The structure of glycine-water H-bonded complexes

Hydrogen-bond energies and geometric structures for the 1 : 1 cyclic complexes of water with the three possible conformers of glycine have been determined. Ab initio gradient optimization at the self-consistent field (SCF) level in a double-zeta plus polarization-functions basis set was followed by single-point post-SCF second-order Møller-Plesset (MP2) calculations for the lowest energy structures. The most stable complex involves a doubly hydrogen-bonded arrangement of water with the carboxyl group of glycine monomer I, which is also the lowest energy conformer of the glycine monomer. The calculated MP2 binding energy of 11.0 kcal/mol (uncorrected for basis set superposition which is expected to be small) is substantial and this complex should be observed experimentally. MP2 reduces the shorter SCF geometry optimized H-bond length by ≈ 0.1 Å.