Kinetics of ion transport in lipid membranes induced by lysine-valinomycin and derivatives.

Abstract

Lysine-valinomycine and two N epsilon-acyl derivatives are compared with respect to their potency to transport Rb+ ions across thin lipid membranes. Lysine-valinomycin acts as a neutral ion carrier only above a pH of about 7 of the aqueous solutions, while at lower pH the molecules seem to be positively charged due to a protonation of the epsilon-NH2 group of the lysine residue. A kinetic analysis based on voltage jump relaxation experiments and on the nonlinearity of the current-voltage characteristics showed that the conductance increment delta per carrier molecule for uncharged lysine-valinomycin is similar to that of natural valinomycin. The attachment of a rather bulky side group such as the dansyl or para-nitrobenzyloxycarbonyl group reduced delta by approximately one order of magnitude. Some of the relaxation data of the valinomycin analogues were influenced by an unspecific relaxation of the pure lipid membrane. This structural relaxation represents a limitation to the possibility of analyzing specific transport systems in thin lipid membranes by the voltage jump or charge pulse techniques. It is shown that the time dependence of this structural relaxation--which was first published by Sargent (1975)--is at variance with a three capacitor equivalent circuit of the membrane, which was suggested by Coster and Smith (1974) on the basis of a.c. measurements. A modified equivalent circuit has been found to represent a satisfactory analogue for the current relaxation in the presence of valinomycin. It turned out, however, that such an equivalent circuit provides little insight into the molecular mechanism of transport.