INTERACTION OF CALCIUM AND MAGNESIUM-IONS WITH MALONATE AND THE ROLE OF THE WATERS OF HYDRATION - A QUANTUM-MECHANICAL STUDY

The structure of malonate interacting with either a Ca(II) or a Mg(II) ion in a chelation bidentate orientation has been studied with ab initio molecular orbital theory. In addition, the optimized chelation bidentate structures have also been determined for the enol tautomer of malonate ion and two different deprotonated forms of the malonate-metal ion complex. All malonate-derived structures were optimized with and without four waters of hydration about the divalent metal ion. The enol tautomer of malonate was found to be less stable than the keto form complexed with either Ca(II) or Mg(II) ions. Computations have demonstrated that removal of a proton from one of the waters of hydration about the divalent metal ion leads to a structure that is calculated to be more stable than the structure resulting from the deprotonation on carbon of the malonate complex. A mechanism for the chemical modification and proton exchange of proteins containing gamma-carboxyglutamic acid (e.g., prothrombin), for which malonic acid serves as a model, is proposed.