Chiral differentiation ofd- andl-alanine by permethylated β-cyclodextrin: IRMPD spectroscopy and DFT methods

Abstract

The gaseous chiral differentiation of alanine by permethylated β-cyclodextrin was studied using IRMPD spectroscopy and density functional theory calculations. The protonated non-covalent complexes of permethylated β-cyclodextrin and d- or l-alanine were mass-selected and investigated by IR laser pulses in the wavelength region of 2650-3800 cm. The remarkably different features of the IRMPD spectra for d- and l-alanine are described, and their origin is elucidated by quantum chemical calculations. We show that the differentiation of the experimentally observed spectral features is the result of different local interactions of d- and l-alanine with permethylated β-cyclodextrin. We also assign the extremely high-frequency (>3700 cm) bands in the observed spectra to the stretch motions of completely isolated alanine -OH groups.

Noncovalent Complexes of Cyclodextrin with Small Organic Molecules: Applications and Insights into Host–Guest Interactions in the Gas Phase and Condensed Phase

“…The experimental observation of the very unstable (i.e., of unusually high Gibbs free energy) conformer ( Figure 12 d), rather than the thermodynamically favorable conformer ( Figure 12 c), was quite a surprise. Similar results were noted for gaseous structures that possessed very high Gibbs energy values such as per-β-CD/H + /amino acid complexes [ 59 , 60 ].…”

“…IRMPD spectroscopy has provided an important tool for the structural elucidation of gaseous CD complexes containing guest molecules [ 56 , 59 , 60 ]. Permethylated β-CD (per-β-CD) complexes with H 2 O guest molecules, which is the simplest guest molecule available, was the first system to be analyzed using this spectroscopic technique [ 56 ].…”

“…Other references suggest that the complementarity of the cavity size is also an important factor for the complexation. Mechanism studies for CD complexes in the gas phase have been performed using a combination of the mass spectrometric methods, namely, gas chromatography–mass spectrometry (GC–MS), ion mobility mass spectrometry (IMS–MS), infrared multiphoton dissociation (IRMPD) spectroscopy, and theoretical calculations involving various combinations of molecular dynamics (MD), semi-empirical, and density functional theory (DFT) methods [ 2 , 26 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 ]. Recently, structural analysis was conducted using action–FRET (Fluorescence Resonance Energy Transfer), a gas phase version of FRET in which chromophore-tagged β-cyclodextrin and amyloid-β fragment peptides were utilized [ 61 ].…”

Spectroscopic Discrimination of Diastereomeric Complexes Involving an Axially Chiral Receptor

“…The IRMPD spectra of ESI-formed diastereomeric[ M aR ·H·D D ] + and [M aR ·H·D L ] + complexes were consistent with the III-type structures, in which protonated D wasa ccommodatedo utside the flat cavity of the macrocycle M aR and hydrogen-bonded to its O 2 center.T he absence of IRMPD signals attributable to the more stable I-a nd II-type structures was not surprising and could be explained as follows. First, as reported in previous studies, [17,[31][32][33][34][35] it might be that, as the mixture of M aR /D was transferredi nt he gas phase, the proton-bound complexd id not have enough time and proper pathways to fully relaxt o the more stable structures, but was kinetically trapped in the originall ocal minimum. Second, even once formed, the more stable I-a nd II-type structures might be spectroscopically inactive, that is, stable enough to resist IR-induced fragmentation.…”

“…[15] Only in rare cases have clear IRMPD spectral differences been discerned for proton-bound diastereomeric complexes involving receptors with three-dimensional chiral cavities. [16,17] Herein, we provide a detaileda nalysis of the specific interactions involved in protonbound diastereomeric complexes between several aminoa cids and an axially chiral receptor endowed with af lat cavity.…”