Effect of metal Ions (Ni²⁺, Cu²⁺ and Zn²⁺) and water coordination on the structure of L-phenylalanine, L-tyrosine, L-tryptophan and their zwitterionic forms.

Methods of quantum chemistry have been applied to double-charged complexes involving the transition metals Ni(2+), Cu(2+) and Zn(2+) with the aromatic amino acids (AAA) phenylalanine, tyrosine and tryptophan. The effect of hydration on the relative stability and geometry of the individual species studied has been evaluated within the supermolecule approach. The interaction enthalpies, entropies and Gibbs energies of nine complexes Phe•M, Tyr•M, Trp•M, (M = Ni(2+), Cu(2+) and Zn(2+)) were determined at the Becke3LYP density functional level of theory. Of the transition metals studied the bivalent copper cation forms the strongest complexes with AAAs. For Ni(2+)and Cu(2+) the most stable species are the NO coordinated cations in the AAA metal complexes, Zn(2+)cation prefers a binding to the aromatic part of the AAA (complex II). Some complexes energetically unfavored in the gas-phase are stabilized upon microsolvation.