Tautomerism of Orotic Acid Dianion. Effect of Calcium and Magnesium Cations on the Tautomeric Constant and on Tautomerization Dynamics

Article abstract of DOI:10.1021/ja00529a030

Orotic acid dianion (HL^2-) exists in basic aqueous solutions as a mixture of two isomeric structures (K_T = / = 0.4 +/- 0.08, ΔH^T = -0.3 +/- 0.05 kcal, ΔS^T = -2 +/- 0.4 eu).Its tautomeric interconversion as investigated by T-jump relaxation is fast (τ never exceeds 30 μs in D2O) and is catalyzed by OD^- OD- = (3.3 +/- 0.2) <*> 10⁸ M^-1 s^-1>, by D2O acting as an acid D2O = (3 +/- 0.3) <*> 10⁴ s^-1> and by orotate monoanion H2L- = (4 +/- 0.7) <*> 10⁶ M^-1 s^-1>.The orientation of orotic acid methylation is discussed in terms of the change in tautomer populations caused by temperature.The tautomeric equilibrium constant is markedly affected by alkaline earth cations (Ca²⁺ and Mg²⁺) owing to a specific complexation of the N(3)H form.Estimates of the complex formation constants, K_comp, are (1.7 +/- 0.2) <*> 10² and (5 +/- 0.5) <*> 10² M^-1 for Ca²⁺ and Mg²⁺, respectively.The relaxation time for the complexation process is shorter than 1 μs for Ca²⁺, whereas it is in the millisecond range for Mg²⁺ with a second order rate constant equal to (5.5 +/- 0.6) <*> 10⁵ M^-1 s^-1.