Conformational studies on the gramicidin A transmembrane channel in lipid micelles and liposomes

The interaction of gramicidin A with lysolecithin micelles and with lecithin liposomes is demonstrated by circular dichroism to result in several metastable conformational states. A stable state can be obtained after extensive heating when the gramicidin A was added dry or in ethanol solution to the phospholipid dispersion but the stable state is readily obtained when gramicidin A is added in a trifluoroethanol solution. The circular dichroism of the stable conformational states is characterized by negative ellipticity below 205 nm and principally by a positive 220 nm band on which is superposed a weak 230 nm band (the latter likely arising from tryptophan side chains). The stable conformational state is considered to be that of the functional transmembrane channel primarily on the basis of extensive studies on its interaction with sodium ions.