Speciation in Vanadium Bioinorganic Systems. 1. A Potentiometric and 51V NMR Study of Aqueous Equilibria in the H+--Vanadate(V)--L-alpha-Alanyl-L-histidine System.

Abstract

The full speciation in the system H+-H2VO4--L-alpha-alanyl-L-histidine(Ah) has been determined from potentiometric (glass electrode) and V-51 NMR measurements. The study was performed in 0.6 M Na(Cl) medium at 25-degrees-C. Data cover the range 2 less-than-or-equal-to pH less-than-or-equal-to 9.5 and were evaluated with the computer program LAKE, able to treat combined EMF and NMR data. The pK(a)-values for Ah were determined as 8.08, 6.85 and 2.76. In the ternary system, two complexes, (H+)p(H2VO4-)q(Ah)r, having the (p,q,r) values (0,1,1) and (1,1,1) with log beta0,1,1=2.55 +/- 0.04 and log beta1,1,1 = 9.44 +/- 0.05 (pK(a) = 6.89) explain all data. The errors given are 3sigma. The complexes formed are fairly strong. At a ratio Ah/V = 4:1 about 60% of V is bound in ternary complexes, with the optimum amount at around pH 6. Equilibrium conditions are illustrated in distribution diagrams and structures of the complexes formed are proposed. The system H+-H2VO4--L-histidine(His) has been partly studied. The pK(a)-values for His were determined as 9.103, 6.146 and 1.778. His has a maximum complexation to vanadate near pH 5, and the complexes formed are weaker than those for Ah. Spontaneous reduction was, however, found to occur, so accurate quantitative evaluation could not be performed on the H2VO4--His system.