Stochastic Dynamics of Electrical Membrane with Voltage-Dependent Ion Channel Fluctuations

Brownian ratchet like stochastic theory for the electrochemical membrane system of Hodgkin-Huxley (HH) is developed. The system is characterized by a continuous variable $Q_m(t)$, representing mobile membrane charge density, and a discrete variable $K_t$ representing ion channel conformational dynamics. A Nernst-Planck-Nyquist-Johnson type equilibrium is obtained when multiple conducting ions have a common reversal potential. Detailed balance yields a previously unknown relation between the channel switching rates and membrane capacitance, bypassing Eyring-type explicit treatment of gating charge kinetics. From a molecular structural standpoint, membrane charge $Q_m$ is a more natural dynamic variable than potential $V_m$; our formalism treats $Q_m$-dependent conformational transition rates $\lambda_{ij}$ as intrinsic parameters. Therefore in principle, $\lambda_{ij}$ vs. $V_m$ is experimental protocol dependent,e.g., different from voltage or charge clamping measurements. For constant membrane capacitance per unit area $C_m$ and neglecting membrane potential induced by gating charges, $V_m=Q_m/C_m$, and HH's formalism is recovered. The presence of two types of ions, with different channels and reversal potentials, gives rise to a nonequilibrium steady state with positive entropy production $e_p$. For rapidly fluctuating channels, an expression for $e_p$ is obtained.Comment: 8 pages, two figure

The Role of Exopolyphosphatase in Neisseria meningitidis Infection

The development of vaccines against serogroup B Neisseria meningitidis to reduce the morbidity and mortality of meningococcal disease is a major public health priority. We developed a novel genetic screen for immunogens present on the bacterial surface using human immune sera with bactericidal activity. We found that two mutants lacking nmb1467 survived in high concentrations of sera from two patients, while the wild-type strain was killed. Biochemical assays using purified recombinant NMB1467 indicated that nmb1467 encodes an exopolyphosphatase (PPX) with the ability to hydrolyse inorganic polyphosphate (poly P). In addition, we demonstrated that the Δppx mutant has at least 2-fold more poly P than the wild-type strain. Therefore, we designated NMB1467 as PPX. We showed that N. meningitidis mutant lacking the ppx had an increased resistance against normal human complement system. Substitution of the glutamic acid at residue 147 of PPX with an alanine significantly reduced the enzymatic activity in vitro, and contributed to increased level of poly P in N. meningitidis and the resistance of bacteria against the complement-mediated killing. Levels of polysaccharide capsule and lipopolysaccharide (LPS) sialylation, two important virulence factors, were not affected by the loss of ppx in N. meningitidis. Using flow cytometry, we demonstrated that binding of factor H (fH), the negative regulator of the alternative pathway of complement activation, to the bacterial surface was increased in the strain lacking PPX. By Western blot analysis, we did not detect a significant change in the expression of the fH receptor, indicting another mechanism is involved in the fH binding to the bacterial surface and resistance of bacteria against complement-mediated killing

Angular Momentum of Phonons and Einstein-de Haas Effect

We study angular momentum of phonons in a magnetic crystal. In the presence of a spin-phonon interaction, we obtain a nonzero angular momentum of phonons, which is an odd function of magnetization. At zero temperature, phonon has a zero-point angular momentum besides a zero-point energy. With increasing temperature, the total phonon angular momentum diminishes and approaches to zero in the classical limit. The nonzero phonon angular momentum can have a significant impact on the Einstein-de Haas effect. To obtain the change of angular momentum of electrons, the change of phonon angular momentum needs to be subtracted from the opposite change of lattice angular momentum. Furthermore, the finding of phonon angular momentum gives a potential method to study the spin-phonon interaction. Possible experiments on phonon angular momentum are also discussed.Comment: Accepted by Phys. Rev. Lett. Detailed supplementary file is include

Valley contrasting chiral phonons in monolayer hexagonal lattices

In monolayer hexagonal lattices, two inequivalent valleys appear in the Brillouin zone. With inversion symmetry breaking, we find chiral phonons with valley contrasting circular polarization and ionic magnetic moment. At valley centers, there is a three-fold rotational symmetry endowing phonons with a quantized pseudo angular momentum, which includes spin and orbital parts. From conservation of the pseudo angular momentum, crystal momentum and energy, selection rules in intervalley scattering of electrons by phonons are obtained. The chiral valley phonons are verified and the selection rules are predicted in monolayer Molybdenum disulfide. Due to valley contrasting phonon Berry curvature, one can also detect a valley phonon Hall effect. The valley-contrasting chiral phonon, together with phonon circular polarization, ionic magnetic moment, phonon pseudo angular momentum, valley phonon Hall effect, will form the basis for valley-based electronics and phononics applications in the future