17O NMR studies of the effect of mixed ligands on the bound water in aqueous solutions of amino acids and Co(II)

Abstract

AbstractThe effect of mixed ligands on the lifetime and 17O hyperfine coupling constant of water bound to Co2+ in aqueous solutions of proline, glycine and alanine was determined, as a function of pH, by means of 17O NMR. The lifetime of the coordinated water molecules decreased from 4.9 X 10−7 s (for the pure aqueous complex) to 3.5 × 10−8 at pH 8, where more than one complex of proline were present. Smaller changes were measured for the 17O hyperfine coupling constant of water [from ‐ 1.7 × 107 Hz for Co(H2O) to ‐1.45 × 107 Hz measured at pH 6 in the presence of proline], and the enthalpy of activation of the first‐order exchange from the first coordination sphere of Co2+ (from 7.4 ± 0.2 kcal mol−1 for the pure aqueous complex to 6.2 ± 0.4 kcal mol−1 at pH 8 in the presence of proline).