Enthalpic control of zinc-water acidity in complexes of tris(2-aminoethyl)amine and tris(2-(dimethylamino)ethyl)amine

James Canary, Jinqi Xu, Jesus M. Castagnetto, Dionisios Rentzeperis, Luis A. Marky

Research output: Contribution to journalArticle

Abstract

The enthalpies of ionization (ΔHa) of zinc-bound water in the complexes [Zn(tren)(OH2)]2+ and [Zn(Me6-tren)(OH2)]2+ have been determined by isothermal titration calorimetry and potentiometric methods in 1 M NaClO4. The ΔG° (14.6 kcal/mol) for [Zn(tren)(OH2)]2+ is mainly due to large positive ΔH (11.0 kcal/mol), while that for [Zn(Me6tren)(OH2)]2+ (12.1 kcal/mol) is attributable equally to positive ΔH (6.1 kcal/mol) and negative ΔS at 298 K. The increase in acidity of [Zn(Me6tren)(OH2)]2+ relative to [Zn(tren)(OH2)]2+ corresponds to a ΔΔG° of -2.5 kcal/mol. The ΔΔH is -4.9 kcal/mol, and it is partially offset by a 2.4 kcal/mol contribution from Δ(TΔS). 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 in the two ionization reactions are discussed. The results of the study invalidate the original data that led to the view that solvent structure mediates the pKa of zinc-bound water in zinc hydrolytic enzymes.

Original languageEnglish (US)
Pages (from-to)11545-11547
Number of pages3
JournalJournal of the American Chemical Society
Volume117
Issue number46
StatePublished - Nov 22 1995

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Acidity
Zinc
Amines
Ionization
Water
Calorimetry
Titration
Thermodynamics
Enthalpy
Enzymes
tris(2-aminoethyl)amine
tris(2-(dimethylamino)ethyl)amine

ASJC Scopus subject areas

  • Chemistry(all)

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Enthalpic control of zinc-water acidity in complexes of tris(2-aminoethyl)amine and tris(2-(dimethylamino)ethyl)amine. / Canary, James; Xu, Jinqi; Castagnetto, Jesus M.; Rentzeperis, Dionisios; Marky, Luis A.

In: Journal of the American Chemical Society, Vol. 117, No. 46, 22.11.1995, p. 11545-11547.

Research output: Contribution to journalArticle

Canary, James ; Xu, Jinqi ; Castagnetto, Jesus M. ; Rentzeperis, Dionisios ; Marky, Luis A. / Enthalpic control of zinc-water acidity in complexes of tris(2-aminoethyl)amine and tris(2-(dimethylamino)ethyl)amine. In: Journal of the American Chemical Society. 1995 ; Vol. 117, No. 46. pp. 11545-11547.
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