Structural relationships in small molecule interactions governing gas-phase enantioselectivity and zwitterionic formation.

Gas-phase zwitterionic amino acids were formed in complexes of underivatized beta-cyclodextrin through reactions with a neutral base, n-propylamine. The reaction was performed in the analyzer cell of an electrospray ionization-Fourier transform mass spectrometer. Most of the natural amino acids were studied with three cyclodextrin hosts including alpha-, beta-, and gamma-cyclodextrin to understand better the structural features that lead to the stabilization of the zwitterionic complexes. Molecular dynamics calculations were performed to provide insight into the structural features of the complexes. The rate constants of the reactions were obtained through kinetic plots. Examination of both L- and D-enantiomers of the amino acid showed that the reaction was enantioselective. The reaction was then employed to analyze mixtures of Glu enantiomers naturally occurring in the bacteria Bacillus licheniformis.