STRUCTURE AND STABILITY OF COMPLEXES OF GLYCINE AND GLYCINE METHYL ANALOGS WITH H+, LI+, AND NA+

The gas-phase structures of complexes of glycine and glycine methyl analogues with H+, Li+, and Na+ have been investigated by ab initio calculations. Geometries have been optimized at the Hartree-Fock level with the 6-31G* basis set and relative energies determined with the MP2/6-31G* method. Higher level calculations showed that relative energies are rather insensitive to enlargement of the basis set and further inclusion of electron correlation, but the semiempirical MNDO method is of insufficient accuracy for being useful. A total of nine structures for protonated glycine are found, three corresponding to N-protonation and six corresponding to O-protonation. Thirteen structures of glycine-Li+ have been optimized, and the lowest energy species corresponds to a five-membered ring in which lithium is coordinated to both nitrogen and oxygen in the neutral form of the amino acid. Four other structures are within approximately 10 kcal/mol of the global minimum. The lowest energy Na+ complex is the same as that for Li+, although species in which sodium is coordinated to both oxygens are only slightly higher in energy. For the low energy lithium complexes, the effect of substituting a hydrogen with a methyl either at C, O, or N was probed. Both N- and C-methylation stabilize preferentially the doubly oxygen coordinated form.