Investigation of dialkyl tartrate molecular recognition in cluster ions by Fourier transform mass spectrometry: a comparison of chirality effects in gas and liquid phases

Electrospray ionization of equimolar solutions of S-dimethyl tartrate and R-d6-dimethyl tartrate in methanol/water/acetic acid/salt solutions was utilized to investigate molecular recognition processes in solution. Pronounced chirality effects previously reported for formation of the protonated dimer by ion molecule reactions in the sas phase are quantitatively reproduced in experiments which sample solution phase protonated dimers. Ab initio quantum calculations demonstrate that hydrogen bonds in the protonated cluster are responsible for molecular recognition and that Li+ bound clusters, which do not exhibit chiral recognition, are primarily bound by electrostatic forces. In contrast with gas phase studies of alkali and ammonium ion core dimers of dimethyl tartrate-which show no chirality effects-ions electrosprayed from solutions containing trace amounts of these ions show pronounced chirality. With increasing salt concentration the apparent chirality effect disappears and a statistical distribution identical to that found for the gas phase is obtained. These observations are rationalized by a kinetic model that considers the displacement of protons by alkali ions in the final stages of desolvation of microdroplets formed in the electrospray process. (C) 1999 Elsevier Science B.V.