Molecular dynamics simulation of phosphatidylcholine membrane in low ionic strengths of sodium chloride.

The one-microsecond molecular dynamics simulations of a membrane-protein complex investigate the influence of the aqueous sodium chloride solutions on the structure and dynamics of a palmitoyl-oleoyl-phosphatidylcholine bilayer membrane. The simulations were performed on five different concentrations (40, 150, 200, 300, and 400 mM) in addition to a salt-free system by using the charmm36 force field for all atoms. Four biophysical parameters, (membrane thicknesses of annular and bulk lipids, and the area per lipid of both leaflets), were computed separately. Nevertheless, the area per lipid was expressed by using the Voronoi algorithm. All time-independent analyses were carried out for the last 400 ns trajectories. Different concentrations revealed dissimilar membrane dynamics before equilibration. The biophysical properties of the membrane (thickness, area-per-lipid, and order parameter) have non-significant changes with increasing ionic strength, however, the 150 mM system had exceptional behavior. Sodium cations were dynamically penetrating the membrane forming weak coordinate bonds with single or multiple lipids. Nevertheless, the binding constant was unaffected by the cation concentration. The electrostatic and Van der Waals energies of lipid-lipid interactions were influenced by the ionic strength. On the other hand, the Fast Fourier Transform was performed to figure out the dynamics at the membrane-protein interface. The nonbonding energies of membrane-protein interactions and order parameters explained the differences in the synchronization pattern. All results were consensus with experimental and theoretical works.Communicated by Ramaswamy H. Sarma.

First principles and mean field study on the magnetocaloric effect of YFe3 and HoFe3 compounds.

In this work, the magnetothermal characteristics and magnetocaloric effect in YFe3 and HoFe3 compounds are calculated as function of temperature and magnetic field. These properties were investigated using the two-sublattice mean field model and the first-principles DFT calculation using the WIEN2k code. The two-sublattice model of the mean-field theory was used to calculate the temperature and field-dependences of magnetization, magnetic heat capacity, magnetic entropy, and the isothermal change in entropy ∆Sm. We used the WIEN2k code to determine the elastic constants and, subsequently, the bulk and shear moduli, the Debye temperature, and the density-of-states at Ef. According to the Hill prediction, YFe3 has bulk and shear moduli of roughly 99.3 and 101.2 GPa respectively. The Debye temperature is ≈ 500 K, and the average sound speed is ≈ 4167 m/s. In fields up to 60 kOe and at temperatures up to and above the Curie point for both substances, the trapezoidal method was used to determine ∆Sm. For instance, the highest ∆Sm values for YFe3 and HoFe3 in 30 kOe are approximately 0.8 and 0.12 J/mol. K, respectively. For the Y and Ho systems, respectively, the adiabatic temperature change in a 3 T field decreases at a rate of around 1.3 and 0.4 K/T. The ferro (or ferrimagnetic) to paramagnetic phase change in these two compounds, as indicated by the temperature and field dependences of the magnetothermal and magnetocaloric properties, ∆Sm and ∆Tad, is a second-order phase transition. The Arrott plots and the universal curve for YFe3 were also calculated and their features give an additional support to the second order nature of the phase transition.