Antimony(III)-D, L-tartrates exhibit proton-assisted enantioselective binding in solution and in the gas phase

Abstract

The negative ion mode ESI mass spectral analysis of antimony(III)-D-and -L-tartrate ("tartar emetic"), in association with leucine enantiomeric isotopomers, revealed remarkable protonassisted enantioselective molecular recognition phenomena. The current study infers that recognition of amino acids by antimony(III)-D,L-tartrate complexes requires that the chiral selector associate a proton to become enantioselective. The dianionic selector itself failed to show enantiomeric discrimination capacity. This observation was shown to be consistent both in solution-phase targeting full scan and gas-phase targeting collision threshold dissociation (CTD) experiments. Importantly, this disparity in enantioselective binding capacity between the dianionic and the protonated monoanionic representatives of antimony(III)-D-and -L-tartrates could only be clearly revealed by ESI-MS and tandem mass spectrometry experiments as described herein. This finding urges a more in-depth study of mechanisms associated with exhibited enantiomeric resolving capacity of antimony tartrates in HPLC and CE applications, as well as in former ESI-MS association studies. (J Am Soc Mass Spectrom 2009, 20, 2100 -2105

ESI‐MS investigation of solvent effects on the chiral recognition capacity of tartar emetic towards neutral side‐chain amino acids

“…It is clearly observed that regardless of the solvent system utilized, previously reported protonassisted enantioselectivity is maintained. 28 Close inspection of adjusted enantioselectivity values in Table 1 22 ion forms showed no significant selectivity towards the amino acid enantiomers studied. This observation is also consistent with the previous work which signifies that absence of a proton associated to the {[D,L-Sb 2 -tar 2 ] } 22 ionic core abolishes enantiomeric discrimination character towards neutral side-chain amino acids.…”

“…This observation is also consistent with the previous work which signifies that absence of a proton associated to the {[D,L-Sb 2 -tar 2 ] } 22 ionic core abolishes enantiomeric discrimination character towards neutral side-chain amino acids. 28 It is also apparent that the variation in solvent system does not alter this effect. It was also observed that amino acid enantiomers employed in this study showed a general trend of increasing adjusted selectivity in an increasing order of Val Ala < Leu Phe.…”

“…Enantiomerically-pure deuterated amino acid standards L-2,3, To characterize the compounds using methods described in our previous report, 28 ESI-mass spectra of the selectors were obtained. Both negative ionization mode ESI-mass spectra of synthesized bis(sodium) D 4,8,10, and 8 for the amino acids Ala, Val, Leu, and Phe, respectively).…”

“…It was assumed that utility of 75% of the organic modifiers in these solutions were sufficient to represent their effect to these chiral molecular recognition systems. Formaldehyde incorporation (100 mM) to enhance spectral quality of previously reported experiments was not practiced in these experiments, 28,32 because the study was focused on studying the effects by electrospray friendly organic solvent modifiers on these chiral recognition systems. It was assumed that formaldehyde incorporation with a high concentration (100 mM) would influence the solvent properties and alter the original form of the solvent system.…”

“…26,27 Our recent studies in single-stage ESI-MS experiments of D,L-antimony(III)-tartrates and enantiomers of leucine showed previously unprecedented proton-assisted enantioselectivity. 28 More specifically, it was observed that proton-associated L-antimony(III)-tartrate showed selective binding towards D-Leu over L-Leu. However, when there was no proton association, D,L-antimony(III)-tartrate dianionic hosts failed to show enantioselective molecular recognition phenomena toward the amino acid guests.…”