Hydrogen bonds of a water molecule in the second coordination sphere of amino acid metal complexes: Influence of amino acid coordination.

The hydrogen bonds of free and coordinated amino acids with water molecule were studied by analyzing data in the crystal structures from the Cambridge Structural Database (CSD) and by quantum chemical calculations. The CSD data indicate bifurcated NH/O hydrogen bonds and O1/HO hydrogen bonds of coordinated oxygen. The O/HO hydrogen bonds of free zwitterions and non-coordinated carbonyl oxygen (O2/HO) in metal complexes form primarily linear, non-bifurcated hydrogen bonds. Calculated M06L-GD3/def2-TZVPP interaction energies for free zwitterions (glycine, cysteine, phenylalanine and, serine) and water molecule are in the range from -5.1 to -9.6 kcal/mol for NH/O and from -6.9 to -7.6 kcal/mol for O/HO interactions. Coordinated amino acids in neutral octahedral cobalt(III) complexes have NH/O interaction energies ca. -7.4 kcal/mol, independent of the amino acid. The singly and doubly charged complexes have stronger NH/O interactions; the strongest has energy of -16.9 kcal/mol. In the case of O1/HO hydrogen bond, the interaction energy decreases upon coordination; interactions are quite weak for neutral complexes (-2.2 to -2.6 kcal/mol). For O2/HO hydrogen bonds, all amino acids except serine show slightly stronger interaction in singly negative complexes (-6.3 to -8.0 kcal/mol), while interactions are weaker for neutral complexes (-2.8 to -4.4 kcal/mol), comparing to zwitterions.