Imaging biomolecular interactions by fast three-dimensional tracking of laser-confined carrier particles

The quantitative study of the near-equilibrium structural behavior of individual biomolecules requires high-resolution experimental approaches with longtime stability. We present a new technique to explore the dynamics of weak intramolecular interactions. It is based on the analysis of the 3D Brownian fluctuations of a laser-confined glass bead that is tethered to a flat surface by the biomolecule of interest. A continuous autofocusing mechanism allows us to maintain or adjust the height of the optical trap with nanometer accuracy over long periods of time. The resulting remarkably stable trapping potential adds a well-defined femto-to-piconewton force bias to the energy landscape of molecular configurations. A combination of optical interferometry and advanced pattern-tracking algorithms provides the 3D bead positions with nanometer spatial and >120 Hz temporal resolution. The analysis of accumulated 3D positions has allowed us not only to identify small single biomolecules but also to characterize their nanomechanical behavior, for example, the force-extension relations of short oligonucleotides and the unfolding/refolding transitions of small protein tethers.     

Thermal analysis of amorphous phase in a pharmaceutical drug

Thermally Stimulated Current (TSC) spectroscopy and Differential Scanning Calorimetry (DSC) have been applied to the characterization of the microstructure of a pharmaceutical drug.The dielectric relaxation spectrum shows two modes located in the temperature range of the glass transition. They have been attributed to the molecular mobility in the true amorphous phase and in the rigid amorphous region.     

Intermolecular interactions and phase equilibria in nitrocellulose - s-diethyldiphenylurea system

Values of the Flory-Huggins interaction parameters c were predicted on the base of mixing enthalpy H ^M for nitrocellulose-s-diethyldiphenylurea system. The phase diagram of the system and the glass transition temperature of mixtures T _g12 were estimated using calculated c parameters. The predicted glass transition temperatures were in accordance with values determined experimentally. This revised version was published online in July 2006 with corrections to the Cover Date.     

Many-body interactions between particles in a polydisperse polymer fluid

We use a continuum chain model and develop an analytical theory for the interaction between many spheres immersed in a fluid of ideal polydisperse polymers. Assuming local spherical symmetry of the polymer field about each particle, combined with a local approximation, compact expressions are derived for the many-body interaction between the spheres. We use a mean-field approximation to investigate the fluid-fluid phase diagram for the mixture.     

Phases and phase transitions in a nematic lyotropic system with a biaxial phase

NMR spectroscopy and optical microscopy have been used to study phase transitions and structure in nematic lyotropic mesophases formed by potassium laurate, decylammonium hydrochloride and water. The different mesophases obtained in a well-defined composition range have been characterized, by deuterium NMR, following the evolution of the D₂O spectra as a function of temperature and of the orientation of the samples with respect to the magnetic field. Wide ranges of biaxiality have been found and the asymmetry parameter of the averaged electrical field gradient tensor on the deuterium of the D₂O molecule has been determined. The presence of the different mesophases has always been confirmed by observing oriented samples under the polarizing optical microscope.     

Low energy charged particles interacting with amorphous solid water layers

The interaction of charged particles with condensed water films has been studied extensively in recent years due to its importance in biological systems, ecology as well as interstellar processes. We have studied low energy electrons (3-25 eV) and positive argon ions (55 eV) charging effects on amorphous solid water (ASW) and ice films, 120-1080 ML thick, deposited on ruthenium single crystal under ultrahigh vacuum conditions. Charging the ASW films by both electrons and positive argon ions has been measured using a Kelvin probe for contact potential difference (CPD) detection and found to obey plate capacitor physics. The incoming electrons kinetic energy has defined the maximum measurable CPD values by retarding further impinging electrons. L-defects (shallow traps) are suggested to be populated by the penetrating electrons and stabilize them. Low energy electron transmission measurements (currents of 0.4-1.5 μA) have shown that the maximal and stable CPD values were obtained only after a relatively slow change has been completed within the ASW structure. Once the film has been stabilized, the spontaneous discharge was measured over a period of several hours at 103 ± 2 K. Finally, UV laser photo-emission study of the charged films has suggested that the negative charges tend to reside primarily at the ASW-vacuum interface, in good agreement with the known behavior of charged water clusters.     

Theory of ordering in three-dimensional polymer systems with local orientational-deformational interactions

The spontaneous ordering of a three-dimensional polymer system composed of flexible segments with a fixed mean-square length in the presence of local intra- and interchain orientational-deformational interactions was considered. The fixation of the segment chain length on average is possible only during relatively weak interchain interactions or at high temperatures, i.e., in the isotropic state. The three-dimensional model, unlike its two-dimensional version, suggests the existence of the critical point at which a second-order phase transition from the isotropic state to an ordered state takes place. The critical behavior of the multichain model is described by the spherical approximation for an anisotropic Heisenberg ferromagnetic. The dependence of the critical point and the parameters of short- and long-range dipole and quadrupole orientational orders on the chain rigidity and magnitude of interchain interactions was determined. In the isotropic state, orientation correlations of segments decline according to the Ornstein-Zernike law, as in the 3D model of Gaussian subchains without fixation of their mean-square length. In the ordered state, the correlation functions tend to a finite limiting value corresponding to the presence of long-range order, as in the case of the multichain model of rigidchain segments in the strong-order approximation. A comparison of the short-range and long-range orders in the model of chains composed of segments with a fixed mean-square length and undeformable (rodlike) elements in the mean-field approximation, on the one hand, and multichain models, on the other hand, showed their equivalence, especially at high degrees of ordering.     

Complex phase behavior of the system of particles with smooth potential with repulsive shoulder and attractive well

We report a detailed simulation study of the phase behavior of core-softened system with attractive well. Different repulsive shoulder widths and attractive well depths are considered which allows to monitor the influence of repulsive and attractive forces on the phase diagram of the system. Thermodynamic anomalies in the systems are also studied. It is shown that the diffusion anomaly is stabilized by small attraction.     

Second-order phase transitions in amorphous gallium clusters

Ion mobility and calorimetry measurements have been used to probe the nature of the phase transitions in gallium clusters with 29-55 atoms. While most clusters appear to undergo a first-order transition between solidlike and liquidlike phases, a few show the signature of melting without a significant latent heat. These transitions appear to be the finite size analogue of a second-order phase transition, and they presumably occur for some cluster sizes because their solidlike phase is amorphous.     

A second-order phase transition in a monolayer of rectangle particles

Summary To describe the surface pressure-area isotherms of monolayers of amphiphilic molecules a two-dimensional gas of rectangular particles is proposed. Using theZwanzig's simplication of theOnsager's virial expansion analysis we found a second-order transition. Then introducing attractive potentials of different shapes, surrounding the hard core we may change the position of the critical points. The calculated isotherms have similar profiles than those obtained experimentally. With the more refined potential well, this depending on the area overlap of the wells, the interaction parameters have the same order of magnitude than those given in the literature. It is shown that this orientational transition is obtained only with particles which are asymmetrical enough.On leave from UER Pluridisciplinaire de Luminy Université d'Aix-Marseille II.     

Shear-induced morphology transition and microphase separation in a lamellar phase doped with clay particles

We report on the influence of shear on a nonionic lamellar phase of tetraethyleneglycol monododecyl ether (C12E4) in D2O containing clay particles (Laponite RD). The system was studied by means of small-angle light scattering (SALS) and small-angle neutron scattering (SANS) under shear. The SANS experiments were conducted using a H2O/D2O mixture of the respective scattering length density to selectively match the clay scattering. The rheological properties show the familiar shear thickening regime associated with the formation of multilamellar vesicles (MLVs) and a shear thinning regime at higher stresses. The variation of viscosity is less pronounced as commonly observed. In the shear thinning regime, depolarized SALS reveals an unexpectedly strong variation of the MLV size. SANS experiments using the samples with lamellar contrast reveal a change in interlamellar spacing of up to 30% at stresses that lead to MLV formation. This change is much more pronounced than the change observed, when shear suppresses thermal bilayer undulations. Microphase separation occurs, and as a consequence, the lamellar spacing decreases drastically. The coincidence of the change in lamellar spacing and the onset of MLV formation is a strong indication for a morphology-driven microphase separation.     

Calculating activation energy of amorphous phase with the Lambert W function

A new approach for determining the activation energy of amorphous alloys is developed. Setting the second order differential coefficient of heterogeneous reaction rate equation of non-isothermal heating as zero at extreme points of DSC curve, we obtain the new correlation taking form:

Water vapor permeability of shape memory polyurethane with amorphous reversible phase

Shape memory thermoplastic polyurethanes (TPUs), based on amorphous soft segment from the reaction of hexamethylene diisocyanate and 1,2‐butane diol, and the crystalline hard segment from 4,4′‐methylenediphenyl diisocyanate and 1,6‐hexanediol, were modified by hydrophilic segments, diol‐terminated poly(ethylene oxide) or dimethylol propionic acid (DMPA). Differential scanning calorimetry, dynamic mechanical testing, tensile testing, and the measurement of shape memory effect, water swell, and water vapor permeability were carried out to examine these TPUs. The hydrophilic segment increased the hysterisis in shape memory effect by reducing the crystallinity of the hard segment. The neutralized DMPA unit enhanced the sensitivity of the thermoresponsive water vapor permeability (WVP) by amplifying the increase of WVP at the temperature range above the glass transition temperature. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3009–3017, 2000     

Viscoelastic and structural properties of a phenyl-modified polysiloxane system with a three-dimensional structure

The relationships between the viscoelastic and structural properties of glass-forming materials with polysiloxane bonds, which serve as network formers, and phenyl groups, which act as network terminators, are examined based on shear viscoelasticity, (29)Si MAS NMR, and GPC measurements during the early stages of the network-forming process. The viscosities of the present samples do not depend on the frequency at temperatures up to 200 degrees C, suggesting that the origin of the viscous flow does not include intermolecular entanglement. According to the results of the strain dependence of the elastic modulus, the bridging-oxygen number, and molecular weight, the present polysiloxane system has a complex structure, or distribution of various-sized molecules composed of a polysiloxane network with various dimensionalities, and furthermore an elementary process of the viscosity is simple flow of these molecules. The structural factors that determine the viscosity and its temperature dependence are categorized into the molecular size and the intramolecular structure by using a theory based on the free-volume model. The relationship between the viscosity and the structure around the glass transition temperature is quantitatively examined and it is concluded that introducing larger numbers of Ph groups makes the viscosity less sensitive to structural factors.     

Liquid crystal mediated interactions between nanoparticles in a nematic phase

A continuum theory is used to study the interactions between nanoparticles suspended in nematic liquid crystals. The free energy functional that describes the system is minimized using an Euler-Lagrange approach and an unsymmetric radial basis function method. It is shown that nanoparticle liquid-crystal mediated interactions can be controlled over a large range of magnitudes through changes of the anchoring energy and the particle diameter. The results presented in this work serve to reconcile past discrepancies between theoretical predictions and experimental observations, and suggest intriguing possibilities for directed nanoparticle self-assembly in liquid crystalline media.     

Fragment model as a phase analysis method for diffraction amorphous materials

The main theoretical principles of the fragment model are presented. It is shown that in constructing the model radial distribution functions of atoms the parameters of the experimental radial distribution function are not used, and the model can be used to identify diffraction amorphous nanodispersed materials, to analyze the atomic structure of classic glasses, and also in the phase analysis of amorphous materials. The procedure is easy to use and does not require cumbersome calculations; the necessity to perform relaxation is absent.     

Heat capacities of a networked system of single-molecule magnet with three-dimensional structure

A magnetic-fields dependence of heat capacity of [Mn₅(hmp)₄(OH)₂{N(CN)₂}₆]2MeCN·2THF (hmp=hydroxymethylpyridinate) is investigated by the thermal relaxation calorimetry technique. This compound is a three-dimensional system consisting of Mn₄ single-molecule magnet (SMM) units and Mn²⁺ ions, which are linked by the dicyanamide ligands to form a coordination network structure. A sharp peak of C _p being associated with the formation of three-dimensional long-range order is observed around 1.96 K. The thermodynamic discussion based on the magnetic entropy suggests that both SMMs and Mn²⁺ ions are involved in the formation of the anitiferromagnetic spin ordering. However, this long-range ordering is very sensitive to the external magnetic fields which work to change the magnitude of the Zeeman splitting of the SMM levels. The behavior under magnetic fields is similar to that of the two-dimensional Mn₄-network system studied previously.