Peer-reviewed Publications |
Awali, S., Mestdagh, J. - M., Gaveau, M. - A., Briant, M., Soep, B., Mazet, V., & Poisson, L. (2021). Time-Resolved Observation of the Solvation Dynamics of a Rydberg Excited Molecule Deposited on an Argon Cluster. II. DABCO* at Long Time Delays. J Phys Chem A, 125(20), 4341–4351.
Résumé: The real-time dynamics of DABCO-argon clusters is investigated in a femtosecond pump-probe experiment where the pump excites DABCO to the S1 state within the argon cluster. The probe operates by photoionization and documents the energy and angular distributions of the resulting photoelectrons. The present work complements a previous work from our group [Awali Phys. Chem. Chem. Phys., 2014, 16, 516-526] where this dynamics was probed at short time, up to 4 ps after the pump pulse. Here, the dynamics is followed up to 500 ps. A multiscale dynamics is observed. It includes a jump between two solvation sites (time scale 0.27 ps) followed by the relaxation of the solvation cage excess vibrational energy (time scale 14 ps) and then by that of DABCO (time scale >150 ps). Polarization anisotropy, double polarization, and angular anisotropy effects are reported also. They are interpreted (quantitatively for the former effect) in terms of decoherence of rotational alignment, driven by the overall rotation of the DABCO-argon clusters. A tomographic view of the DABCO excited orbital, provided by the double anisotropy effect, is discussed on a qualitative basis.
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Gutiérrez-Quintanilla, A., Platakyte, R., Chevalier, M., Crépin, C., & Ceponkus J. (2021). Hidden Isomer of Trifluoroacetylacetone Revealed by Matrix Isolation Infrared and Raman Spectroscopy. J Phys Chem A, 125, 2249–2266.
Résumé: Enol forms of trifluoroacetylacetone (TFacac) isolated in molecular and rare gas matrices were studied using infrared (IR) and Raman spectroscopy. Additionally, calculations using DFT B3LYP and M06-2X as well as MP2 methods were performed in order to investigate the possibility of coexistence of more than one stable enol form isomer of TFacac. Calculations predict that both stable enol isomers of TFacac, 1,1,1-trifluoro-4-hydroxy-3-penten-2-one (1) and 5,5,5-trifluoro-4-hydroxy-3-penten-2-one (2), could coexist, especially in matrices where the room temperature population is frozen, 1 being the most stable one. Raman and IR spectra of TFacac isolated in nitrogen (N2) and carbon monoxide (CO) matrices exhibit clear absorption bands, which cannot be attributed to this single isomer. Their relative band positions and intensity profiles match well with the theoretical calculations of 2. This allows us to confirm that in N2 and CO matrices both isomers exist in similar amounts. Careful examination of the spectra of TFacac in argon, xenon, neon, normal, and para-hydrogen (Ar, Xe, Ne, nH2, and pH2 respectively) matrices revealed that both isomers coexist in all the explored matrices, whereas 2 was not considered in the previous spectroscopic works. The amount of the second isomer (2) in the as-deposited samples depends on the host. The analysis of TFacac spectra in the different hosts and under various experimental conditions allows the vibrational characterization of both chelated isomers. The comparison with theoretical predictions is also investigated.
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Jahnigen, S., Zehnacker, A., & Vuilleumier, R. (2021). Computation of Solid-State Vibrational Circular Dichroism in the Periodic Gauge. J. Phys. Chem, 12(30), 7213–7220.
Résumé: We introduce a new theoretical formalism to compute solid-state vibrational circular dichroism (VCD) spectra from molecular dynamics simulations. Having solved the origin-dependence problem of the periodic magnetic gauge, we present IR and VCD spectra of (1S,2S)-trans-1,2-cyclohexanediol obtained from first-principles molecular dynamics calculations and nuclear velocity perturbation theory, along with the experimental results. Because the structure model imposes periodic boundary conditions, the common origin of the rotational strength has hitherto been ill-defined and was approximated by means of averaging multiple origins. The new formalism reconnects the periodic model with the finite physical system and restores gauge freedom. It nevertheless fully accounts for nonlocal spatial couplings from the gauge transport term. We show that even for small simulation cells the rich nature of solid-state VCD spectra found in experiments can be reproduced to a very satisfactory level.
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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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Pérez-Mellor, A., Alata, I., Lepere, V., Spezia, R., & Zehnacker-Rentien, A. (2021). Stereospecific collision-induced dissociation and vibrational spectroscopy of protonated cyclo (Tyr-Pro). International Journal of Mass Spectrometry, 465, 116590.
Résumé: The protonated cyclo (LTyr-LPro) and cyclo (LTyr-DPro) dipeptides based on a diketopiperazine (DKP) ring are studied by tandem mass spectrometry in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Collision-induced dissociation (CID) and infrared multiple-photon dissociation (IRMPD) spectroscopy results are interpreted with the aid of quantum chemical calculations and chemical dynamics simulations. All the conformers identified for each diastereomer, denoted c-LLH+ and c-LDH+, respectively, are protonated on the carbonyl group of the tyrosine. The most stable form has an extended structure with the aromatic ring oriented outside the DKP ring; it is stabilized by an OH+…π interaction. Distinct IR signatures are obtained for the extended conformers of c-LLH+ and c-LDH+, which differ by the strength of the OH+…π interaction, much stronger in c-LLH+. Less stable species with the aromatic ring folded over the DKP ring are kinetically trapped in our experimental conditions, but their IR spectrum is identical for c-LLH+ and c-LDH+. The main collision-induced dissociation products of the protonated dipeptides are analyzed using chemical dynamics simulations. More efficient CID is observed for c-LDH+, in particular for the formation of the iminium ion of tyrosine. In contrast to the monomers, the protonated dimers of c-LLH+ and c-LDH+ show identical IR spectra. This is explained in terms of a structure involving a single strong OH+…O interaction between subunits not sensitive to the absolute configuration of the residues, i.e., from a folded protonated monomer to an extended neutral monomer.
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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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Pollet R, & Chin, W. (2021). Reversible Hydration of alpha-Dicarbonyl Compounds from Ab Initio Metadynamics Simulations: Comparison between Pyruvic and Glyoxylic Acids in Aqueous Solutions. J Phys Chem B, 125(11), 2942.
Résumé: Glyoxylic and pyruvic oxoacids are widely available in the atmosphere as gas-phase clusters and particles or in wet aerosols. In aqueous conditions, they undergo interconversion between the unhydrated oxo and gem-diol forms, where two hydroxyl groups replace the carbonyl group. We here examine the hydration equilibrium of glyoxylic and pyruvic acids with first-principles simulations in water at ambient conditions using ab initio metadynamics to reconstruct the corresponding free-energy landscapes. The main results are as follows: (i) our simulations reveal the high conformational diversity of these species in aqueous solutions. (ii) We show that gem-diol is strongly favored in water compared to its oxo counterpart by 29 and 16 kJ/mol for glyoxylic and pyruvic acids, respectively. (iii) From our atomic-scale simulations, we present new insights into the reaction mechanisms with a special focus on hydrogen-bond arrangements and the electronic structure of the transition state.
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