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International audience; The thermal behaviours of racemic sibutramine hydrochloride monohydrate as well as that of the anhydrous state were investigated by DSC. A new anhydrous solid was then put in evidence and its crystal structure was solved from XRPD data. This new phase was found to be the conglomerate. Depending on the heat treatment , the initial anhydrous racemic compound may be completely resolved. DSC analyses have shown that the anhydrous conglomerate melts at a higher temperature than the anhydrous racemic compound.

International audience; In this work, we studied the interaction of two oxidized lipids, PoxnoPC and PazePC, with POPC phospholipid. Mean molecular areas obtained from (π–A) isotherms of mixed PoxnoPC–POPC and PazePC–POPC monolayers revealed different behaviors of these two oxidized lipids: the presence of PoxnoPC in the monolayers induces their expansion, mean molecular areas being higher than those expected in the case of ideal mixtures. PazePC–POPC behave on the whole ideally. This difference can be explained by a different conformation of oxidized lipids. Moreover the carboxylic function of PazePC is protonated under our experimental conditions, as shown by (π–A) isotherms of PazePC at different pH values. Both oxidized lipids induce also an increase of the monolayer elasticity, PoxnoPC being slightly more efficient than PazePC. These monolayers were transferred from the air–water interface onto mica supports for a study by AFM. AFM images are on the whole homogenous, suggesting the presence of only one lipid phase in both cases. However, in the case of PazePC–POPC monolayers, AFM images show also the presence of areas thicker of 7 nm to 10 nm than the surrounding lipid phase, probably due to the local formation of multilayer systems induced by compression.

International audience; Cancer cell transformation is often accompanied by a modification of their viscoelastic properties. When capturing the stress-to-strain response of primary chronic myelogenous leukemia (CML) cells, from two data sets of CD34+ hematopoietic cells isolated from healthy and leukemic bone marrows, we show that the mean shear relaxation modulus increases upon cancer transformation. This stiffening of the cells comes along with local rupture events, detected as reinforced sharp local maxima of this modulus, suggesting that these cancer cells respond to a local mechanical stress by a cascade of local brittle failure events.

International audience; The crystal structure of p-synephrine was solved from a high-resolution X-ray powder diffraction pattern optimized by energy-minimization calculations using the Dreiding force field. The title compound crystallizes in a monoclinic system (space group P2 1 /c, Z = 4, with a = 8.8504(11) Å, b = 12.1166(15) Å, c = 9.7820(11) Å, ␤ = 122.551(2) • , V = 884.21(19) Å 3 and d = 1.256 g cm −3). Since p-synephrine degrades upon melting, its melting data were determined from DSC experiments carried out as a function of the heating rate. This method allowed determining a melting temperature and enthalpy equal to 199.8 ± 1.3 • C and 57 ± 3 kJ mol −1 , respectively.

International audience; Prednisolone is known to exist in two anhydrous solid polymorphic forms. The substance is known to degrade upon melting, resulting in erroneous melting data, as shown by the widely scattered results reported in the literature. In this article, thermal analyses carried out at different scan rates show that the onset temperature and the enthalpy value of the signal increase with the scan rate and reach plateau values for high scan rates. Owing to flash scanning calorimetry, the plateau value for the temperature has been identified as the “true” temperature of melting of both polymorphs. This consistent set of new thermodynamic data on the two solid forms leads to the conclusion that both forms are unambiguously enantiotropes of each other. The solid–solid transition has been observed experimentally for the first time and has been confirmed by calculation.

International audience; no abstract

International audience; Abiotic aromatic oligoamide foldamers are shown to self-assemble in solution to form a double helix, which can accommodate a bichromophoric thread in its central void. While in solution reversible electronic energy transfer is instilled between chromophoric termini of the free, flexible thread as evidenced through delayed luminescence, upon rigidification of the rod the chromophores are mutually distanced and effectively decoupled. Consequently, the chromophores display their individual photophysical characteristics. The observed conformation-dependent changes of dynamic luminescence properties, which are particularly sensitive to distance, offers a new strategy for lifetime-based detection of geometry on the molecular scale as demonstrated through real-time luminescence detection of molecular complexation leading to foldaxane formation.

International audience; no abstract

Complementary techniques had to be applied to investigate the binary system tetradecanedioic acid (C14H26O4)–hexadecanedioic acid (C16H30O4), because all the forms observed have the same space group (P21/c; Z = 2). We studied the polymorphism of the two single compounds and of their mixtures by X-ray powder diffraction, differential-scanning calorimetry (DSC), infrared spectroscopy (IR), scanning electron microscopy (SEM), and thermo-optical microscopy (TOM). The two diacids were found to be isopolymorphic. At low temperature, they crystallize in the same ordered C-form, and, on heating, adopt the ordered Ch-form, 1° below their melting point. In contrast to similar compounds (unbranched alkanes, alkanols, and fatty acids), the solid–solid and solid–liquid phase-transition temperatures decrease with increasing chain length. At low temperature, a new monoclinic form, Ci, appears as a result of the disorder of composition in the mixed samples. There are two [C + Ci]-type solid–solid domains. On heating, the solid domains are related to solid–liquid domains by a peritectic invariant for compositions rich in C14H26O4, and by a eutectic invariant for compositions rich in C16H30O4. At higher temperature, there appears a second peritectic invariant for compositions rich in C14H26O4, together with a metatectic invariant for compositions rich in C16H30O4. All the solid forms observed in this binary system are isostructural. Nevertheless, the equilibrium between them is complex near the melting point, and their miscibility in the solid state is reduced.

Polyethylene, cellulose and their blends with different compositions (5–30% cellulose content) were exposed to UV radiation or composted in soil for bio-degradation. Both types of degradation were carried out under laboratory conditions (room temperature, air atmosphere). The course of degradation processes has been studied using ATR-FTIR and Raman spectroscopies. There are practically no changes in spectra of photo- or bio-degraded pure PE films and cellulose powders. In contrast, the PE/cellulose blends are less stable than pure components and the processes of photodegradation seem to depend on blend composition and irradiation time. Prolonged UV exposure (100 h) leads to efficient photo-oxidation of blends studied. Biodegradation effect was also pronounced in PE films containing 30% cellulose.

In this study, the measurement of the true vibrational circular dichroism (VCD) spectrum is considered from an experimental and theoretical approach for any general anisotropic thin solid sample exhibiting linear as well as circular birefringence (LB, CB) and dichroism (LD, CD) properties. For this purpose, we have made use of a simple model alpha-helix polypeptide, namely, the poly(gamma-benzyl-L-glutamate) or PBLG, reference sample possessing a well-known VCD spectrum and giving rise to slightly oriented films by deposition onto a solid substrate. Also, we have used a different Fourier transform infrared modulation of polarization (PM-FTIR) optical setup with two-channel electronic processing in order to record the PM-VLD and PM-VCD spectra for various sample orientations in its film plane. All the corresponding general relations of the expected intensities in these experiments and the related properly designed calibration measurements were established using the Stokes-Mueller formalism; in addition, the residual birefringence of the optical setup and the transmittance anisotropy of the detector were estimated. From a comparative study of the results obtained in solution and in the solid state, we then propose a simple new experimental procedure to extract the true VCD spectrum of an oriented PBLG thin film: its consists of calculating the half-sum of two spectra recorded at theta and at theta +/- 90 degrees sample orientations. Moreover, the complete linear and circular birefringence and dichroism properties of the ordered PBLG thin film are estimated in the amide I and amide II vibrational regions. This allows us to establish for any sample orientation various theoretical simulations of the VCD spectra that agree nicely with the observed experimental results; this confirms that the measurement of LD and LB is in this case a prerequisite in simulating the true VCD spectrum of a partly oriented anisotropic sample. This validates our combined experimental and theoretical approach and opens the route to promising future vibrational CD studies on other macroscopic anisotropic thin film samples.

Recent experiments on tunneling systems in insulating glasses showed an unexpected magnetic-field dependence of spontaneous polarization echoes. Besides a strong overall increase with the magnetic field, the echo amplitude slowly oscillates with a frequency that is proportional to B. We develop a theory that couples the tunneling motion to nuclear quadrupoles and obtain the beat frequency and dephasing in terms of the nuclear magnetic and quadrupolar moments, the waiting time, and the magnetic field. Our results compare favorably with measurements on various amorphous solids.

The dynamical behaviour of an entangled network, a melt made of long polymer chains, with an oscillating nanotip, is investigated to provide evidence for a close relationship between the spatial scale and dynamics of the network, from the molecular to the mesoscopic scale. The experimental results show how a dynamic force microscope is able to discriminate between slow and fast relaxation processes related to molecular motion at different scales. In particular, it is shown that within a very small variation of the tip indentation depth, a few angstroms, the relaxation times involved in the process of dissipation vary by more than three orders of magnitude.

We consider a ferromagnetic film, with perpendicular magnetization, only coupled by the magnetic field, to a superconducting film. Our results show how the superconducting film thickness influences the equilibrium domain structure of the ferromagnetic film.

Monomolecular films of valine gramicidin A (VGA) were investigated in situ at the air-water interface by x-ray reflectivity and x-ray grazing incidence diffraction as well as polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). These techniques were combined to obtain information on the secondary structure and the orientation of VGA and to characterize the shoulder observed in its π-A isotherm. The thickness of the film was obtained by x-ray reflectivity, and the secondary structure of VGA was monitored using the frequency position of the amide I band. The PM-IRRAS spectra were compared with the simulated ones to identify the conformation adopted by VGA in monolayer. At large molecular area, VGA shows a disordered secondary structure, whereas at smaller molecular areas, VGA adopts an anti-parallel double-strand intertwined β5.6 helical conformation with 30° orientation with respect to the normal with a thickness of 25Å. The interface between bulk water and the VGA monolayer was investigated by x-ray reflectivity as well as by comparing the experimental and the simulated PM-IRRAS spectra on D2O and H2O, which suggested the presence of oriented water molecules between the bulk and the monolayer.

We determine the phase diagram of the ferromagnetic (F)/superconducting (S) atomic superlattice with arbitrary coupling between the layers. The exchange field acting on F layers may induce different kinds of modulated superconducting phases with either in-plane modulation or a phase shift between adjacent S layers. When the exchange field is increased, the re-entrance of the superconductivity emerges and the "π phase" where the superconducting order parameter changes its sign in successive S layers is realized.

The recent extension of time-resolved femtosecond optical techniques to the investigation of the ultrafast electron scattering processes in metal clusters offers the unique possibility to follow their evolution from a bulk to a confined metal. The size dependent results obtained in model materials, the noble metals, are presented, focusing on the impact of the confinement on energy redistribution processes (electron–electron and electron–phonon coupling). Their application to the investigation of the acoustic vibration property of cluster and to the cluster-surrounding matrix energy transfers are also discussed. To cite this article: N. Del Fatti, F. Vallee, C. R. Physique 3 (2002) 365–380.

This work contains a short overview on two phenomenological models aiming to evaluate the loss of energy when a nanotip oscillates at the proximity of a surface. In one case, the surface is described as a continuous medium with a local stiffness ks and a local damping term γs. The viscoelastic properties allows an easy evaluation of the energy loss as a function of the surface relaxation times τs=γs/ks. The main purpose of the second approach is to evaluate the effect of large fluctuations on the dynamic properties of the oscillator. The surface is built of atoms, each of them lying in a symmetric double-well potential and the oscillating nanotip produces a stochastic resonance process. The linear response theory is applied to evaluate fluctuations of the particle corresponding to interwell motions. Then from the amplitude, fluctuation is calculated the loss of energy. The result shows that a stochastic resonance process can increase the amount of dissipate energy of two orders of magnitude at a forcing frequency much smaller than the usual ones corresponding to atom motions.

In conducting two-dimensional laminar and turbulence experiments, use of a vertical flowing soap film is often a good choice. However, one of the most frequently encountered and yet highly nontrivial problems is to measure the thickness of the film precisely. We propose a solution to this problem based on the strong absorbtion of infrared light by the water molecules in the film. At λ≃3 μm, a thin sheet of water is essentially opaque. The extinction length of 0.9 μm serves as a precise ruler for gauging the film thickness. Although only the time-averaged, single-point measurements are presented, the technique is general and can be used for multipoint measurements to investigate turbulent driven spatiotemporal fluctuations of the film thickness.

DsRed, a tetrameric fluorescent protein cloned from the Discosoma genus of coral, has shown promise as a longer-wavelength substitute for green fluorescent protein (GFP) mutants for in vivo protein labeling. Bulk and single-molecule studies of the recombinant protein revealed that the DsRed chromophore shows high stability against photobleaching as compared to GFP mutants. Stark modulation spectra confirm that the electronic structure of the DsRed chromophore is similar to that of GFP. However, the tetrameric nature of DsRed leads to intersubunit energy transfer, as evidenced by the molecule's unusually low fluorescence anisotropy when immobilized (0.23 ± 0.02). This value is approximately consistent with an estimate of the energy transfer rates based on preliminary crystallographic information. The fluorescence emission bleaches at a rate linear in the applied excitation intensity, implying that the cessation of emission during pumping at 532 nm is light-driven and, consistent with the tetrameric structu...

We study the dynamic response of a [111] quantum impurity, such as lithium or cyanide in alkali halides, with respect to an external field coupling to the elastic quadrupole moment. Because of the particular level structure of a eight-state system on a cubic site, the elastic response function shows a biexponential relaxation feature and a van Vleck type contribution with a resonance frequency that is twice the tunnel frequency $\Delta/\hbar$. This basically differs from the dielectric response that does not show relaxation. Moreover, we show that the elastic response of a [111] impurity cannot be reduced to that of a two-level system. In the experimental part, we report on recent sound velocity and internal friction measurements on KCl doped with cyanide at various concentrations. At low doping (45 ppm) we find the dynamics of a single [111] impurity, whereas at higher concentrations (4700 ppm) the elastic response rather indicates strongly correlated defects. Our theoretical model provides a good description of the temperature dependence of $\delta v/v$ and $Q^{-1}$ at low doping, in particular the relaxation peaks, the absolute values of the amplitude, and the resonant contributions. From our fits we obtain the value of the elastic deformation potential $\gamma_t=0.192$ eV., Comment: 19 pages, 5 figures

We report a shear-induced sponge (L3) to lamellar (L(alpha)) transition in a surfactant system. Under a constant shear rate, after a delay time t(n) we observe random nucleation and subsequent growth of the L(alpha) phase, demonstrating that the shear-induced transition is first order. A simple argument for the energy of a two-dimensional nucleus accounts for the observed delay and its shear-rate dependence.

We present a frequency-domain phase measurement technique coupled with a numerical chirp processing of the recorded sonogram to realise a pulse characterisation. Applications to the propagation of ultrashort pulses in linear and nonlinear media are investigated. Calibration of the experimental setup is performed using spectral interferometry technique. Results on linear dispersion glass and Kerr effect in vitreous samples are presented.

In this work, we study borate glasses doped with a low concentration of iron oxide by X band (9.5 GHz) electron magnetic resonance. These glasses (composition: 0.63B2O3–0.37Li2O–0.75×10−3 Fe2O3 in mole %) were annealed at increasing temperatures Ta, starting at the glass transition temperature. A new composite resonance at gef≈2.0 arises in the spectra measured at room temperature (300 K). The narrow component of this resonance is predominant in glasses annealed at lower Ta while the broad component increases in intensity as Ta increases. This resonance is ascribed to an assembly of superparamagnetic nanoparticles of a crystalline iron-containing compound. Numerical simulations assuming a lognormal particle volume distribution show that the mean particle diameter increases from 5.3 to 8.5 nm as Ta increases from 748 to 823 K. The integrated spectra intensity shows that the total number of spins in the nanoparticles increases rapidly with Ta. At lower anneal temperatures Ta, a striking increase occurs in t...

Time-resolved shadow imaging and novel two-color interferometric imaging have been developed for the investigation of optical breakdown induced in water by 100 fs laser pulses with a spatial resolution better than 5 μm. With a femtosecond time resolution, we clearly identified induced-plasma and filamentation formations, as well as the shock-wave expansion. Using the two-color interferometric imaging technique together with a conventional Mach–Zehnder interferometer, we investigated the phase shift of the interference fringes produced in water by the femtosecond optical breakdown. For different pump energies, at 40 ns after the breakdown, we estimated the water temperature to be between 30°C and 60°C and the water pressure between 10 and 40 MPa at the shock-wave front.

Molecular ordering of a 2D array of hydrocarbon chains in alkane crystals or Langmuir monolayers arises from the anisotropic pair interaction for rotations about the molecular axes. Starting from atom–atom potentials for H and C, we determine the relevant Fourier coefficients of the molecular pair interaction and, in particular, their distance dependence. In terms of a molecular-field approximation (MFA), we find two first-order transitions between a high-temperature hexagonal phase, an intermediate orthorhombic phase, and at lower T a herringbone structure with a nonhomogeneous order parameter, in agreement with the phase diagrams of various systems. It turns out that molecular ordering is closely related to an orthorhombic distortion of the hexagonal lattice. The calculated distortion agrees well, without free parameters, with measured values. In order to account for the weak variation of the critical temperatures with the chain length, we propose a simple mean-field model for conformational defects tha...

We present a lattice model based on two n→0 spin vectors, capable of treating the thermodynamics of living networks in micellar solutions at any surfactant concentration. We establish an isomorphism between the coupling constants in the two spin vector Hamiltonian and the surfactant energies involved in the micellar situation. Solving this Hamiltonian in the mean-field approximation allows one to calculate osmotic pressure, aggregation number, free end and cross-link densities at any surfactant concentration. We derive a phase diagram, including changes in topology such as the transition between spheres and rods and between saturated and unsaturated networks. A phase separation can be found between a saturated network and a dilute solution composed of long flexible micelles or a saturated network and a solution of spherical micelles.

Quasi-two-dimensional decaying turbulence is studied in a flowing soap film by measuring the moments of the probability density function P(δv(r)) for the longitudinal velocity differences δv(r) on a scale r. As in three-dimensional (3-D) turbulence, P becomes non-Gaussian with decreasing r. The third moment S3(r)≡〈(δv(r))3〉 is small and negative at small scales, but becomes positive at larger scales. The exact calculation of S3(r) for 2-D homogeneous isotropic turbulence relates this change in sign to the development of the velocity correlation function as the turbulence decays.

Efficient collection and detection of fluorescence coupled with careful minimization of background from impurities and Raman scattering now enable routine optical microscopy and study of single molecules in complex condensed matter environments. This ultimate method for unraveling ensemble averages leads to the observation of new effects and to direct measurements of stochastic fluctuations. Experiments at cryogenic temperatures open new directions in molecular spectroscopy, quantum optics, and solid-state dynamics. Room-temperature investigations apply several techniques (polarization microscopy, single-molecule imaging, emission time dependence, energy transfer, lifetime studies, and the like) to a growing array of biophysical problems where new insight may be gained from direct observations of hidden static and dynamic inhomogeneity.