Pore formation in BLM studied by means of alternate current

Porin form pores which act as molecular sieves to allow the diffusion of small, hydrophilic solutes across the outer membrane of gram-negative bacteria and mitochondria. The addition of porin to the bathing solutions of a BLM results in the formation of ion permeable pores. Pore incorporation kinetic, as detected measuring electrical membrane current, is S-shaped and a steady state is reached after many minutes depending on the porin concentration, the nature of BLM and the composition of the media. Porin incorporation kinetic was analyzed considering the phenomenon studied by means of alternate current. The membrane equivalent circuit, constituted by a parallel resistance and capacitance, was in series with an analogous circuit of the measuring device and the input voltage was applied to the whole series. The pore dependence of membrane conductance and capacitance was determined from geometrical considerations and single channel electrical characteristics. A non linear decreasing of membrane resistance, a linear decreasing of capacitance, and then a non linear increasing of reactance, are obtained increasing the number of the pores. Analytical time dependence of pore formation was introduced with a four-parameter logistic equation or some exponential functions. A set of differential equations describes the circuit. The current depends not only on the membrane resistance and capacitance but, during kinetic phase, also on the time derivative of the capacitance. Because of this time dependence of membrane electrical parameters the phase angle varies during channel formation.