Mass Spectrometry Study and Infrared Spectroscopy of the Complex Between Camphor and the Two Enantiomers of Protonated Alanine: The Role of Higher‐Energy Conformers in the Enantioselectivity of the Dissociation Rate Constants

Abstract

The properties of the protonated complexes built from S camphor and R or S alanine were studied in a Paul ion trap at room temperature by collision-induced dissociation (CID) and infrared multiple-photon dissociation spectroscopy (IRMPD), as well as molecular dynamics and ab initio calculations. While the two diastereomer complexes display very similar vibrational spectra in the fingerprint region, in line with similar structures, and almost identical calculated binding energies, their collision-induced dissociation rates are different. Comparison of the IRMPD results to computed spectra shows that the SS and SR complexes both contain protonated alanine strongly hydrogen-bonded to the keto group of camphor. The floppiness of this structure around the NH⁺...O=C hydrogen bond results in a complex potential energy surface showing multiple minima. Calculating the dissociation rate constant within the frame of the transition state theory shows that the fragmentation rate larger for the heterochiral SR complex than the homochiral SS complex can be explained in terms of two almost isoenergetic low-energy conformers in the latter that are not present for the former.

Theoretical studies of infrared signatures of proton‐bound amino acid dimers with homochiral and heterochiral moieties

“…Many conventional methods employed for molecular structure elucidation are insensitive to chirality. In contrast to chiral molecules, the diastereomers formed upon enantiomer‐selective reactions in chiral environment can potentially have specific fragmentation patterns, ion mobility, infrared (IR) spectra features, etc, and thus can be probed with traditional techniques such as mass spectrometry and IR spectroscopy. One example of diastereomers is proton‐bound homochiral and heterochiral dimers of amino acids.…”

Chirally sensitive collision induced dissociation of proton‐bound diastereomeric complexes of tryptophan and 2‐butanol

“…This small difference can be converted into detectable differences in fragment‐ion branching ratios . In a recent study, Zehnacker‐Rentien and co‐workers investigated systems without a transition metal and detected differences in the collision‐induced dissociation rates of hetero‐ and homochiral complexes of camphor and alanine . The observation of a higher fragmentation efficiency for the heterochiral complex, despite it being calculated to be slightly more stable than the homochiral system, was explained by the presence of a second, lower‐energy, conformer of the homochiral complex.…”

Chirality effects in gas-phase spectroscopy and photophysics of molecular and ionic complexes: contribution of low and room temperature studies

“…MS approaches of chiral recognition have been the subject of several review articles [35,[47][48][49]. However, studies of chiral recognition coupling spectroscopy and MS are still scarce [22,50,51] and this review will focus on them as well as on the photophysical properties of the studied systems. Low temperature as achieved in supersonic expansions and room-temperature experiments, in ion trap or in the bulk, give complementary information.…”

“…One therefore expects a strongly NH + …O hydrogen-bonded complex to be formed, for which conformational mobility is limited to flexibility around the intermolecular hydrogen bond. This system provides therefore a mean of studying the influence of conformational mobility along a single degree of freedom on CID efficiency and allows assessing the influence of the presence of higher energy conformers on the chirality dependence of CID efficiency [51].…”