Chiral differentiation of <scp>d</scp> - and <scp>l</scp> -isoleucine using permethylated β-cyclodextrin: infrared multiple photon dissociation spectroscopy, ion-mobility mass spectrometry, and DFT calculations

摘要： Chiral di?erentiation of protonated isoleucine (Ile) using permethylated b-cyclodextrin (perCD) in the gas-phase was studied using infrared multiple photon dissociation (IRMPD) spectroscopy, ion-mobility, and density functional theory (DFT) calculations. The gaseous protonated non-covalent complexes of perCD and D-Ile or L-Ile produced by electrospray ionization were interrogated by laser pulses in the wavenumber region of 2650 to 3800 cm?1. The IRMPD spectra showed remarkably di?erent IR spectral features for the D-Ile or L-Ile and perCD non-covalent complexes. However, drift-tube ion-mobility experiments provided only a small di?erence in their collision cross-sections, and thus a limited separation of the D- and L-Ile complexes. DFT calculations revealed that the chiral distinction of the D- and L-complexes by IRMPD spectroscopy resulted from local interactions of the protonated Ile with perCD. Furthermore, the theoretical results showed that the IR absorption spectra of higher energy conformers (by B13.7 kcal mol?1) matched best with the experimentally observed IRMPD spectra. These conformers are speculated to be formed from kinetic-trapping of the solution-phase conformers. This study demonstrated that IRMPD spectroscopy provides an excellent platform for di?erentiating the subtle chiral di?erence of a small amino acid in a cyclodextrin-complexation environment; however, drift-tube ion-mobility did not have su?cient resolution to distinguish the chiral di?erence.