DISTINCTION OF AMINO-ACID ENANTIOMERS BASED ON THE BASICITY OF THEIR DIMERS

Mixtures of several amino acid pairs, in all four chiral combinations, were studied, The protonated trimers (A(2)-BH+) fragment, forming ABH (+) and A(2)H(+) dimers, Abundance ratios of these fragments were meas ured in the mass-analyzed ion kinetic energy spectra of the trimers. T hese were found to depend on the stereochemistry (homo- or heterochira l form) of the ABH(+) dimer, The results were evaluated using the kine tic method, and the chiral discrimination was related to a difference in gasphase basicity (GB) between the homo- and the heterochiral dimer s. Four amino acid pairs (proline-tryptophan, phenylalanine-alanine, p henylalanine-proline, and phenylalanine-valine) were studied, Chiral d iscriminations were observed in all cases, relating to 0.4-4 kJ/mol di fferences in GE, The technique described here can generally be used to study enantiomers by mass spectrometry and is capable of reliably dis tinguishing energy differences as small as 0.2 kJ/mol in cluster ions.