Kinetic resolution of D,L-amino acids based on gas-phase dissociation of copper(II) complexes

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 [A(2)BCu(II) - H](+) dissociate to form [A(2)Cu(II) - H](+) and [ABCu( II) - H](+) upon collision-induced dissociation (CID) in a quadrupole ion t rap. The abundance ratios of these fragments depend strongly on the stereoc hemistry of the ligands in the [A(2)BCu(II) - H](+) complex ion. The kineti c method was used to calculate relative Cu ion affinities (Delta Cu(II)') f or homo- and heterochiral copper(II)-bound dimeric cluster ions as the indi cator of chiral discrimination. Six amino acids of four different types sho wed chiral distinctions which ranged from 0 to 6.5 kJ/mol in terms of value s of Delta Cu(II)' with abundance ratios, referenced to the other enantiome r, ranging from 1 to 9.2. Amino acids with aromatic substituents displayed the largest chiral distinction, which correlates well with reported chromat ographic results. The methodology presented here provides a sensitive means to study enantiomers by mass spectrometry, and initial results show that i t is applicable to measurement of enantiomeric excess.