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Published December 1, 1973 | public
Journal Article

The nature of the voltage-dependent conductance induced by alamethicin in black lipid membranes


Alamethicin induces a conductance in black lipid films which increases exponentially with voltage. At low conductance the increase occurs in discrete steps which form a pattern of five levels, the second and third being most likely. The conductance of each level is directly proportional to salt concentration, inversely proportional to solution viscosity, and nearly independent of voltage. The probability distribution of the five steps is not a function of voltage, but as the voltage is increased, more levels begin to appear. These can be explained as superpositions of the original five, both in position and relative probability. This suggests that the five levels are associated with a physical entity which we call a pore. This point of view is confirmed by the following measurements. The kinetic response of the current to a voltage step is first order, and shows an exponential increase in rate of pore formation and an exponential decrease in rate of pore disappearance with voltage. If these rates are statistical, the number of pores should fluctuate about a voltage-dependent mean. High conductance current fluctuations are too large to be explained by fluctuation in the number of pores alone. But if fluctuations among the five levels are included, the magnitude of the fluctuations at high conductance is accurately predicted. Alamethicin adsorbs reversibly to the membrane surface, and the conductance at a fixed voltage depends on the ninth power of alamethicin concentration and on the fourth power of salt concentration, in the aqueous phase. In our bacterial phosphatidyl ethanolamine membranes, alamethicin added to one side of the membrane produces elevated conductance only when the voltage on that side is increased.

Additional Information

© 1973 by Springer-Verlag New York Inc. Received 12 June 1973. We thank Professor Max Delbrück for many useful suggestions and for critical readings of the manuscript. We are grateful to Dr. Ed Lipson for suggesting the simple derivation of Eq.(17) and to Professor Leon Bruner for suggesting the glycerol experiments. We also thank Mr. H. M. Simpson for the skillful design and construction of several membrane cells. M. E. is grateful to the Rockefeller Foundation for support and J. E. H. acknowledges the support of the Sloan Foundation and a National Institutes of Health Fellowship, No. 1 F03 GM55043-01.

Additional details

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