Evaluation of chiral recognition characteristics of metal and proton complexes of di- o-benzoyl-tartaric acid dibutyl ester and L-tryptophan in the gas phase

Abstract

Chiral recognition of di- o-benzoyl-tartaric acid dibutyl ester (T) was achieved in the gas phase by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. In this method two divalent transition metal cations, zinc(II) and copper(II), were used as binding metal ions, and L-tryptophan (A) was used as a chiral reference. Multimeric complexes were readily formed by electrospray ionization of a methanol:water (50:50) solution containing metal ion, L-tryptophan and T. These multimeric complexes included singly charged protonated dimeric [TAH] +, doubly charged copper(II) bound tetrameric [TACu-H] 2 2+ and doubly charged zinc(II) bound tetrameric [TAZn-H] 2 2+, together with other complexes. The mass-selected complex, i.e., [TAH] +, [TACu-H] 2 2+ and [TAZn-H] 2 2+, was used to acquire the second stage mass spectra. The chiral recognition capability of these three complexes was evaluated using the abundance ratios of daughter ion to parent ion. A high degree of chiral recognition ability was observed in [TACu-H] 2 2+ and [TAZn-H] 2 2+. It was found that the type of binding ion played an important role in the chiral recognition. Different binding ions exhibited distinctive dissociation pathways and unique chiral recognition characteristics. The present method is based not only on whole-molecule loss but also on fractional-molecule loss. In addition, the reproducibility of the chiral recognition method was confirmed by several determinations of the abundance ratios of daughter ion to parent ion with a fixed activation energy and with five different activation energies. It was also shown that this chiral recognition method can tolerate acid interference.