The mechanisms of ion movement through lipid bilayers will be studied with emphasis on ion transport through, and ion selectivity of, polypeptide channels incorporated into the bilayer. Specifically the behavior of channels formed by gramicidin A will be investigated in detail. Ion movement through gramicidin A channels in diphytanoylphosphatidylcholine membranes will be studied for the alkali metal cations and other monovalent cations using single-channel current-voltage characteristics, ion tracer fluxes, as functions of the aqueous activity of the permeant ion, as well as the equilibrium binding of the ion to the channel as the primary experiments. These data will be used to develop a model for ion movement through gramicidin A channels. It should then be possible to predict the ion-selectivity properties of the channels, these predictions will be tested by biionic potential measurements as well as single-channel conductance measurements in symmetrical mixtures of two permeant ions. Serious discrepancies between predictions and experiment will be investigated further to characterize their molecular basis, in particular to see if ion-induced structural changes can occur. Other experiments will be concerned with the molecular behavior of the gramicidin A channels, the kinetics of channel formation and channel dissociation will be studied as functions of the type and concentration of the permeant ions, to see to what extent permeating ions can influence the channel behavior on a rather gross level. Experiments with different gramicidin A analogs, and with membranes of different lipid composition will be used to obtain a clearer picture of the molecular details of ion movement through these channels. The information thus obtained will be used to illustrate principles of ion movement through single-filing channels. They will also be used to elucidate how phloretin, a powerful membrane-modifier, will affect a transmembrane channel.
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