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Eur. J. Mass Spectrom. 8, 107 - 115 (2002) |
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Chiral preferences in the dissociation of homogeneous amino acid/metal ion clusters | ||
W. Andy Tao and R. Graham Cooks* |
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ABSTRACT: | ||
Collision-induced dissociation (CID) of alanine or phenylalanine aggregates with various cations, including the proton, alkali metal ions, calcium and zinc, as well as the transition metals Co(II), Ni(II) and Cu(II), has been studied by tandem mass spectrometry. In these experiments, an isotopically labeled, chirally-pure amino acid (L-Ala-d3 and L-Phe-d5) is mixed with its unlabeled enantiomer and a metal salt and the clusters generated by electrospray ionization are mass-selected and subjected to CID. The relative stability of the precursor ions, as well as product-ion abundance ratios in the tandem mass spectra, are used as indicators of chiral effects on the dissociation process. It is shown that the stability of alanine aggregates during the kinetic dissociation process is independent of chirality, whereas a large chiral effect (Rchiral ranging from 0.31 to 8.1) is observed for the clusters assembled from phenylalanine and transition metal ions. When two bulky phenylalanine molecules are covalently bound to a metal ion, the heterochiral cluster is more stable than the homochiral one. Clusters containing three or four phenylalanine molecules are loosely bound to the core ion and the homochiral cluster is more stable than the heterochiral one. It is suggested that non-covalent interactions, in particular π-cation interactions, are significant contributors to the large chiral effects. It is further suggested that the relationship between the branching ratio in the tandem mass spectrum and chirality can be used to determine the absolute configuration of chiral molecules. | ||
Keywords: homochirogenesis, chirality, kinetic method, amino acids, tandem mass spectrometry, self assembly, isotope effects |
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