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Some Further Experiments on Bimolecular Lipid Membranes

T. Hanai,D. Haydon,Janet Taylor

Published 1965 in The Journal of General Physiology

ABSTRACT

The authors of this paper have previously reported some electrical properties of simple lipoidal bimolecular leaflets in aqueous solutions (1). Recently, further investigations along similar lines have been carried out, the results of which are summarized in the following paragraphs. In the earlier experiments it was established that a bilayer of pure egg phos-phatidyl choline containing less than 50 per cent by volume of n-decane had a capacitance of 0.38 /ff/cm 2 , this value being independent of applied voltage up to at least 50 my and independent of frequency between 200 and 5 X 106 cs. It was also unaffected by the nature and concentration of the surrounding electrolyte solution for NaCl, KC1, and CaC1 2. In an attempt to discover more about the electrical properties of the film at frequencies below 200 cps the time dependence of the response to a sudden change in the D.C. potential across the film was examined. The transients were characterized by a single time constant and, using the measured value of the D.C. resistance, a value of the capacitance of the film was calculated. The capacitance was thus found to be not significantly different from 0.38 #uf/cm 2 at frequencies of from 0.16 to 0.001 cs and applied voltages of up to 50 my. The consideration of these results in terms of the molecular structure of the bilayer raises the question of the relative contributions to the membrane capacitance and conductance of the hydrocarbon and phosphatidyl choline layers respectively. It has already been inferred from experimental results (1) that both the capacitance and conductance of the polar group layer are completely masked by the hydrocarbon region, and that the measurements therefore relate only to the hydrocarbon region. As the aqueous phase, polar group layer, and hydrocarbon constitute a three layer dielectric (see for instance Schwan (2)), the detailed equations for this structure were derived. By inserting reasonable values for the capacitance and conductance of the polar group layer it has been shown that although the relaxation frequency introduced by the polar groups may be either high or low, depending on their 59 The Journal of General Physiology

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