Results Of High-pressure Studies Research Articles

Dielectric Relaxation Study at Ambient and Elevated Pressure of the Modeled Lipophilic Drug Fenofibrate.

We have investigated the molecular dynamics in supercooled liquid and glassy state of the pharmaceutical agent fenofibrate. To do that, dielectric relaxation studies at ambient and elevated pressure were performed. Data collected at atmospheric pressure were found to be in good agreement with that already reported in the literature. High-pressure studies enable us to distinguish the secondary relaxation processes of the different molecular origin. This includes (i) pressure insensitive γ-relaxation of the activation energy Ea = 28 kJ/mol and (ii) β-relaxation that senses the density increase and originates most probably from the intermolecular movements (Ea = 77 kJ/mol at 480 MPa). The results of high-pressure studies have also revealed the validity of isochronal superposition and decoupling between α-relaxation and translational motions of charged species reflected by dc conductivity. The latter one was found to intensify with increasing pressure. Finally, we also show that the presence of quite complex inter- and intramolecular hydrogen bonds might still exert some subtle effect on the molecular dynamics of supercooled fenofibrate.

Allosteric Effectors Influence the Tetramer Stability of Both R- and T-states of Hemoglobin A

The contribution of heterotropic effectors to hemoglobin allostery is still not completely understood. With the recently proposed global allostery model, this question acquires crucial significance, because it relates tertiary conformational changes to effector binding in both the R- and T-states. In this context, an important question is how far the induced conformational changes propagate from the binding site(s) of the allosteric effectors. We present a study in which we monitored the interdimeric interface when the effectors such as Cl-, 2,3-diphosphoglycerate, inositol hexaphosphate, and bezafibrate were bound. We studied oxy-Hb and a hybrid form (alphaFeO2)2-(betaZn)2 as the T-state analogue by monitoring heme absorption and Trp intrinsic fluorescence under hydrostatic pressure. We observed a pressure-dependent change in the intrinsic fluorescence, which we attribute to a pressure-induced tetramer to dimer transition with characteristic pressures in the 70-200-megapascal range. The transition is sensitive to the binding of allosteric effectors. We fitted the data with a simple model for the tetramer-dimer transition and determined the dissociation constants at atmospheric pressure. In the R-state, we observed a stabilizing effect by the allosteric effectors, although in the T-analogue a stronger destabilizing effect was seen. The order of efficiency was the same in both states, but with the opposite trend as inositol hexaphosphate > 2,3-diphosphoglycerate > Cl-. We detected intrinsic fluorescence from bound bezafibrate that introduced uncertainty in the comparison with other effectors. The results support the global allostery model by showing that conformational changes propagate from the effector binding site to the interdimeric interfaces in both quaternary states.

High pressure effect on magnetic properties and volume anomalies of Ce2Fe17

We present the results of high pressure studies of magnetization, magnetic phase transitions, and related volume anomalies for the intermetallic compound Ce2Fe17 under hydrostatic pressures up to 10 kbar. The giant negative pressure effect on the ferromagnetic ordering temperature ΘT=94 K was determined: dΘT/dP=−(38±2) K/kbar. This extraordinary decrease of ΘT under pressure is not accompanied by the related pronounced decrease of the saturated magnetization under pressure below 2.5 kbar. The effect of pressure on the Néel temperature TN=206 K is less pronounced: dTN/dP=−(1.7±0.2) K/kbar. A large positive spontaneous volume magnetostriction that is observed below TN is suppressed by the application of high pressure and it disappears above 2.5 kbar. A competition of positive and negative exchange interactions between iron atoms is discussed and is thought to be the main reason for the volume anomalies observed. The role of the f electrons of Ce, their possible hybridization with d states of Fe, and their role in magnetic properties of the Ce2Fe17 compound is not yet clear.

MgSiO3 ilmenite: Heat capacity, thermal expansivity, and enthalpy of transformation

A new determination, using high temperature drop-solution calorimetry, of the enthalpy of transformation of MgSiO3 pyroxene to ilmenite gives ΔH298 = 59.03 ±4.26 kJ/mol. The heat capacity of the ilmenite and orthopyroxene phases has been measured by differential scanning calorimetry at 170–700 K; Cp of MgSiO3 ilmenite is 4–10 percent less than that of MgSiO3 pyroxene throughout the range studied. The heat capacity differences are consistent with lattice vibrational models proposed by McMillan and Ross (1987) and suggest an entropy change of -18 ± 3 J-K-1 ·mol-1, approximately independent of temperature, for the pyroxene-ilmenite transition. The unit cell parameters of MgSiO3 ilmenite were measured at 298–876 K and yield an average volume thermal expansion coefficient of 2.44 × 10-5 K-1. The thermochemical data are used to calculate phase relations involving pyroxene, β-Mg2SiO4 plus stishovite, Mg2SiO4 spinel plus stishovite, and ilmenite in good agreement with the results of high pressure studies.

Pressure effects in intermetallic uranium compounds

High-pressure experiments are reported on various series of ferromagnetic intermetallic uranium compounds. Pressure effects in these compounds are compared with the results of high-pressure studies on 3d and 4f metals in order to establish the itinerant or localised nature of ferromagnetism in these compounds. Special attention is devoted to the equiatomic UPT compound in which complex magnetic phenomena are observed, including pressure-induced hysteretic effects.

Molecular reorientation in liquids by Rayleigh scattering: Pressure dependence of rotational correlation functions

Results of high pressure study of the depolarized Rayleigh scattering in acetone, benzene, and methyl iodide are presented. The density dependences of the angular velocity and angular position correlation functions are determined. The angular velocity correlation function of each liquid shows a negative overshoot. In addition, benzene at all pressures and methyl iodide at high pressures exhibit additional oscillations in the angular velocity correlation function. The details of the correlation functions are discussed in terms of ''collision interrupted'' models such as the well known J-diffusion model. It is concluded that a more general description than J diffusion capable of allowing incomplete loss of momentum upon collision is needed. The results are compared to studies of reorientation by means of NMR. Finally, the relation between shear viscosity and reorientation in these liquids is discussed.