Mobility Of Molecules In Solution Research Articles

Dynamics of collagen molecules in aqueous solutions containing metal ions with different ionic radii

The dynamic parameters of collagen molecules in aqueous solutions were assessed by the photon-correlation spectroscopy method. The dependences of translational diffusion coefficients as a function of pH in pure aqueous collagen solutions and in solutions containing salts of different metals such as Na, Ca, K, Pb were established. It has been reported that the size of a metal’s ionic radius affects intermolecular interactions and the mobility of collagen molecules in solution.

Phase Behavior of Ranitidine HCl in the Presence of Degradants and Atmospheric MoistureImpact on Chemical Stability

For hydrophilic organic solids, it is well recognized that degradation is often promoted by exposure to humid conditions. Although this is an important issue for certain classes of materials, in particular pharmaceuticals, the factors which dictate the sensitivity of a given compound to moisture are not well understood. The goal of this work was to elucidate the synergistic influence of self-originating impurities and water vapor on the degradation kinetics of the histamine H2 receptor antagonist, ranitidine HCl. Physical mixtures of the drug and each of three major degradation products were subjected to conditions of elevated temperature and relative humidities. Pure samples showed a sigmoidal-shaped degradation profile for all storage conditions studied. During the lag time, the pure drug gained minimal quantities of moisture. Once degradation commenced, the samples started to absorb moisture. When mixed with the degradant, the lag period was eliminated for all storage conditions, even at low partial pressures of water. The extent of moisture gain by samples containing impurities could not be attributed to the presence of the impurity alone. It was found that the presence of impurities in contact with the surface of the drug, in combination with water vapor, promoted a phase transition of the crystalline material to the solution phase. A ternary phase diagram was constructed to visualize the proportion of the drug in the solid and solution phases as a function of impurity and moisture content. The increased mobility of molecules in solution presumably leads to enhanced reactivity relative to the crystalline material.

Confined molecular motions of globular proteins studied in powder samples and in solution

Internal molecular equilibrium fluctuations have been studied for α-amylase in two different types of samples. The comparison of incoherent neutron scattering from the enzyme in hydrated powders and in solutions gives insights into the protein - solvent interaction. Although the protein is already fully hydrated at h = 0.4 in a powder sample (the protein surface is completely covered with solvent), the proteins are characterised by a significantly larger structural flexibility in solution as compared proteins in powder samples. As obtained from model fits describing local diffusion, the results indicate that the higher mobility of solvent molecules in solution initiate additional fluctuations in the protein structure. Due to much more solvent surrounding the protein in solutions, the picosecond fluctuations of the enzyme seem to be damped, indicated by smaller amplitudes and a weaker temperature dependence as compared to powder samples.

Effects of sugars and polymers on crystallization of poly(ethylene glycol) in frozen solutions: phase separation between incompatible polymers.

This study examined the effect of third components (low-molecular-weight saccharides and polymers) on the crystallization of poly(ethylene) glycol (PEG) in frozen solutions, focusing on the relationship between their crystallization-inhibiting ability and molecular compatibility. Effects of sugars and polymers on the crystallization of PEG 3000 in frozen solution were monitored by differential scanning calorimetry (DSC). Pulsed-NMR was employed to monitor the molecular mobility of water and solutes in the frozen solutions. Miscibility between PEG and third components in aqueous solution was estimated from the lowering of cloud point of PEG 20,000. Thermal analysis of frozen solutions containing some non-crystallizing solutes was used to examine the possibility of phase separation in frozen solutions. Some sugars and polymers inhibited the crystallization of PEG and formed practically stable amorphous phases among ice crystals. The mobility of solute molecules in the amorphous phase increased above the softening temperature of maximally concentrated solutions (Ts), whereas that of water molecules appeared at a lower temperature. Mono- and disaccharides that are relatively less miscible with PEG in solution inhibit PEG crystallization to a lesser degree. Two Ts regions were observed in frozen solutions containing both polyvinylpyrrolidone (PVP) and dextran, at much lower concentrations than those causing aqueous two-phase separation at ambient temperatures. Ice crystallization raises the concentration of solutes in the remaining solution, which can lead to phase separation in the amorphous phase. Molecular compatibility between components is an important factor determining their propensity to phase separate and crystallize.

Effect of water content on the mobility of solute molecules and of protein side chains in casemate preparations

SummaryThe effect of water content on the mobility of small solute molecules and of casein side chains was studied by electron spin resonance in caseinate/water systems. The immobilization of nitroxide probes of different sizes and properties were followed during progressive dehydration of concentrated caseinate preparations (3 g H2O/g dry protein). Above a characteristic minimum moisture content a linear relationship was observed between the rotational diffusivity and water content. The slope of the straight lines depended upon the size of the probe and upon the nature of the interactions with the protein. Nitroxide labels were covalently bound to caseinate and the changes in the flexibility of the side chains as hydration changed were followed. The results are discussed in relation to the solvent properties of water, and to recent suggestions concerning the significance, for stability, of the glass transition phenomenon in such materials.