Journal of the American Chemical Society, Vol.117, No.46, 11545-11547, 1995
Enthalpic Control of Zinc Water Acidity in Complexes of Tris(2-Aminoethyl)Amine and Tris(2-(Dimethylamino)Ethyl)Amine
The enthalpies of ionization (Delta H-a) of zinc-bound water in the complexes [Zn(tren)(OH2)](2+) and [Zn(Me(6)tren)(OH2)](2+) have been determined by isothermal titration calorimetry and potentiometric methods in 1 M NaClO4. The Delta G degrees (14.6 kcal/mol) for [Zn(tren)(OH2)](2+) is mainly due to large positive Delta H (11.0 kcal/mol), while that for [Zn(Me(6)tren)(OH2)](2+) (12.1 kcal/mol) is attributable equally to positive Delta H (6.1 kcal/mol) and negative Delta S at 298 K. The increase in acidity of [Zn(Me(6)tren)(OH2)](2+) relative to [Zn(tren)(OH2)](2+) corresponds to a Delta Delta G degrees of -2.5 kcal/mol. The Delta Delta H is -4.9 kcal/mol, and it is partially offset by a 2.4 kcal/mol contribution from Delta(T Delta S). These values differ from ones previously reported, particularly in that the thermodynamic basis of the increased acidity of the latter complex is not related to entropic effects, as was previously believed. Several possible origins of the enthalpic difference ill the two ionization reactions are discussed. The results of the study invalidate the original data that led to the vi;ew that solvent structure mediates the pK(a) of zinc-bound water in zinc hydrolytic enzymes.