Fourier transform infrared spectroscopic studies of the interaction of theantimicrobial peptide gramicidin S with lipid micelles and with lipid monolayer and bilayer membranes

We have utilized Fourier transform infrared spectroscopy to study the inter action of the antimicrobial peptide gramicidin S (GS) with lipid micelles a nd with Lipid monolayer and bilayer membranes as a function of temperature and of the phase state of the lipid. Since the conformation of GS does not change under the experimental conditions employed in this study, we could u tilize the dependence of the frequency of the amide I band of the central b eta-sheet region of this peptide on the polarity and hydrogen-bonding poten tial of its environment to probe GS interaction with and location in these lipid model membrane systems. We find that the GS is completely or partiall y excluded from the gel states of all of the lipid bilayers examined in thi s study but strongly partitions into lipid micelles, monolayers, or bilayer s in the liquid-crystalline state. Moreover, in general, the penetration of GS into zwitterionic and uncharged lipid bilayer coincides closely with th e gel to liquid-crystalline phase transition of the lipid. However, GS begi ns to penetrate into the gel-state bilayers of anionic phospholipids prior to the actual chain-melting phase transition, while in cationic lipid bilay ers, GS does not partition strongly into the liquid-crystalline bilayer unt il temperatures well above the chain-melting phase transition are reached. In the liquid-crystalline state, the polarity of the environment of GS indi cates that this peptide is located primarily at the polar/apolar interfacia l region of the bilayer near the glycerol backbone region of the lipid mole cule. However, the depth of GS penetration into this interfacial region can vary somewhat depending on the structure and charge of the lipid molecule. In general, GS associates most strongly with and penetrates most deeply in to more disordered bilayers with a negative surface charge, although the de tailed chemical structure of the lipid molecule and physical organization o f the lipid aggregate (micelle versus monolayer versus bilayer) also have m inor effects on these processes.