Conformation of Gramicidin in Monolayers, Organic Solvents and Phospholipid Bilayers

The properties of gramicidin and its action on membranes were investigated by the use of monolayers, organic solvents and phospholipid vesicles in relation to cation-binding. The behavior of a gramicidin monolayer was strongly dependent on the organic solvent in which it was dissolved, showing the dependence of the solvent history. A gramicidin monolayer prepared from an ethanol solution caused a large conformational change upon the binding of K+, whereas that prepared from a 2,2,2-trifluoroethanol (TFE) solution showed no conformational change upon the binding of K+. In two organic solvents, gramicidin showed a different calorimetric behavior, due to a difference in conformation. Gramicidin in ethanol markedly decreased the phase-transition temperature and transition enthalpy (ΔH) upon the binding of K+, whereas gramicidin in TFE did not change in calorimetric behavior upon the binding of K+. Incorporating gramicidin into lipid bilayers led to a broadening of the transition and a decrease in the ΔH of the lipid, showing that about 20 lipid molecules per gramicidin molecule were immobilized. By binding K+ to the gramicidin channel, the perturbing effect of gramicidin for the lipid bilayers was depressed, implying aggregation among the peptide channels.