Bilayer Lipid Membrane as Memcapacitance: Capacitance-Voltage Pinched Hysteresis and Negative Insertion Conductance

Effect of colloidal magnetite (Fe3O4) nanoparticles on the electrical characteristics of the azolectin bilayer in a static inhomogeneous magnetic field

This work is devoted to the study of the combined effects of applied magnetic field and MNPs on the electrical characteristics of bilayer lipid membranes. We present results of the study of electrical parameters of azolectin membranes in a static inhomogeneous magnetic field at the one-sided addition of positively charged quasi-spherical superparamagnetic magnetite nanoparticles with a diameter of about 4 nm. The magnet was located at different distances from the membrane, and the magnetic field attracted the nanoparticles to the membrane surface with different strengths. We observed three pronounced effects that depended on the external magnetic field. Firstly, after addition of nanoparticles in a magnetic field, the conductance of the membranes increased. A smooth increase in conductance was accompanied in some cases by the appearance of current jumps, which can be associated with the formation of through pores with a radius of no more than 1 nm. The conductance increased with increasing magnetic field gradient. Secondly, at zero command voltage, a negative current through the membrane was observed. Although our experiments did not allow us to unambiguously determine which particles create this current, we believe that this current is associated with the penetration of particles through the membrane. This effect intensified with increasing magnetic field gradient. Thirdly, we observed a sharp change in the nonlinear dependence of capacitance on voltage associated both with the change in the surface potential of the azolectin membrane and with the effect of MNP binding to the membrane surface on the apparent membrane capacitance.

Physical insights into biological memory using phospholipid membranes

Electrical signals may propagate along neuronal membranes in the brain, thus enabling communication between nerve cells. In doing so, lipid bilayers, fundamental scaffolds of all cell membranes, deform and restructure in response to such electrical activity. These changes impact the electromechanical properties of the membrane, which then physically store biological memory. This memory can exist either over a short or long period of time. Traditionally, biological memory is defined by the strengthening or weakening of transmissions between individual neurons. Here, we show that electrical stimulation may also alter the properties of the lipid membrane, thus pointing toward a novel mechanism for memory storage. Furthermore, based on the analysis of existing electrophysiological data, we study molecular mechanisms underlying the long-term potentiation in phospholipid membranes. Finally, we examine possible relationships between the memory capacitive properties of lipid membranes, neuronal learning, and memory.

Effect of Cytochrome C on the Conductance of Asolectin Membranes and the Occurrence of Through Pores at Different pHs

The study of the electrical parameters of asolectin bilayer lipid membranes in the presence of cytochrome c (cyt c) at various concentrations showed that an increase in the concentration of cyt c leads to an increase in the membrane conductance and the appearance of through pores. The studied membranes did not contain cardiolipin, which is commonly used in studying the effect of cyt c on membrane permeability. In the presence of cyt c, discrete current fluctuations were recorded. The occurrence of these fluctuations may be associated with the formation of through pores. The diameter of these pores was ~0.8 nm, which is smaller than the size of the cyt c globule (~3 nm). Measurements carried out at pH values from 6.4 to 8.4 showed that the concentration dependence of the membrane conductance increases with increasing pH. To assess the binding of cyt c to the bilayer, we measured the concentration and pH dependences of the difference in surface potentials induced by the unilateral addition of cyt c. The amount of bound cyt c at the same concentrations decreased with increasing pH, which did not correspond to the conductance trend. An analysis of conductance traces leads to the conclusion that an increase in the integral conductance of membranes is associated with an increase in the lifetime of pores. The formation of "long-lived" pores, of which the residence time in the open state is longer than in the closed state, was achieved at various combinations of pHs and cyt c concentrations: the higher the pH, the lower the concentration at which the long-lived pores appeared and, accordingly, a higher conductance was observed. The increase in conductance and the formation of transmembrane pores are not due to the electrostatic interaction between cyt c and the membrane. We hypothesize that an increase in pH leads to a weakening of hydrogen bonds between lipid heads, which allows cyt c molecules to penetrate into the membrane. This disrupts the order of the bilayer and leads to the occurrence of through pores.