THEORETICAL-STUDY OF PHOSPHATE INTERACTION WITH NH4+, WITH NA+, AND WITH MG2+ IN THE PRESENCE OF WATER

The interactions of the phosphate anion with NH4+, Na+ and Mg2+ are studied by ab initio molecular orbital theory. Regions of the potential energy surfaces for anion-cation complex formation with 2 water molecules present are investigated. The theory predicts that the most stable structures for the monocation complexes have the water molecules partially between the anion and the cation. These bridging water molecules increase the separation distance between the ionic species in order to optimize the water-ion interactions. The addition of 2 water molecules is not predicted to affect the geometry of the magnesium phosphate complex. Implications for the association of cations with nucleic acids are discussed.