Biomedical subjects

M Plazanet

Publications and source records attributed to M Plazanet.

4 recordsLinked to original sources

PubMed2006

Crystallization on heating and complex phase behavior of alpha-cyclodextrin solutions.

Solutions composed of alpha-cyclodextrin (alpha-CD), water, and various methylpyridines, in particular, 4-methylpyridine (4MP), undergo reversible liquid-solid transitions upon heating, the crystalline solid phases undergoing further phase transformations at higher temperatures. This unusual behavior has been characterized by an ensemble of measurements, including solubility, differential scanning calorimetry, quasielastic neutron scattering, as well as x-ray powder diffraction. For the alpha-CD/4MP system five crystalline phases have been identified. The unit cell parameters and corresponding changes with temperature indicate a scenario for the crystallization process. A simple model is proposed that mimics the observed disorder-order transition.

Journal Article↗

PubMed2005

Neutron vibrational spectroscopy gives new insights into the structure of poly(p-phenylene terephthalamide).

The vibrational spectra of benzanilide and poly(p-phenylene terephthalamide) have been measured using inelastic neutron scattering. These compounds have similar hydrogen-bond networks, which, for poly(p-phenylene terephthalamide), lead to two-dimensional hydrogen-bonded sheets in the crystal. Experimental spectra are compared with solid-state, quantum chemical calculations based on density functional theory (DFT). Such "parameter-free" calculations allow the structure-dynamics relation in this type of compound to be quantified, which is demonstrated here for benzanilide. In the case of poly(p-phenylene terephthalamide), vibrational spectroscopy and DFT calculations help resolve long-standing questions about the packing of hydrogen-bonded sheets in the solid state.

Journal Article↗

PubMed2004

Freezing on heating of liquid solutions.

We report a reversible liquid-solid transition upon heating of a simple solution composed of a-cyclodextrine (alpha CD), water, and 4-methylpyridine. These solutions are homogeneous and transparent at ambient temperature and solidify when heated to temperatures between 45 degrees and 75 degrees. Quasielastic and elastic neutron scattering show that molecular motions are slowed down in the solid and that crystalline order is established. The solution "freezes on heating." This process is fully reversible, on cooling the solid melts. A rearrangement of hydrogen bonds is postulated to be responsible for the observed phenomenon.

Crystallization↗

PubMed2004

Rotation-libration and rotor-rotor coupling in 4-methylpyridine.

The low temperature rotational dynamics of methyl groups in 4-methylpyridine is analyzed in terms of a model potential including rotation-libration and rotor-rotor coupling. The parameters of the model potential are adjusted by comparison of calculated with published and newly recorded inelastic neutron scattering spectra. Initial evaluations of the potential parameters of the model are obtained from molecular mechanics calculations. Experimental spectra are calculated from these potentials by numerical solution of Schrödinger's equation for clusters of coupled rotors embedded in a bigger ensemble of rotors treated in the mean field approximation. Adjustment of the potential parameters leads to excellent agreement with the experimental spectra of protonated 4-methylpyridine, measured at well-defined spin temperatures. At higher levels of deuteration, agreement with experiment is qualitative, only. The observed deviations are attributed to the increasing frustration of the system of coupled methyl groups and mutual localization, effects leading to a phase transition around 5.5 K in isotopic mixtures, as shown in diffraction experiments.

Journal Article↗