Solvent history dependence of gramicidin-lipid interactions: a Raman and infrared spectroscopic study

We have investigated the interactions between gramicidin and a model membrane composed of one phospholipid, dimyristoylphosphatidylcholine, as a function of the cosolubilization solvent and incubation time used in the sample preparation. Three organic solvents have been used; trifluoroethanol, a mixture of methanol/chloroform (1:1 v/v), and ethanol. Using Fourier transform infrared spectroscopy, we have demonstrated that the conformation adopted by gramicidin in the membrane is dependent upon the cosolubilization solvent used, and, only with trifluoroethanol, it is possible to incorporate gramicidin entirely as a beta 6.3-helix. Moreover, Raman spectroscopy results indicate that the orientation of the tryptophan side chains in gramicidin and their interaction with the hydrocarbon chains and the carbonyl groups of the lipids are also dependent on the cosolubilization solvent. On the other hand, the effect of the incorporation of gramicidin on the thermotropism of the lipid bilayer was found to be dependent upon the conformation of gramicidin in the lipid bilayers.