Kinetic Resolution of d,l-Amino Acids Based on Gas-Phase Dissociation of Copper(II) Complexes

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Abstract
Chiral recognition of d- and l-amino acids is achieved in the gas phase on the basis of the kinetics of competitive fragmentations of trimeric Cu(II)-bound complexes. The singly charged copper(II)−amino acid trimeric cluster ions [A2BCuII − H]+ dissociate to form [A2CuII − H]+ and [ABCuII − H]+ upon collision-induced dissociation (CID) in a quadrupole ion trap. The abundance ratios of these fragments depend strongly on the stereochemistry of the ligands in the [A2BCuII − H]+ complex ion. The kinetic method was used to calculate relative Cu ion affinities (ΔCu(II)‘) for homo- and heterochiral copper(II)-bound dimeric cluster ions as the indicator of chiral discrimination. Six amino acids of four different types showed chiral distinctions which ranged from 0 to 6.5 kJ/mol in terms of values of ΔCu(II)‘ with abundance ratios, referenced to the other enantiomer, ranging from 1 to 9.2. Amino acids with aromatic substituents displayed the largest chiral distinction, which correlates well with reported chromatographic results. The methodology presented here provides a sensitive means to study enantiomers by mass spectrometry, and initial results show that it is applicable to measurement of enantiomeric excess.