Deprotonation of Water in the Presence of Carboxylate and Magnesium Ions

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Abstract

The effects of a metal ion-bound carboxylate group on the acidity of a water molecule bound to the same cation have been assessed by ab initio molecular orbital calculations. In the hexahydrate Mg[H2O]62+ the free energy required to deprotonate one coordinated water molecule is only 40% of that required to deprotonate a free water molecule, indicating that the presence of the magnesium ion facilitates the ionization of water. However, if one of the water molecules in this hexahydrate Mg[H2O]62+ is replaced by a carboxylate ligand, the energy required to dissociate a proton from a metal ion-bound water molecule is increased by approximately 80 kcal/mol and is intermediate between the energy required to deprotonate one water molecule in Mg[H2O]62+ and that for a free water molecule. This effect of the carboxylate group on the pKa of metal ion-bound water appears to be primarily the result of a reduction of the net positive charge of the overall Mg[H2O]52+−(RCOO-) complex rather than any changes in the electronic structure of the magnesium cation itself, since a Mg2+-coordinated chloride ion has a similar influence on acidity. Two aquated magnesium−carboxylate motifs have been identified in crystal structures of small molecules and in proteins. One is a magnesium-bound hydrated carboxylate motif with an internal hydrogen bond. Formation of the hydrogen bond within this motif does not appear to appreciably affect the pKa of the metal ion-bound water molecule. A major role of such a motif, found in many protein crystal structures, may be to help align the rather rigid magnesium coordination octahedron, thus positioning appropriate functional groups for efficient catalytic activity. A second motif, which involves a carboxylate group bound to two metal ion-bound water molecules, is also found in several protein crystal structures. It is, however, more flexible in conformation than is the first motif and therefore cannot exert such rigid orientational powers. Thus, metal ion-bound water molecules and carboxylate groups can interact in a synergistic fashion to assist in the catalytic activity of enzymes by altering the pKa of the water molecule and by providing a means for aligning required functional groups in a stereochemically precise manner (“coordination clamping”).

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