Modeling of ionic relaxation around a biomembrane disk

2D measurement of ion currents associated to the signal transduction of the phototactic alga Chlamydomonas reinhardtii

Our objective was to develop a simple procedure for the detection of light-induced ion currents of photomotile cells in two dimensions. The novel technique was based on the light gradient method (LGM), and the model object was Chlamydomonas reinhardtii, a phototactic unicellular alga, ideal for such experiments. The conventional LGM cuvette was modified such that the electrode pair could be rotated around the sample and pick up the electric signals from arbitrary directions. The experiments were performed with and without the application of an auxiliary light beam preorienting the motile cells. The analysis of the detected traces revealed two main vectorial components of the signal by the help of singular value decomposition (SVD), in concert with previous experimental findings and theoretical considerations suggesting different origins of the "fast" and "slow" components of the photoelectric response of Chlamydomonas and Haematococcus cells. Using plausible assumptions, our method allowed a quantitative analysis of the signal, assigning size and direction to the two vectorial components. The method allows a rapid and accurate way to measure electric signals of photomotive cells in 2D, and particularly to test the physiological activity and in vivo-kinetics of site-directed mutants of ChR1 or ChR2, providing novel photo-electrophysiological methods with important quantitative information.

Brownian dynamics investigation of magnetization and birefringence relaxations in ferrofluids

Brownian dynamics simulations are used to investigate the dynamics of orientational properties of real charge-stabilized ferrofluids, i.e. stable colloidal dispersions of magnetic nanoparticles. The relaxation times of the magnetization and of the birefringence, data accessible by experimental techniques, have been computed at several volume fractions. Besides, the effect of hydrodynamic interactions has been investigated. Equilibrium simulations without field are found to be inadequate to determine the aforementioned relaxation times for the systems under study, the dipolar interaction being too weak. Thus a nonequilibrium simulation procedure that mimics the experimental operating mode has been developed. After equilibrium simulations under a magnetic field, both birefringence and magnetization decays are recorded once the field is suppressed. Birefringence and magnetization decays are markedly impeded as the volume fraction increases, whereas they are barely enhanced when the intensity of the initial magnetic field is increased at a fixed volume fraction. Eventually, hydrodynamic interactions exhibit a slight but systematic lengthening of the relaxation times.

Relaxation of the electrical double layer after an electron transfer approached by Brownian dynamics simulation

In this paper, the dynamical properties of the electrochemical double layer following an electron transfer are investigated by using Brownian dynamics simulations. This work is motivated by recent developments in ultrafast cyclic voltammetry which allow nanosecond time scales to be reached. A simple model of an electrochemical cell is developed by considering a 1:1 supporting electrolyte between two parallel walls carrying opposite surface charges, representing the electrodes; the solution also contains two neutral solutes representing the electroactive species. Equilibrium Brownian dynamics simulations of this system are performed. To mimic electron transfer processes at the electrode, the charge of the electroactive species are suddenly changed, and the subsequent relaxation of the surrounding ionic atmosphere are followed, using nonequilibrium Brownian dynamics. The electrostatic potential created in the center of the electroactive species by other ions is found to have an exponential decay which allows the evaluation of a characteristic relaxation time. The influence of the surface charge and of the electrolyte concentration on this time is discussed, for several conditions that mirror the ones of classical electrochemical experiments. The computed relaxation time of the double layer in aqueous solutions is found in the range 0.1 to 0.4 ns for electrolyte concentrations between 0.1 and 1 mol L(-1) and surface charges between 0.032 and 0.128 C m(-2).