A potentiometric and V-51 NMR study of the aqueous H+/H2VO4-/H2O2/L-alpha-alanyl-L-histidine system

The speciation in the quaternary aqueous H+/H2VO(4)(-)/H2O2/L-alpha -alanyI -L-histidine (Ah) system has been determined from quantitative V-51 NMR mea surements and potentiometric data (glass electrode). The study was performe d in 0.150M Na(CI) medium at 25 degreesC. Data were evaluated with the comp uter program LAKE, which is able to treat combined EMF and NMR data. The pK (a) values for Ah were determined as 8.06, 6.72 and 2.64. In the ternary H/ H2VO4-/Ah system, two complexes, (H+)(p)(H2VO4-)q(Ah)(r) for which (p, q, r) values of (0, 1, 1) and (1, 1, 1) with log beta (0,1,1) = 2.52 +/-0.03 and log(beta1,1,1) = 9.40 +/- 0.05 (pK(a)=6.88), respectively, explain all data. The errors given are 3 sigma. In the quaternary H+/H2VO4-/H2O2/Ah sys tem, eight complexes were determined in addition to. all binary and ternary complexes, four with a V/X/Ah ratio 1:1:1 and four with a ratio 1:2:1 (X= peroxo ligand), VX(2)Ah(2-) and VX(2)Ah(-)(pK(a) = 8,19) are the main compl exes and predominate in the pH range 5 to 9. Three additional minor species have also been found but their compositions could not be determined owing to their small amounts. Equilibria are slow, significant decomposition of p eroxide occurs only in acidic solutions. Data in the pH range 5 to 10 have been used for the LAKE calculations. Chemical shifts, compositions, and for mation constants for the eight quaternary complexes are given, and equilibr ium conditions are illustrated in distribution diagrams. Structural proposa ls for VX(2)Ah(2-) and VX(2)Ah(-) are made from H-1 and C-13 NMR measuremen ts.