Tunable Voltage Clamp Method: Application to Photoelectric Effects in Pigmented Bilayer Lipid Membranes

Abstract

Pulsed dye laser excitation of magnesium porphyrins contained in bilayer lipid membranes produced a transient electrical response across the membrane caused by electron transfer from the pigment to the acceptor (ferrocyanide ion) in the adjacent aqueous phase, followed by their return. The photoresponse is measured by a tunable voltage clamp (TVC) method which is essentially a potentiostatic method with adjustable access impedance (time resolution 150 nsec). Comparison with the ideal potentiostatic and the ideal galvanostatic methods shows that the TVC method is superior in providing a complete characterization of the system in terms of an irreducible equivalent circuit. An intrinsic relaxation time constant identified with the reverse interfacial electron transfer is uniquely determined. This time constant can be decomposed into a resistive and a capacitative element. This novel chemical capacitance is required to account for the experimental observations. The model and the method are relevant to vision and photosynthesis research. The TVC method is also potentially applicable to photoelectrochemistry wherever an interfacial photoreaction is present.