M.-H. Baron

Inelastic neutron-scattering study of the proton dynamics in N-methylacetamide at 20 K

Abstract Inelastic neutron-scattering (INS) spectra of four isotopic derivatives of N-methylacetamide (CH 3 CONHCH 3 , CD 3 CONHCH 3 , CH 3 CONHCD 3 and CD 3 CONHCD 3 ) at 20 K are presented from 30 to 4000 cm −1 . The band frequencies are compared with those observed in the infrared and Raman at low temperature. The quantitative simulation of the INS intensities, in the harmonic force field approximation, shows that the proton dynamics for the (N)H proton are totally different from those proposed previously. The valence-bond approach is not consistent with observation and the proton dynamics are independent of the molecular frame. A phenomenological approach is proposed in terms of localized modes. The calculated intensities reveal that the (N)H stretching mode is at ∼ 1575 cm −1 . This is a dramatic change compared to all former assignments at ∼ 3250 cm −1 based on the infrared and Raman data. These unforeseen proton dynamics are associated with the weakening of the NH bond due to the ionic character of the hydrogen bond (N δ− …H + …O δ′− ) and proton transfer. The infrared and Raman spectra are reconsidered and a new assignment scheme is proposed for the amide bands in terms of dynamicalproton exchange between the amidic (…OCNH…) and imidolic (…HOCN…) forms in infinite chains of hydrogen-bonded molecules.

Vibrational spectra and dynamics of conformation and hydrogen bonding of n-methylacetamide. I. Conformational dynamics of the CH3CONHCH3 molecule and NH out of plane band splitting

Abstract The infrared spectra of CH3CONHCH3 and CH3CONDCH3 have been investigated as low temperature crystals, pure liquids and solutions in various solvents in the 400-800 cm−1 range. A new assignment of the bands associated with the NH group is given. Multiplets of γ NH and γ ND fundamentals have generally been observed and have been interpreted in terms of a double minimum potential function of the γ NH mode with a tunnelling between two minima. The potential functions of γ NH and τ CN modes are similar and can be combined to give a potential surface with four minima corresponding to four molecular conformations. The influence of the hydrogen bonding on the γ NH splitting and barrier height is discussed.

Vibrational Spectroscopic Study of Glutathione Complexation in Aqueous Solutions

A spectroscopic study of glutathione (GSH) and glutathione disulfide (GSSG) has been performed using Fourier-transformed infrared absorption and Raman scattering in order to pinpoint the sites of complexation of these two species with water and particularly with H2O2. Molecules of GSH and GSSG were studied in KBr pellets, and in aqueous solutions of H2O, D2O, and H2O with H2O2 (1 mol L(-1)) to characterize the specific influence of the solvent molecules. A time-resolved Raman study was performed for GSH/H2O2, in aqueous solution at 1:1 molar ratio in order to observe the formation of GSSG and to discuss the mechanism of this redox reaction.

Spectroscopic study of N-acetylcysteine and N-acetylcystine/hydrogen peroxide complexation

Abstract A spectroscopic study of N-acetylcysteine (RSH) and N-acetylcystine (RSSR) has been performed using infrared absorption and Raman scattering in order to pinpoint the sites of complexation of these two species with H 2 O 2 . Molecules of RSH and RSSR were studied in KBr pellets, and in aqueous solutions of H 2 O, D 2 O and H 2 O with H 2 O 2 (1 mol l −1 ) to characterize the specific influence of the solvent molecules. A time-resolved Raman study was performed for RSH-H 2 O 2 in aqueous solution at 1:1 molar ratio in order to observe the formation of RSSR and to discuss the mechanism of this redox reaction.

Neutron Compton scattering study of proton-transfer dynamics in partially deuterated N-methylacetamide: CD3CONHCD3

Abstract Neutron Compton scattering measurements for the partially deuterated N-methylacetamide molecule (CD3CONHCD3) at 4.5 K give the powder averaged mean frequency of the photon modes: 1457 ± 38 cm−1. This value is lower than that anticipated from the assignment scheme based on infrared and Raman spectra ( ⋍1790 cm −1 ). Anharmonicity and vibrational coupling are such that that assignment scheme must be rejected. The mean frequency obtained with the assignment based on inelastic neutron scattering (INS) ( ⋍1230 cm −1 ) appears realistic. These conclusions are in accord with force-field calculations in the impulse approximation.

Proton transfer dynamics in polyglycine

Abstract Inelastic neutron-scattering spectra from 30 to 4000 cm−1 of partially deuterated polyglycine I and II of (—COCD2-NH—)n were obtained at ∼ 25 K using the TFXA spectrometer. The dynamics of the (N)H proton are similar in the two structures, but totally different from those proposed previously. These dynamics are associated with a weakening of the N H bond due to the ionic character of the hydrogen bond (Nδ−⋯+⋯Oδ−) and proton transfer.

Approach of the Anti-Bacterial Properties of Fluoroquinolones using SERS Spectroscopy

The activity of fluoroquinolones depends on two factors: their ability to enter bacteria and their specific inhibition on the DNA gyrase-catalysed supercoiling. Both factors depend on the presence of Magnesium.1,2,3 Actually we attempt to display if the use of SERS (Surface-Enhanced Raman Spectroscopy) could give valuable information on the DNA/DNA-Gyrase/Fluoroquinolone complexes with or without magnesium, at biological activity concentrations.

A New View of Proton Transfer Dynamics in Amides and Peptides: Vibrational Spectroscopy with Neutrons

Proton transfer is of considerable importance to much of chemistry and biology.1–7 The transfer frequently occurs along an existing hydrogen bond (AH…B). Then, it is thought to be governed by a double-minimum potential. Previous studies of proton transfer1–7 have been hampered by quantum effects (tunnelling) and strong interactions with the motions of surrounding atoms. The “vibration assisted tunnelling mechanism” has emerged from extensive studies (mostly of carboxylic acid dimers) with nuclear magnetic resonance (NMR)1–5 and quasi-elastic neutron-scattering (QENS),6,7 which probe time-scales considerably longer than that of proton dynamics. Consequently, the information they provide is averaged over many excursions of the proton between the two wells, and quantum effects are then not observed directly. Neutron scattering enables us to measure these quantum effects directly and reveals that the effects of such interactions have been overestimated in the past.8

Inelastic Neutron-Scattering Study of Proton Dynamics in Polyglycine II at 20 K

Inelastic neutron-scattering (INS) spectra of polyglycine II (-CO-CH2-NH-)n and the partially deuterated analogs, (-CO-CH2-ND-)n* and (-CO-CD2-NH-)n, at 20 K are presented (figs 1, 2 and 3). Band frequencies are compared to those observed in the infrared and Raman. Assignments in terms of group vibrations are proposed (table 1). These mostly resemble previous assignment schemes, 1 except for the Amide bands.