No abstract

In order to study the heating process of water by the microwaves of 2.5-20 GHz frequencies, the authors have performed molecular dynamics simulations by adopting a nonpolarizable water model that has fixed point charges on a rigid-body geometry. All runs are started from the equilibrated states derived from the I(c) ice with given density and temperature. In the presence of microwaves, the molecules of liquid water exhibit rotational motion whose average phase is delayed from the microwave electric field. Microwave energy is transferred to the kinetic and intermolecular energies of water, where one-third of the absorbed microwave energy is stored as the latter energy. The water in ice phase is scarcely heated by microwaves because of the tight hydrogen-bonded network of water molecules. Dilute salt water is significantly more heated than pure water because of the field-induced motion of salt ions, especially that of large-size ions, by the microwave electric field and energy transfer to water molecules by collisions.

We report molecular dynamics simulation of the (overall neutral) system consisting of an immobile macroion surrounded by the electrolyte of multivalent counterions and monovalent coions. As expected theoretically, counterions adsorb on the macroion surface in the amount much exceeding neutralization requirement, thus effectively inverting the sign of the macroion charge. We find two conditions necessary for charge inversion, namely, counterions must be multivalently charged and Coulomb interactions must be strong enough compared to thermal energy. On the other hand, coion condensation on the multivalent counterions similar to Bjerrum pairing is the major factor restricting the amount of charge inversion. Depending on parameters, we observe inverted charge up to about 200% the original charge of the macroion in absolute value. The inverted charge scales as ∼ ζ 1/2 when ζ < 1 and crosses over to ∼ ζ for ζ > 1, where ζ = (A0/rs) 2 , rs is the Debye screening length in the electrolyte and A0 is the distance between adsorbed counterions under neutralizing conditions. These findings are consistent with the theory of "giant charge inversion" [Phys. Rev.Lett., 85, 1568].

A new source mechanism is proposed for the ‘reflected’ ion beams observed in the foreshock region of the earth's bow shock. In our model the beams originate in the magnetosheath downstream of the quasi‐perpendicular portion of the shock. The quasi‐perpendicular shock transition is characterized by two downstream ion populations including high‐energy gyrating ions in addition to the directly transmitted anisotropic ions. We show by particle simulations that this highly anisotropic downstream ion distribution (T⊥/T∥ » 1) can excite electromagnetic ion cyclotron waves which, in turn, pitch angle scatter the gyrating ions in a few ion gyroperiods. As a result, some ions acquire large parallel velocities and move fast enough along the convecting downstream magnetic field to escape back across the bow shock into the upstream region. The distribution of escaping ions calculated by using the pitch‐angle‐scattered ions, as a source, becomes a beam with a large temperature anisotropy T⊥ ∼ 3–5 T∥ and a mean velocity along the magnetic field of about twice that of the solar wind velocity. A significant result is the presence of the maximum angle θnB = θc above which no ions can escape, where θnB is the angle between the shock normal and the interplanetary magnetic field. A wide peak of constant escaping ion flux is formed below θc whose number density is 1–2% of that of the solar wind. These results are in general agreement with the ISEE observations of the ‘reflected’ ions.

Ganglioside GM1 mediates the amyloid beta (Abeta) aggregation that is the hallmark of Alzheimer's disease (AD). To investigate how ganglioside-containing lipid bilayers interact with Abeta, we examined the interaction between Abeta40 and supported planar lipid bilayers (SPBs) on mica and SiO(2) substrates by using atomic force microscopy, fluorescence microscopy, and molecular dynamics computer simulations. These SPBs contained several compositions of sphingomyelin, cholesterol, and GM1 and were treated at physiological salt concentrations. Surprisingly high-speed Abeta aggregation of fibril formations occurred at all GM1 concentrations examined on the mica surface, but on the SiO(2) surface, only globular agglomerates formed and they formed slowly. At a GM1 concentration of 20mol%, unique triangular regions formed on the mica surface and the rapidly formed Abeta aggregations were observed only outside these regions. We have found that some unique surface-induced phase separations are induced by the GM1 clustering effects and the strong interactions between the GM1 head group and the water layer adsorbed in the ditrigonal cavities on the mica surface. The speed of Abeta40 aggregation and the shape of the agglomerates depend on the molecular conformation of GM1, which varies depending on the substrate materials. We identified the conformation that significantly accelerates Abeta40 aggregation, and we think that the detailed knowledge about the GM1 molecular conformation obtained in this work will be useful to those investigating Abeta-GM1 interactions.