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 co mplexes [Zn(tren)(OH2)](2+) and [Zn(Me(6)tren)(OH2)](2+) have been det ermined 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 t hat for [Zn(Me(6)tren)(OH2)](2+) (12.1 kcal/mol) is attributable equal ly to positive Delta H (6.1 kcal/mol) and negative Delta S at 298 K. T he increase in acidity of [Zn(Me(6)tren)(OH2)](2+) relative to [Zn(tre n)(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 fro m ones previously reported, particularly in that the thermodynamic bas is of the increased acidity of the latter complex is not related to en tropic effects, as was previously believed. Several possible origins o f the enthalpic difference ill the two ionization reactions are discus sed. 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 wat er in zinc hydrolytic enzymes.