Semiconductive and photoconductive properties of bimolecular lipid membranes

Abstract

Some lipids, both in the solid state and as bimolecular lipid membranes (a good model system for biological membranes), have a temperature dependence of conductivity given by the compensation law equation typical of organic semiconductors: σ(T)=σ^′₀ exp [E/2 kT₀], exp [–E/2 kT], where E and T₀ are identical in both states. These lipids form donor-acceptor complexes with a number of acceptor molecules as I₂, 2,4-DNP, picric acid, CLCCP etc. The conductivity of the complexes in both states is many orders of magnitude higher than that of the lipid alone. Only E is decreased by complexation; T₀ and σ₀ are unchanged. Acceptor molecules added to one side of a bilayer produces semiconductive rectifier behaviour, Zener diode-like characteristics, and mechanical and electrical oscillations. With some different molecules on opposite sides of the bilayer, applied electric fields produce interesting electrochemical changes at the membrane-water interface, including precipitates which change the electrical properties of the membrane, and electrodeposition of specularly reflecting metallic mirrors. These results support the contention that both electronic and ionic carriers can migrate through the membrane. For the former, an electric field-controlled oxidation-reduction system on opposite sides of the membrane must occur.

In the presence of ferric and ceric ions in solution, large photo-e.m.f. changes ( >40mV) and photoconductance changes ( >100x) occur when the membrane is irradiated. It is suggested that donation to the excited ionic species of an electron by the membrane accounts for these effects.