Comprehensive ab initio study on the conformations of L-Threonine and L-allo-Threonine and related species in gas phase

Systematic conformational searches have been performed for various species of L-Threonine (L-Thr) and L-allo-Threonine (L-allo-Thr) in gas phase. The trial structures were generated by allowing for all combinations of internal single-bond rotamers. The trial structures were optimized at the HF/3-21G(d) level and then subjected to further optimization at the B3LYP/6-311++G(d, p) level. A total of 74 canonical neutral, 12 protonated, 22 deprotonated and 4 secondary deprotonated conformers are found for the L-Thr species, while 75 canonical neutral, 14 protonated, 22 deprotonated, and 6 secondary deprotonated conformers are found for the L-allo-Thr species. However, no stable zwitterionic structures of isolated threonine may exist. The conformational energies are determined by the MP2/6-311+G(2df, p) calculations. The intramolecular hydrogen bonds were characterized. The equilibrium conformer distributions at different temperatures are shown. The proton affinities, gas phase basicities, proton dissociation energies and gas phase acidities are determined and compared with the experiments. The conformations of threonine hydrated with one water molecule were also systematically searched. The zwitterionic structures may be stabilized by one water molecule, but their energies are relatively high. The infrared spectra of the most stable structures of all species are presented.