The mechanosensitive channel of small conductance (MscS) belongs to a family of membrane proteins that are gated in response to changes in membrane tension, thereby protecting the cell from hypo-osmotic shock. Here we report on passive ion transport simulations of MscS in a POPC bilayer using a coarse-grained particle-based description based on the Boltzmann transport Monte Carlo method. Single channel current-voltage curves are computed over hundreds of nanoseconds for channel conformations derived from all-atom molecular dynamics simulations reaching an overall simulation time of over 5 micros. Channel conformations similar to that of the crystal structure exhibit low conductance, whereas conformations reached after opening the channel by means of steered molecular dynamics simulations match experimentally determined conductances. However, while experiments indicate a slight preference for anionic currents, the simulated channel strongly selects anions over cations and the direction of rectification at high voltages is opposite to what is observed in experiments. Three-dimensional maps of time-averaged ion distribution and equilibrium occupancy profiles constructed from trajectory data indicate separation of anions and cations inside and in the immediate vicinity of the large cytoplasmic domain of MscS, in accordance with earlier molecular dynamics simulations. This separation arises from the distribution of ionizable residues of MscS and suggests a specific, yet unknown, functional purpose.

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The radial structure of a low-pressure cylindrical post-cathode direct-current magnetron discharge is investigated using a one-dimensional, electrostatic particle-in-cell code, incorporating non-periodic boundary conditions and an external circuit. Electron and ion collisions with a background gas of argon are modelled using Monte Carlo techniques. The radial structure of the discharge is examined for a range of operating conditions. Profiles of the electric potential, electric field and space charge density are found to vary systematically with the pressure p and magnetic field strength B, in a way which corresponds to a transition from the usual positive space charge mode at low values of B/p to a higher impedance negative space charge mode at higher values of B/p. This is consistent with a continuous and considerable reduction in the ratio of the electron-to-ion classical cross-field transport coefficients with increasing B/p. Results of a fluid model also predict the transition to the negative space charge mode.

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