Homochiral heterochiral preference in chiral self-recognition of cyclic diols.

The structure and clustering propensity of a chiral derivative of -1,2-cyclohexanediol, namely, 1-phenyl--1,2-cyclohexanediol (-PCD), has been studied under supersonic expansion conditions by combining laser spectroscopy with quantum chemistry calculations. The presence of the phenyl substituent induces conformational locking relative to -1,2-cyclohexanediol (-CD), and only one conformer of the bare molecule is observed by both Raman and IR-UV double resonance spectroscopy. The homochiral preference inferred for the dimer formation at low enough temperature is in line with the formation of a conglomerate in the solid state.

Induced photoelectron circular dichroism onto an achiral chromophore.

An achiral chromophore can acquire a chiral spectroscopic signature when interacting with a chiral environment. This so-called induced chirality is documented in electronic or vibrational circular dichroism, which arises from the coupling between electric and magnetic transition dipoles. Here, we demonstrate that a chiroptical response is also induced within the electric dipole approximation by observing the asymmetric scattering of a photoelectron ejected from an achiral chromophore in interaction with a chiral host.

Non-covalent interactions reveal the protein chain conformation in a flexible single-residue model.

The conformation is a local secondary structure in proteins that implicates a π N-H⋯N interaction between a backbone N atom and the NH of the following residue. Small-molecule models thereof have been limited so far to rigid proline-type compounds. We show here that in derivatives of a cyclic amino acid with a sulphur atom in the γ-position, specific side-chain/backbone N-H⋯S interactions stabilize the conformation sufficiently to allow it to compete with classical C5 and C7 H-bonded conformers.

How Crystal Symmetry Dictates Non-Local Vibrational Circular Dichroism in the Solid State.

Solid-State Vibrational Circular Dichroism (VCD) can be used to determine the absolute structure of chiral crystals, but its interpretation remains a challenge in modern spectroscopy. In this work, we investigate the effect of a twofold screw axis on the solid-state VCD spectrum in a combined experimental and theoretical analysis of P2 crystals of (S)-(+)-1-indanol. Even though the space group is achiral, a single proper symmetry operation has an important impact on the VCD spectrum, which reflects the supramolecular chirality of the crystal.

How change in chirality prevents β-amyloid type interaction in a protonated cyclic dipeptide dimer.

The protonated dimers of the diketopiperazine dipeptide cyclo (LPhe-LHis) and cyclo (LPhe-DHis) are studied by laser spectroscopy combined with mass spectrometry to shed light on the influence of stereochemistry on the clustering propensity of cyclic dipeptides. The marked spectroscopic differences experimentally observed in the hydride stretch region are well accounted for by the results of DFT calculations. Both diastereomeric protonated dimers involve a strong ionic hydrogen bond from the protonated imidazole ring of one monomer to the neutral imidazole nitrogen of the other.

Competition between inter and intramolecular hydrogen bond evidenced by vibrational circular dichroism spectroscopy: The case of (1S,2R)-(-)-cis-1-amino-2-indanol.

The infrared (IR) absorption and vibrational circular dichroism (VCD) spectra of an intramolecularly hydrogen-bonded chiral amino-alcohol, (1S,2R)-(-)-cis-1-amino-2-indanol, are studied in DMSO-d . The spectra are simulated at the density functional theory (DFT) level within the frame of the cluster-in-the-liquid model. Both IR and VCD spectra show a clear signature of the formation of intermolecular hydrogen bonds at the detriment of the intramolecular OH … N interaction present in the isolated molecule.

Conformation control through concurrent N-H⋯S and N-H⋯O[double bond, length as m-dash]C hydrogen bonding and hyperconjugation effects.

In addition to the classical N-H⋯O[double bond, length as m-dash]C non-covalent interaction, less conventional types of hydrogen bonding, such as N-H⋯S, may play a key role in determining the molecular structure. In this work, using theoretical calculations in combination with spectroscopic analysis in both gas phase and solution phase, we demonstrate that both these H-bonding modes exist simultaneously in low-energy conformers of capped derivatives of Attc, a thietane α-amino acid. 6-Membered ring inter-residue N-H⋯S interactions (C6), assisted by hyperconjugation between the thietane ring and the backbone, combine with 5-membered ring intra-residue backbone N-H⋯O[double bond, length as m-dash]C interactions (C5) to provide a C5-C6 feature that stabilizes a planar geometry in the monomer unit.

A theoretical and experimental case study of the hydrogen bonding predilection of S-methylcysteine.

S-containing amino acids can lead to two types of local NH···S interactions which bridge backbone NH sites to the side chain to form either intra- or inter-residue H-bonds. The present work reports on the conformational preferences of S-methyl-L-cysteine, Cys(Me), using a variety of investigating tools, ranging from quantum chemistry simulations, gas-phase UV and IR laser spectroscopy, and solution state IR and NMR spectroscopies, on model compounds comprising one or two Cys(Me) residues. We demonstrate that in gas phase and in low polarity solution, the C- and N-capped model compound for one Cys(Me) residue adopts a preferred C5-C6γ conformation which combines an intra-residue N-H···O=C backbone interaction (C5) and an inter-residue N-H···S interaction implicating the side-chain sulfur atom (C6γ).

Assessing cluster models of solvation for the description of vibrational circular dichroism spectra: synergy between static and dynamic approaches.

Solvation effects are essential for defining the shape of vibrational circular dichroism (VCD) spectra. Several approaches have been proposed to include them into computational models for calculating VCD signals, in particular those resting on the "cluster-in-a-liquid" model. Here we examine the capabilities of this ansatz on the example of flexible (1S,2S)-trans-1-amino-2-indanol solvated in dimethyl sulfoxide (DMSO).

Solid-state synthesis of cyclo LD-diphenylalanine: A chiral phase built from achiral subunits.

The solid-state structure of LL/DD or LD/DL diphenylalanine diluted in KBr pellets is studied by infrared (IR) absorption and vibrational circular dichroism (VCD) spectroscopy. The structure depends on the absolute configuration of the residues. The natural LL diphenylalanine exists as a mixture of neutral and zwitterionic structures, depending on the humidity of the sample, while mostly the zwitterion is observed for LD diphenylalanine whatever the experimental conditions.

Effect of puckering motion and hydrogen bond formation on the vibrational circular dichroism spectrum of a flexible molecule: the case of (S)-1-indanol.

The influence of flexibility and hydrogen bond formation on the IR absorption and vibrational circular dichroism (VCD) spectrum of a floppy protic molecule, namely, (S)-1-indanol, is studied in both non-polar CCl4 and polar DMSO solvents. The experimental IR absorption and VCD spectra obtained by Fourier transform spectroscopy are interpreted using both static density functional theory (DFT) calculations and first principles molecular dynamics (FPMD) within DFT, using the nuclear velocity perturbation theory (NVPT). Simulation of the spectra based on static optimised geometries is not sufficient in CCl4 and going beyond static calculations is mandatory for satisfactorily reproducing the VCD spectra.

Photofragmentation mechanisms in protonated chiral cinchona alkaloids.

The photo-stability of protonated cinchona alkaloids is studied in the gas phase by a multi-technique approach. A multi-coincidence technique is used to demonstrate that the dissociation is a direct process. Two dissociation channels are observed.

Chirality-dependent structuration of protonated or sodiated polyphenylalanines: IRMPD and ion mobility studies.

Ion mobility experiments are combined with Infra-Red Multiple Photon Dissociation (IRMPD) spectroscopy and quantum chemical calculations for assessing the role of chirality in the structure of protonated and sodiated di- or tetra-peptides. Sodiated systems show a strong chirality dependence of the competition between Na(+)O and Na(+)π interactions. Chirality effects are more subtle in protonated systems and manifest themselves by differences in the secondary interactions such hydrogen bonds between neutral groups or those involving the aromatic rings.

Exotic Protonated Species Produced by UV-Induced Photofragmentation of a Protonated Dimer: Metastable Protonated Cinchonidine.

A metastable protonated cinchona alkaloid was produced in the gas phase by UV-induced photodissociation (UVPD) of its protonated dimer in a Paul ion trap. The infrared multiple photon dissociation (IRMPD) spectrum of the molecular ion formed by UVPD was obtained and compared to DFT calculations to characterize its structure. The protonation site obtained thereby is not accessible by classical protonation ways.

How do pseudoenantiomers structurally differ in the gas phase? An IR/UV spectroscopy study of jet-cooled hydroquinine and hydroquinidine.

The gas-phase structures of the cinchona alkaloids, hydroquinine and its pseudoenantiomer hydroquinidine, are studied in a supersonic expansion by means of laser-induced fluorescence and IR/UV double-resonance spectroscopy. Vibrational spectroscopy combined with density functional calculations show that the conformational properties of the two pseudoenantiomers are identical. In both cases, they exist in two isoenergetic forms, with similar IR spectra.

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.

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.

Structural rearrangement in the formation of jet-cooled complexes of chiral (S)-1,2,3,4-tetrahydro-3-isoquinolinemethanol with methyl lactate: chirality effect in conformer selection.

The jet-cooled complexes between the two enantiomers of methyl lactate (ML) and (S)-1,2,3,4-tetrahydro-3-isoquinolinemethanol (THIQM) are studied by double resonance spectroscopy combined with ab initio calculations. Both diastereomer complexes exist in different isomers, involving either direct addition of THIQM on ML with no structural rearrangement of the subunits or formation of very stable structures involving multiple intermolecular hydrogen bonds and extensive deformation of the subunits. Competition between these two processes and its dependence upon chirality are discussed.

Conformational analysis of quinine and its pseudo enantiomer quinidine: a combined jet-cooled spectroscopy and vibrational circular dichroism study.

Laser-desorbed quinine and quinidine have been studied in the gas phase by combining supersonic expansion with laser spectroscopy, namely, laser-induced fluorescence (LIF), resonance-enhanced multiphoton ionization (REMPI), and IR-UV double resonance experiments. Density funtional theory (DFT) calculations have been done in conjunction with the experimental work. The first electronic transition of quinine and quinidine is of π-π* nature, and the studied molecules weakly fluoresce in the gas phase, in contrast to what was observed in solution (Qin, W.

The role of weak hydrogen bonds in chiral recognition.

Chiral recognition has been studied in neutral or ionic weakly bound complexes isolated in the gas phase by combining laser spectroscopy and quantum chemical calculations. Neutral complexes of the two enantiomers of lactic ester derivatives with chiral chromophores have been formed in a supersonic expansion. Their structure has been elucidated by means of IR-UV double resonance spectroscopy in the 3 μm region.

Jet-cooled hydrates of chiral (S) 1,2,3,4-tetrahydro-3-isoquinoline methanol (THIQM): structure and mechanism of formation.

The mechanism of formation of hydrates of chiral (S) 1,2,3,4-tetrahydro-3-isoquinoline (THIQM) with two water molecules has been investigated in jet-cooled condition by means of resonance-enhanced two-photon ionization and IR-UV double resonance experiments. Quantum chemical calculations reveal that only one isomer of the THIQM is involved in the THIQM-(H(2)O)(2) complex formation, in contrast with what was observed for THIQM-(H(2)O). Anharmonic vibration calculations allowed unambiguous assignment of THIQM-(H(2)O)(2) to a complex resulting from the addition of a water molecule on the most stable THIQM-(H(2)O) complex.

Role of conformational isomerism in solvent-mediated charge transfer in chiral (S) 1,2,3,4-tetrahydro-3-isoquinoline methanol (THIQM): condensed-phase to jet-cooled spectroscopic studies.

Intramolecular charge-transfer reaction in chiral (S) 1,2,3,4-tetrahydro-3-isoquinoline methanol (THIQM) has been investigated in the condensed phase and in jet-cooled conditions by means of laser-induced fluorescence, dispersed emission, resonance-enhanced two-photon ionization, and IR-UV double resonance experiments, as well as quantum chemical calculations. In the condensed phase, THIQM only shows local emission in nonpolar and protic solvents and dual emission in aprotic polar solvents, where the solvent-polarity dependent Stokes shifted emission is ascribed to a state involving charge transfer from the nitrogen lone pair to the benzene π-cloud. Ab initio calculations reveal two low-energy conformers, which are observed in jet-cooled conditions.

Chiral recognition in cinchona alkaloid protonated dimers: mass spectrometry and UV photodissociation studies.

Chiral recognition in protonated cinchona alkaloid dimers has been studied in mass spectrometry experiments. The experimental setups involved a modified 7T FT-ICR (Fourier transform-ion cyclotron resonance) mass spectrometer (MS) and a modified Paul ion trap both equipped with an electrospray ionization source (ESI). The Paul ion trap has been coupled to a frequency-doubled dye laser.

Chiral recognition in jet-cooled complexes of (1R,2S)-(+)-cis-1-amino-2-indanol and methyl lactate: on the importance of the CH...pi interaction.

Complexation between (1R,2S)-(+)-cis-1-amino-2-indanol (AI) and the two enantiomers of methyl lactate has been studied by means of laser-induced fluorescence, resonance-enhanced two-photon ionisation, and IR-UV double resonance spectroscopy, in the region of 3 microm. Two isomeric complexes have been spectroscopically characterised for each diastereoisomer. Comparison with ab initio calculations shows that the most stable form is an insertion structure, common to the two diastereoisomers, in which the OH group of methyl lactate inserts into the intramolecular bond of AI.

Chirality-dependent hydrogen bond direction in jet-cooled (S)-1,2,3,4-tetrahydro-3-isoquinoline methanol (THIQM): IR-ion dip vibrational spectroscopy of the neutral and the ion.

The structural modifications of (S)-1,2,3,4-tetrahydro-3-isoquinoline methanol (THIQM) upon ionisation have been investigated in jet-cooled conditions, by means of laser-induced fluorescence, REMPI, and IR-UV ion-dip spectroscopy of the neutral ground state and the ion. These results combined with ab initio calculations, support the presence in the jet of two low-energy conformers of THIQM. In the most stable Conformer I, the CH(2)OH substituent acts as a hydrogen bond donor to the nitrogen lone pair in the equatorial position.

Chiral recognition between alpha-hydroxylesters: a double-resonance IR/UV study of the complexes of methyl mandelate with methyl glycolate and methyl lactate.

Chiral recognition between alpha hydroxylesters has been studied in jet-cooled complexes of methyl mandelate with methyl lactate. The complex with nonchiral methyl glycolate has also been studied for the sake of comparison. The hydrogen-bond topology of the complexes has been interrogated by means of IR/UV double-resonance spectroscopy in the range of 3 mum.