Differential scanning calorimetric study of the effect of the antimicrobial peptide gramicidin S on the thermotropic phase behavior of phosphatidylcholine, phosphatidylethanolamine and phosphatidylglycerol lipid bilayer membranes

We have studied the effects of the antimicrobial peptide gramicidin S (GS) on the thermotropic phase behavior of large multilamellar vesicles of dimyr istoylphosphatidylcholine (DMPC), dimyristoylphosphatidylethanolamine (DMPE ) and dimyristoyl phosphatidylglycerol (DMPC) by high-sensitivity different ial scanning calorimetry. We find that the effect of GS on the lamellar gel to liquid-crystalline phase transition of these phospholipids varies marke dly with the structure and charge of their polar headgroups. Specifically, the presence of even large quantities of GS has essentially no effect on th e main phase transition of zwitterionic DMPE vesicles, even after repeating cycling through the phase transition, unless these vesicles are exposed to high temperatures, after which a small reduction in the temperature, entha lpy and cooperativity of the gel to liquid-crystalline phase transitions is observed. Similarly, even large amounts of GS produce similar modest decre ases in the temperature, enthalpy and cooperativity of the main phase trans ition of DMPC vesicles, although the pretransition is abolished at low pept ide concentrations. However, exposure to high temperatures is not required for these effects of CS on DMPC bilayers to be manifested. In contrast, GS has a much greater effect on the thermotropic phase behavior of anionic DMP G vesicles, substantially reducing the temperature, enthalpy and cooperativ ity of the main phase transition at higher peptide concentrations, and abol ishing the pretransition at lower peptide concentrations as compared to DMP C. Moreover, the relatively larger effects of GS on the thermotropic phase behavior of DMPG vesicles are also manifest without cycling through the pha se transition or exposure to high temperatures. Furthermore, the addition o f GS to DMPG vesicles protects the phospholipid molecules from the chemical hydrolysis induced by their repeated exposure to high temperatures. These results indicate that GS interacts more strongly with anionic than with zwi tterionic phospholipid bilayers, probably because of the more favorable net attractive electrostatic interactions between the positively charged pepti de and the negatively charged polar headgroup in such systems. Moreover, at comparable reduced temperatures, GS appears to interact more strongly with zwitterionic DMPC than with zwitterionic DMPE bilayers, probably because o f the more fluid character of the former system. In addition, the general e ffects of GS on the thermotropic phase behavior of zwitterionic and anionic phospholipids suggest that it is located at the polar/apolar interface of liquid-crystalline bilayers, where it interacts primarily with the polar he adgroup and glycerol-backbone regions of the phospholipid molecules and onl y secondarily with the lipid hydrocarbon chains. Finally, the considerable lipid specificity of GS interactions with phospholipid bilayers may prove u seful in the design of peptide analogs with stronger interactions with micr obial as opposed to eucaryotic membrane lipids. (C) 1999 Elsevier Science B .V. All rights reserved.