Stabilities of vanadyl complexes with methionine, phenylalanine, and threonine.

Stepwise formation constants corresponding to 1:1 vanadyl complexes with methionine, phenyl­ alanine and threonine have been determined at 25 °C, and at various ionic concentrations, viz. 0.01, 0.1 and 0.3 M, maintained by the addition of potassium chloride solution. Thermodynamic stability constants have been obtained by extrapolation of log K values to zero ionic strength. Logarithms of the overall stability constants ( log £ (u = 0)) are 7.72 for methionine, 7.70 for phenyl­ alanine and 7.44 for threonine complexes. The corresponding free energy changes (AG°) are — 10.53, —10.51, — 10.15 kcal-mol-1 respectively.