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The interaction of gamma-aminobutiric acid with hydrated Ca2+ and Mg2+. A pseudopotential ab initio study.

As a continuation of a previous work we consider the interaction of Mg2+ and Ca2+ with a neurotransmitter, gamma-aminobutiric acid (GABA). The purpose is twofold, to determine if there is a direct interaction of Ca2+ with the amino acid, which could have some biological relevance, and to find out if such a hypothesis can account for the different role of Ca2+ and Mg2+ on the amino acid's release. We performed ab initio pseudopotential studies of the GABA-ion complex in the presence of the first hydration shell around the interaction's region. We calculated the interaction energy of the hydrated complex by means of a many-body expansion up to three-body terms. We found out that the three-body terms in systems involving the divalent ion have a considerable value and seems to be of a different character for Mg2+ and Ca2+. We found out that the three-body terms are responsible for the observed difference between the coordination properties of Mg2+ and Ca2+ and can lead to a difference in the aminoacid interaction with each ion. The hypothesis of a gamma-aminobutiric acid-Ca2+ complex, that facilitates the aminoacid release has been substantiated. The reasons for the different effect of Ca2+ and Mg2+ has been envisaged but not clearly established.