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

The effect of mixed ligands on the lifetime and 17 O hyperfine coupling constant of water bound to Co 2+ in aqueous solutions of proline, glycine and alanine was determined, as a function of pH, by means of 17 O 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 17 O hyperfine coupling constant of water [from ‐ 1.7 × 10 7 Hz for Co(H 2 O)   6 2+to ‐1.45 × 10 7 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 Co 2+ (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).