotic (3-methyluracil-6-carboxylic) acid dianion gives 1 : 1 complexes with divalent metal ions. Their stability constants were determined by u.v. spectroscopy for Ca²⁺, Mg²⁺, Co²⁺, Ni²⁺, and Cu²⁺. The kinetics of formation of these complexes was investigated by T-jump relaxation in the alkaline pH range. The rate constants thus determined for complexation by Ca²⁺, Mg²⁺, Co²⁺, and Cu²⁺, show that it fits the Eigen mechanism. Complex dissociation was studied by the stopped-flow technique in the acidic pH range. Surprisingly, the observed first-order rate constants for transition metal ions (Co²⁺, Ni²⁺, and Cu²⁺) are found to be dependent on the final acidity. These results indicate a mechanism involving the slow protonation of these complexes (k_H+ < 10⁴ l mol^–1s^–1). A possible explanation of the inhibiting properties of transition-metal ions in the enzyme-catalysed syntheses of 6-carboxyuridine 5′-monophosphate is inferred.