2024 |
Dupont, J., Hartwig, B., Le Barbu-Debus, K., Lepere, V., Guillot, R., Suhm, M. A., & Zehnacker, A. (2024). Homochiral vs. heterochiral preference in chiral self-recognition of cyclic diols. PCCP, 2622(1411), 1061011–1062111.
Résumé: The structure and clustering propensity of a chiral derivative of cis-1,2-cyclohexanediol, namely, 1-phenyl-cis-1,2-cyclohexanediol (cis-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 cis-1,2-cyclohexanediol (cis-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. The change in clustering propensity in cis-PCD relative to trans-1,2-cyclohexanediol (trans-CD), which shows heterochiral preference, is explained by the presence of the phenyl substituent rather than the effect of cis-trans isomerism. Indeed the transiently chiral cis-CD also forms preferentially heterodimers, whose structure is very close to that of the corresponding trans-CD dimer.
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2023 |
Imani, Z., Mundlapati, V. R., Brenner, V., Gloaguen, E., Le Barbu-Debus, K., Zehnacker, A., Robin, S., Aitken, D. J., & Mons, M. (2023). Non-covalent interactions reveal the protein chain δ conformation in a flexible single-residue model. ChemComm, 59, 1161–1164.
Résumé: The δ conformation is a local secondary structural feature in proteins that implicates a πamide N-H···N interaction between a backbone N atom and the NH of the following residue. Small molecule probes of this conformation have been limited so far to rigid proline-type models that may over-emphasize the significance of the interaction. We show here that, in thiacyclic amino acid derivatives with a sulphur atom in the γ-position, specific side-chain/backbone N-H···S interactions stabilize the δ conformation sufficiently to allow it to compete with classical C5 and C7 H-bonding conformers. With support from quantum chemistry, the δ-folded conformers have been characterized by IR spectroscopy in the gas phase. In solution, the IR absorption of the πamide N-H appears at 3450 cm-1, notably less red-shifted than in proline-type models, in a frequency range often considered as implicating a free NH motif and suggestive of very weak hydrogen bonding at best.
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Rouquet, E., Roy Chowdhury, M., Garcia, G. A., Nahon, L., Dupont, J., Lepere, V., Le Barbu-Debus, K., & Zehnacker, A. (2023). Induced photoelectron circular dichroism onto an achiral chromophore. Nat Commun, 1411(1), 629066.
Résumé: 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. In a phenol-methyloxirane complex, removing an electron from an achiral aromatic pi orbital localised on the phenol moiety results in an intense and opposite photoelectron circular dichroism (PECD) for the two enantiomeric complexes with (R) and (S) methyloxirane, evidencing the long-range effect (~5 A) of the scattering chiral potential. This induced chirality has important structural and analytical implications, discussed here in the context of growing interest in laser-based PECD, for in situ, real time enantiomer determination.
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2022 |
Jähnigen, S., Le Barbu-Debus, K., Guillot, R., Vuilleumier, R., & Zehnacker, A. (2022). How Crystal Symmetry Dictates Non-Local Vibrational Circular Dichroism in the Solid State. Angew. Chem. Int. Ed., 62(e20221559).
Résumé: Abstract 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 P21 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. Distinguishing between contributions originating from molecular chirality and from chiral crystal packing, we find that while IR absorption hardly depends on the symmetry of the space group, the situation is different for VCD, where completely new non-local patterns emerge. Understanding the two underlying mechanisms, namely gauge transport and direct coupling, will help to use VCD to distinguish polymorphic forms.
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Le Barbu-Debus, K., Pérez-Mellor, A., Lepère, V., & Zehnacker, A. (2022). How change in chirality prevents β-amyloid type interaction in a protonated cyclic dipeptide dimer. PCCP, 24, 19783–19791.
Résumé: 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. While this strong interaction is accompanied by a single NH⋯O hydrogen bond between the amide functions of the two moieties for the protonated dimer of cyclo (lPhe-dHis), that of cyclo (lPhe-lHis) involves two NH⋯O interactions, forming the motif of an antiparallel β sheet. Therefore, a change in chirality of the residue prevents the formation of the β sheet pattern observed in the amyloid type aggregation. These results emphasize the peculiar role of the histidine residue in peptide structure and interaction.
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2021 |
Le Barbu-Debus, K., & Zehnacker, A. (2021). Competition between inter and intramolecular hydrogen bond evidenced by vibrational circular dichroism spectroscopy: The case of (1S,2R)-(−)-cis-1-amino-2-indanol. Chirality, 33(12), 858–874.
Résumé: Abstract 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-d6. 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. Two solvent molecules are necessary to reproduce the experimental spectra. Whereas the first DMSO molecule captures the main spectral modifications due to hydrogen bond formation between the solute and the solvent, the second DMSO molecule is necessary for a good description of the Boltzmann contribution of the different complexes, based on their Gibbs free energy.
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Mundlapati, V. R., Imani, Z., Goldsztejn, G., Gloaguen, E., Brenner, V., Le Barbu-Debus, K., Zehnacker-Rentien, A., Baltaze, J. - P., Robin, S., Mons, M., & Aitken, D. J. (2021). A theoretical and experimental case study of the hydrogen bonding predilection of S-methylcysteine. Amino Acids, 53(4), 621–633.
Résumé: 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). In contrast, the dominant conformation of the C- and N-capped model compound featuring two consecutive Cys(Me) residues is a regular type I β-turn. This structure is incompatible with concomitant C6γ interactions, which are no longer in evidence. Instead, C5γ interactions occur, that are fully consistent with the turn geometry and additionally stabilize the structure. Comparison with the thietane amino acid Attc, which exhibits a rigid cyclic side chain, pinpoints the significance of side chain flexibility for the specific conformational behavior of Cys(Me).
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Perez-Mellor, A., Le Barbu-Debus, K., Lepere, V., Alata, I., Spezia, R., & Zehnacker, A. (2021). Structure and collision-induced dissociation of the protonated cyclo His-Phe dipeptide: mechanistic studies and stereochemical effects. EUROPEAN PHYSICAL JOURNAL D, 75(6), 1–7.
Résumé: The role of stereochemical factors on the structure and the fragmentation paths of the protonated cyclic dipeptide cyclo histidine-phenylalanine is studied under ion traps conditions by combining tandem mass spectrometry, laser spectroscopy, quantum chemical calculations and chemical dynamics simulations. Vibrational spectroscopy obtained by Infrared Multiple Photon Dissociation (IRMPD) reveals a small difference between the two diastereomers, c-LLH+ and c-LDH+, arising mainly from ancillary CH center dot center dot center dot pi interactions. In contrast, there is a strong influence of the residues chirality on the collision-induced dissociation (CID) processes. Chemical dynamics simulations rationalize this effect and evidence that proton mobility takes place, allowing isomerization to intermediate cyclic structures that are different for c-LLH+ and c-LDH+, resulting in different barriers to proton mobility. This effect is related to the protonation of the imidazole ring. It contrasts with the minute stereochemical effects observed for other cyclic dipeptides in which the proton is borne by an amide CO.
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2020 |
Imani, Z., Mundlapati, V. R., Goldsztejn, G., Brenner, V., Gloaguen, E., Guillot, R., Baltaze, J. P., Le Barbu-Debus, K., Robin, S., Zehnacker, A., Mons, M., & Aitken, D. J. (2020). Conformation control through concurrent N–H⋯S and N–H⋯O=C hydrogen bonding and hyperconjugation effects. CHEMICAL SCIENCE, 11(34), 9191–9197.
Résumé: In addition to the classical N-HMIDLINE HORIZONTAL ELLIPSISO-C non-covalent interaction, less conventional types of hydrogen bonding, such as N-HMIDLINE HORIZONTAL ELLIPSISS, 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 alpha-amino acid. 6-Membered ring inter-residue N-HMIDLINE HORIZONTAL ELLIPSISS interactions (C6(gamma)), assisted by hyperconjugation between the thietane ring and the backbone, combine with 5-membered ring intra-residue backbone N-HMIDLINE HORIZONTAL ELLIPSISO-C interactions (C5) to provide a C5-C6(gamma)feature that stabilizes a planar geometry in the monomer unit. Two contiguous C5-C6(gamma)features in the planar dimer implicate an unprecedented three-centre H-bond of the type C-OMIDLINE HORIZONTAL ELLIPSISH(N)MIDLINE HORIZONTAL ELLIPSISSR2, while the trimer adopts two C5-C6(gamma)features separated by a Ramachandran alpha-type backbone configuration. These low-energy conformers are fully characterized in the gas phase and support is presented for their existence in solution state.
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Le Barbu-Debus, K., Bowles, J., Jähnigen, S., Clavaguéra, C., Calvo, F., Vuilleumier, R., & Zehnacker, A. (2020). Assessing cluster models of solvation for the description of vibrational circular dichroism spectra: synergy between static and dynamic approaches. PCCP, 22, 26047–26068.
Résumé: 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). We compare cluster sets obtained from static calculations with results from explicit molecular dynamics (MD) trajectories based on either force field (FF) or first-principles (FP) methods. While the FFMD approach provides a broader sampling of configurational space, FPMD and time-correlation functions of dipole moments account for anharmonicity and entropy effects in the VCD calculation. They provide a means to evaluate the immediate effect of the solvent on the spectrum. This survey singles out several challenges associated with the use of clusters to describe solvation effects in systems showing shallow potential energy surfaces and non-covalent interactions. Static structures of clusters involving a limited number of solvent molecules satisfactorily capture the main effects of solvation in the bulk limit on the VCD spectra, if these structures are correctly weighted. The importance of taking into consideration their fluxionality, i.e. different solvent conformations sharing a same hydrogen bond pattern, and the limitations of small clusters for describing the solvent dynamics are discussed.
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Le Barbu-Debus, K., Scuderi, D., Lepère, V., & Zehnacker, A. (2020). Homochiral vs. heterochiral sodium core dimers of tartaric acid esters: A mass spectrometry and vibrational spectroscopy study. Journal of Molecular Structure, 1205, 127583.
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Pérez-Mellor, A., Le Barbu-Debus, K., & Zehnacker, A. (2020). Solid-state synthesis of cyclo LD-diphenylalanine: A chiral phase built from achiral subunits. Chirality, 32(5), 693–703.
Résumé: Abstract 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. The system undergoes spontaneous cyclization upon heating at 125°C, resulting to the formation of a diketopiperazine (DKP) dipeptide as the only product. The reaction is faster for LD than for LL diphenylalanine. As expected, LL and DD diphenylalanine react to form the LL and DD enantiomers of cyclo diphenylalanine. Interestingly, the DKP dipeptides formed from the LD or DL diphenylalanine show unexpected optical activity, with opposite VCD spectra for the products formed from the LD and DL reagents. This is explained in terms of chirality synchronization between the monomers within the crystal, which retain the symmetry of the reagent, resulting to the formation of a new chiral phase made from transiently chiral molecules.
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2018 |
Le Barbu-Debus, K., Scherrer, A., Bouchet, A., Sebastiani, D., Vuilleumier, R., & Zehnacker, A. (2018). Effect of puckering motion and hydrogen bond formation on the vibrational circular dichroism spectrum of a flexible molecule: the case of (S)-1-indanol. Phys Chem Chem Phys, 20(21), 14635–14646.
Résumé: 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. The FPMD results obtained in DMSO indicate that (S)-1-indanol is hydrogen-bonded to one DMSO molecule. As a result, static “cluster-in-the-bulk” DFT calculations in which the solute-solvent interaction is modeled as the most stable (S)-1-indanol:DMSO complexes in a DMSO continuum yield satisfactory agreement with the experiment. Correspondence between experimental and simulated spectra is slightly improved when the VCD spectrum is calculated as the summed contributions of snapshots extracted from FPMD trajectories, due to better sampling of the potential-energy surface. Finally, NVPT calculations further improve the description of experimental spectra by taking into account higher-energy structures, which are not necessary local minima.
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2016 |
Kumar, S., Lucas, B., Fayeton, J., Scuderi, D., Alata, I., Broquier, M., Le Barbu-Debus, K., Lepere, V., & Zehnacker, A. (2016). Photofragmentation mechanisms in protonated chiral cinchona alkaloids. Phys. Chem. Chem. Phys., 18(32), 22668–22677.
Résumé: 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. They result from the C-8-C-9 cleavage, accompanied or not by hydrogen migration. The branching ratio between the two photo-fragments is different for the two pseudo-enantiomers quinine and quinidine. Mass spectrometry experiments coupling UV photo-dissociation of the reactants and structural characterization of the ionic photo-products by Infra-Red Multiple Photo-Dissociation (IRMPD) spectroscopy provide unambiguous information on their structure. In addition, quantum chemical calculations allow proposing a reactive scheme and discussing it in terms of the ground-state geometry of the reactant.
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Lepere, V., Le Barbu-Debus, K., Clavaguera, C., Scuderi, D., Piani, G., Simon, A. L., Chirot, F., MacAleese, L., Dugourd, P., & Zehnacker, A. (2016). Chirality-dependent structuration of protonated or sodiated polyphenylalanines: IRMPD and ion mobility studies. Phys. Chem. Chem. Phys., 18(3), 1807–1817.
Résumé: 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+...pi 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.
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2015 |
Alata, I., Scuderi, D., Lepere, V., Steinmetz, V., Gobert, F., Thiao-Layel, L., Le Barbu-Debus, K., & Zehnacker-Rentien, A. (2015). Exotic Protonated Species Produced by UV-Induced Photofragmentation of a Protonated Dimer: Metastable Protonated Cinchonidine. Journal Of Physical Chemistry A, 119(39), 10007–10015.
Résumé: 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. The protonated monomer directly formed in the ESI source or by collision-induced dissociation (CID) of the dimer undergoes protonation at the most basic alkaloid nitrogen. In contrast, protonation occurs at the quinoline aromatic ring nitrogen in the UVPD-formed monomer.
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2013 |
Mahjoub, A., Le Barbu-Debus, K., & Zehnacker, A. (2013). 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. JOURNAL OF PHYSICAL CHEMISTRY A, 117(14), 2952–60.
Résumé: 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. It is shown that the most stable form of the chromophore (THIQMI with an OH···N hydrogen bond) prefers to directly stick to ML to form the addition complex whereas the second conformer (THIQMII with NH···O hydrogen bond) rearranges to form a strongly bound structure. The two structures are formed for the homochiral as well the heterochiral complex, however with different relative abundance. This shows an enantioselective binding preference of ML for one of the conformers of the chromophore.
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Sen, A., Le Barbu-Debus, K., Scuderi, D., & Zehnacker-Rentien, A. (2013). 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. Chirality, 25, 436–443.
Résumé: 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. Chirality 00:000-000, 2013. © 2013 Wiley Periodicals, Inc.
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Sen, A., Lepere, V., Le Barbu-Debus, K., & Zehnacker, A. (2013). How do Pseudoenantiomers Structurally Differ in the Gas Phase? An IR/UV Spectroscopy Study of Jet-Cooled Hydroquinine and Hydroquinidine. CHEMPHYSCHEM, 14(15), 3559–3568.
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2012 |
Sen, A., Bouchet, A., Lepere, V., Le Barbu-Debus, K., Scuderi, D., Piuzzi, F., & Zehnacker-Rentien, A. (2012). Conformational Analysis of Quinine and Its Pseudo Enantiomer Quinidine: A Combined Jet-Cooled Spectroscopy and Vibrational Circular Dichroism Study. JOURNAL OF PHYSICAL CHEMISTRY A, 116(32), 8334–8344.
Résumé: 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 functional theory (DFT) calculations have been done in conjunction with the experimental work. The first electronic transition of quinine and quinidine is of pi-pi* nature, and the studied molecules weakly fluoresce in the gas phase, in contrast to what was observed in solution (Qin, W. W.; et al. J. Phys. Chem. C 2009, 113, 11790). The two pseudo enantiomers quinine and quinidine show limited differences in the gas phase; their main conformation is of open type as it is in solution. However, vibrational circular dichroism (VCD) experiments in solution show that additional conformers exist in condensed phase for quinidine, which are not observed for quinine. This difference in behavior between the two pseudo enantiomers is discussed.
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2011 |
Chakraborty, A., Guchhait, N., Le Barbu-Debus, K., Mahjoub, A., Lepere, V., & Zehnacker-Rentien, A. (2011). 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. JOURNAL OF PHYSICAL CHEMISTRY A, 115(34), 9354–9364.
Résumé: Intramolecular charge-transfer reaction in chiral (S) 1,2,3,4-tetrahydro3-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 pi-cloud. Ab initio calculations reveal two low-energy conformers, which are observed in jet-cooled conditions. In the most stable conformer, THIQM(1), the CH(2)OH substituent acts as a hydrogen bond donor to the nitrogen lone pair in the equatorial position, while the second most stable conformer, THIQM(II), corresponds to the opposite NH center dot center dot center dot O hydrogen bond, with the nitrogen lone pair in the axial position. The two low-energy jet-cooled conformers of THIQM evidenced from the laser-induced fluorescence and dispersed emission spectra only show structured local emission. Complexes with usual solvents reproduce the condensed phase properties. The jet-cooled complex with aprotic polar, solvent acetonitrile shows both local emission and charge transfer emission as observed in solution. The jet-cooled hydrate mainly shows local emission due to the unavailability of the nitrogen lone pair through intermolecular hydrogen bonding.
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Le Barbu-Debus, K., Sen, A., Broquier, M., & Zehnacker, A. (2011). Jet-cooled hydrates of Chiral (S) 1,2,3,4-tetrahydro-3-isoquinoline methanol (THIQM): structure and mechanism of formation. PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 13(31), 13985–13991.
Résumé: 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. A sequential mechanism for complex formation has been deduced from these results.
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Scuderi, D., Le Barbu-Debus, K., & Zehnacker, A. (2011). The role of weak hydrogen bonds in chiral recognition. PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 13(40), 17916–17929.
Résumé: 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 mm region. In both systems described here, the main interaction ensuring the cohesion of the complex is a strong hydrogen bond between the chromophore and methyl-lactate. However, an additional hydrogen bond of much weaker strength plays a discriminative role between the two enantiomers. For example, the 1 : 1 heterochiral complex between R-(+)-2-naphthyl-ethanol and S-(+) methyl-lactate is observed, in contrast with the 1 : 1 homochiral complex which lacks this additional hydrogen bond. On the other hand, the same kind of insertion structures is formed for the complex between S-( +/-)-cis-1-amino-indan-2-ol and the two enantiomers of methyl-lactate, but an additional addition complex is formed for R-methyl-lactate only. This selectivity rests on the formation of a weak CH center dot center dot center dot pi interaction which is not possible for the other enantiomer. The protonated dimers of Cinchona alkaloids, namely quinine, quinidine, cinchonine and cinchonidine, have been isolated in an ion trap and studied by IRMPD spectroscopy in the region of the nu(OH) and nu( NH) stretch modes. The protonation site is located on the alkaloid nitrogen which acts as a strong hydrogen bond donor in all the dimers studied. While the nature of the intermolecular hydrogen bond is similar in the homochiral and heterochiral complexes, the heterochiral complex displays an additional weak CH center dot center dot center dot O hydrogen bond located on its neutral part, which results in slightly different spectroscopic fingerprints in the n( OH) stretch region. This first spectroscopic evidence of chiral recognition in protonated dimers opens the way to the study of the complexes of Cinchona alkaloids involved in enantioselective catalysis. These examples show how secondary hydrogen bonds controlled by stereochemical factors govern molecular recognition processes.
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2010 |
Albrecht, M., Borba, A., Le Barbu-Debus, K., Dittrich, B., Fausto, R., Grimme, S., Mahjoub, A., Nedic, M., Schmitt, U., Schrader, L., Suhm, M. A., Zehnacker-Rentien, A., & Zischang, J. (2010). Chirality influence on the aggregation of methyl mandelate. NEW JOURNAL OF CHEMISTRY, 34(7), 1266–1285.
Résumé: The methyl ester of mandelic acid is investigated by a wide range of techniques to unravel its aggregation pattern and the influence of relative chirality of the aggregating monomers. Matrix isolation confirms that a single monomer conformation prevails. The electronic spectrum of the dimers is strongly affected by the relative monomer chirality. Vibrational effects are more subtle and can be explained in terms of the most stable homo- and heteroconfigurational dimer structures, when compared to results of MP2 and DFT-D computations. Selective IR/UV double resonance techniques and wide-band FTIR spectroscopy provide largely consistent spectroscopic fingerprints of the chirality discrimination phenomena. The dominant homochiral dimer has two intermolecular O-H center dot center dot center dot O=C hydrogen bonds whereas the more strongly bound heterochiral dimer involves only one such hydrogen bond. This is a consequence of the competition between dispersion and intramolecular or intermolecular hydrogen bonding. Aromatic interactions also play a role in trimers and larger clusters, favoring homochiral ring arrangements. Analogies and differences to the well-investigated methyl lactate system are highlighted. Bulk phases show a competition between different hydrogen bond patterns. The enantiopure, racemic, and 3 : 1 crystals involve infinite hydrogen-bonded chains with different arrangements of the aromatic groups. They exhibit significantly different volatility, the enantiopure compound being more volatile than the racemic crystal. The accumulated experimental and quantum-chemical evidence turns methyl mandelate into a model system for the role of competition between dispersion forces and hydrogen bond interactions in chirality discrimination.
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Chakraborty, A., Le Barbu-Debus, K., Zehnacker-Rentien, A., & Guchhait, N. (2010). Laser-induced fluorescence and dispersed fluorescence studies of the donor-acceptor system 4-amino 3-methyl benzoic acid methyl ester and its solvated clusters: Evidence of excited-state charge-transfer reaction. JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY A-CHEMISTRY, 213(2-3), 164–170.
Résumé: Laser-induced fluorescence (LIF) excitation and dispersed fluorescence (OF) spectra of 4-amino 3-methyl benzoic acid methyl ester (AMBME) and its solvated clusters with solvents such as methanol, water and acetonitrile have been investigated in a supersonic expansion. Spectral signature supports the presence of two conformers in the cooled jet which is in line with theoretical calculations. In addition to structured local emission from the Franck Condon excited state, the molecule AMBME shows red-shifted broad emission from the state attributed to the close-lying charge transfer (CT) state, which is facilitated by exciting low-frequency modes. The molecule readily forms clusters with different solvents and the clusters' electronic excitation bands appear in the low-energy side of the transition origin of the bare molecule. Excitation of the clusters leads to the appearance of red-shifted solvent-polarity dependent CT emission. (C) 2010 Elsevier B.V. All rights reserved.
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K. Le Barbu-Debus. (2010). The role of deformation energy of bi-functional entities on the diastereoisomers formation. In Anne Zehnacker (Ed.), Chiral Recognition in the Gas Phase (pp. 47–59).
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Scuderi, D., Maitre, P., Rondino, F., Le Barbu-Debus, K., Lepere, V., & Zehnacker-Rentien, A. (2010). Chiral Recognition in Cinchona Alkaloid Protonated Dimers: Mass Spectrometry and UV Photodissociation Studies. JOURNAL OF PHYSICAL CHEMISTRY A, 114(9), 3306–3312.
Résumé: 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. The fragmentation of protonated dimers made from cinchonidine (Cd) and the two pseudoenantiomers of quinine, namely, quinine (Qn) and quinidine (Qd), has been assessed by means of collision-induced dissociation (CID) as well as UV photodissociation (UVPD). Whereas CID fragmentation of the dimers only leads to the evaporation of the monomers, UVPD results in the additional loss of a neutral radical fragment corresponding to the quinuclidinyl radical. The effect of the excitation wavelength and of complexation with H(2)SO(4) has been studied to cast light on the reaction mechanism. Complexation with H(2)SO(4) modifies the photoreactivity of the dimers; only evaporation of the monomeric fragments, quinine, and cinchonidine is observed. Comparison between the mass spectra of the cinchona alkaloid (CdQnH(+)) or (CdQdH(+)) dimers resulting from the UVPD of (CdQnH(2)SO(4)H(+)) and that of bare (CdQnH(+)) helps propose a fragmentation mechanism, which is thought to involve fast proton transfer from the quinuclidine part of a molecular subunit to the quinoline ring. CID and UV fragmentation experiments show that the homochiral dimer is more strongly bound than the heterochiral adduct.
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2009 |
Le Barbu-Debus, K., Broquier, M., Mahjoub, A., & Zehnacker-Rentien, A. (2009). 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. PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 11(35), 7589–7598.
Résumé: 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 μm. 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. This structure shows almost no chiral discrimination. A secondary structure has been observed, which is specific to each enantiomer. It involves a main hydrogen bond from the OH group of methyl lactate to AI together with weaker hydrogen bonds, which depend on chirality. The enantioselectivity in the hydrogen bond topology is due to a weak stabilizing CH center dot center dot center dot pi interaction, involving the CH located on the asymmetric carbon of methyl lactate, which can be obtained for one of the enantiomers only.
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Mahjoub, A., Chakraborty, A., Lepere, V., Le Barbu-Debus, K., Guchhait, N., & Zehnacker, A. (2009). 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. PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 11(25), 5160–5169.
Résumé: 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. In this case, the nitrogen atom is in ( S) configuration. Conformer II shows the opposite NH center dot center dot center dot O hydrogen bond from the hydrogen atom in the equatorial position of nitrogen to the OH group. In this case, the nitrogen atom is in ( R) configuration. This chirality dependence of the hydrogen bond direction is lost upon ionisation. While ionisation of Conformer II reinforces the NH center dot center dot center dot O hydrogen bond, ionisation of Conformer I induces its isomerisation to the same ion as Conformer II, i.e. a change in hydrogen bond direction.
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2008 |
Le Barbu-Debus, K., Broquier, M., Mahjoub, A., & Zehnacker-Rentien, A. (2008). Chiral recognition between alpha-hydroxylesters: A double-resonance IR/UV study of the complexes of methyl mandelate with methyl glycolate and methyl lactate. JOURNAL OF PHYSICAL CHEMISTRY A, 112(40), 9731–9741.
Résumé: 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 μm. A theoretical approach has been conducted in conjunction with the experimental work to assist in the analysis of the spectra. Owing to the conformational flexibility of the subunits at play, emphasis has been put on the methodology used for the exploration of the potential-energy surface. The hydrogen-bond topology is very similar in the homo- and heterochiral complexes. It involves insertion of the hydroxyl group of methyl mandelate within the intramolecular hydrogen bond of methyl lactate or methyl glycolate, resulting in a five-membered ring. This contrasts with methyl lactate clusters previously studied by FTIR spectroscopy in a filet jet.(1).
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2007 |
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Le Barbu-Debus, K., Guchhait, N., & Zehnacker-Rentien, A. (2007). Electronic and infrared spectroscopy of jet-cooled (+/-)-cis-1-amino-indan-2-ol hydrates. PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 9(32), 4465–4471.
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Mahjoub, A., Le Barbu-Debus, K., & Zehnacker-Rentien, A. (2007). Double resonance IR/UV study of the complexes of methyl mandelate with ethyl glycolate and methyl lactate. In Fundamental and Applied Spectroscopy (Vol. 935, pp. 208–213).
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