Molecular dynamics study of viscosity of aqueous NaCl solution confined in nanoscale channels

A physical model of bulk-nanochannel-bulk with buffer baths is built up using nonequilibrium molecular dynamics （MD） simulation to study the effects of vibrating silicon atoms on the viscosity of aqueous NaCl solutions confined in the nanochannel. The simulation is performed under different moving speeds of the upper wall, different heights and different surface charge densities in the nanochannel. The simulation results indicate that with the increase in the surface charge density and the decrease in the nanochannel height and the shear rate, the vibration effect of silicon atoms on the shear viscosity of the confined fluid in the nanochannel cannot be ignored. Compared with still silicon atoms, the vibrating silicon atoms result in the decrease in the viscosity when the height of the nanochannel is less than 0.8 nm and the shear rate is less than 1.0 ×10^11 s^-1, and the effect of the vibrating silicon atoms on the shear viscosity is significant when the shear rate is small. This is due to the fact that the vibrating silicon atoms weaken the interactions between the counter-ions （Na^＋ ） and the charged surface.

Molecular modeling study of the effect of base methylation on internal interactions and motions in DNA and implication to B-Z conformation change

Methylation in the bases of DNA is known to induce B-Z conformation change. In this work, molecular mechanics and normal mode analysis are used to probe how certain methylation affects the internal interactions and thermodynamic motions in the DNA double helixes in both B and Z conformations, and its implication to B-Z conformation change. By molecular modeling with Insight II, two cases involving cytosine C5 and guanine C8 methylation on both B and Z-form DNA duplex d(CGCGCG)2 are studied in comparison with the corresponding unmethylated duplexes. The internal interaction energies computed based on a molecular mechanics force field and the entropies due to internal motions computed according to a normal mode analysis are in fare agreement with respective observed thermodynamic quantities. The analysis on the computed individual energy terms suggests that the observed B-Z conformation change induced by methylation is primarily driven by enthalpic factors. A combination of changes in Van der Waals interaction, electrostatic interaction and hydrogen bonding likely contributes to the change of enthalpy that favors Z-conformation in the methylated states.

Investigation of the Drug Stability at the Amorphous State Using Thermal Analysis

Formulation of poorly water-soluble crystalline drugs in their amorphous counterpart is a common approach to enhance their biodisponibility. In this study, the amorphous forms of ketoprofen and flurbiprofen were obtained by supercooling of the melt in a DSC （differential scanning calorimetry） apparatus and then investigated, especially under the stability point of view. The average rate of molecular motions at any given temperature is probably the most important parameter to know for amorphous pharmaceutical materials, and it was used to explain and predict the stability of ketoprofen and flurbiprofen. A quantitative estimate of the product＇s behaviour upon storage is obtained with additional data, such as the heat capacity of crystalline and amorphous samples and the distribution of molecular relaxation times. Amorphous flurbiprofen demonstrated greater physical stability at any aging temperature tested, when compared to ketoprofen and a different dependence from aging temperature. Both amorphous drugs could he classified as ＂fragile＂ ones.