Chiral resolution of D- and L-amino acids by tandem mass spectrometry of Ni(II)-bound trimeric complexes

Mass-selected trimeric cluster ions, [Ni-11(A)(ref*)(2)-H](+), where A desi gnates the analyte amino acid and ref* designates the chiral reference amin o acid, undergo competitive collision-induced dissociation to yield two dim eric clusters with a branching ratio dictated by the stereochemistry of bot h the analyte and the reference. This branching ratio (R) is related, using the kinetic method, to an enthalpic term, Delta (NiBDE)-B-11, the differen ce between the formation enthalpies of the two dimeric clusters. Chiral res olution of (D)- acid (L)-amino acids is directly related to the relative st abilities of the two diastereomeric clusters formed by the dissociation of the Ni(II)-bound trimeric clusters. The dimeric product ions differ in enth alpy by only a few kilojoules per mole, but chiral recognition is achieved for all 19 naturally occurring chiral amino acids, using appropriate refere nce amino acids. Because there is evidence that the dissociating trimeric d usters may exist in different isomeric forms, the proportion of analyte ver sus reference amino acids in the mixture was examined to study the effect o f this ratio on the success of chiral recognition. The effect was found to be negligible. This suggests that there is an equilibrium between these iso meric clusters, which is governed by thermochemical properties rather than the relative concentrations of the constituent amino acids. A linear correl ation was observed between In(R) and enantiomeric compositions of the analy te as expected from the kinetic method treatment, acid the direct measureme nt of optical purity to within 3% enantiomeric excess was demonstrated. Wit h this new chiral recognition technique, qualitative and quantitative chira l analysis of amino acids is achieved. (C) 2001 Elsevier Science B.V.