SPECIATION IN VANADIUM BIOINORGANIC SYSTEMS .1. A POTENTIOMETRIC AND V-51 NMR-STUDY OF AQUEOUS EQUILIBRIA IN THE H-VANADATE(V)-L-ALPHA-ALANYL-L-HISTIDINE SYSTEM()

The full speciation in the system H+-H2VO4--L-alpha-alanyl-L-histidine (Ah) has been determined from potentiometric (glass electrode) and V-5 1 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-tha n-or-equal-to 9.5 and were evaluated with the computer program LAKE, a ble to treat combined EMF and NMR data. The pK(a)-values for Ah were d etermined 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) w ith 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 complexe s formed are fairly strong. At a ratio Ah/V = 4:1 about 60% of V is bo und in ternary complexes, with the optimum amount at around pH 6. Equi librium conditions are illustrated in distribution diagrams and struct ures of the complexes formed are proposed. The system H+-H2VO4--L-hist idine(His) has been partly studied. The pK(a)-values for His were dete rmined as 9.103, 6.146 and 1.778. His has a maximum complexation to va nadate near pH 5, and the complexes formed are weaker than those for A h. Spontaneous reduction was, however, found to occur, so accurate qua ntitative evaluation could not be performed on the H2VO4--His system.