NONLAMELLAR PHASES INDUCED BY THE INTERACTION OF GRAMICIDIN-S WITH LIPID BILAYERS - A POSSIBLE RELATIONSHIP TO MEMBRANE-DISRUPTING ACTIVITY

The interactions of the cyclic peptide gramicidin S (GS) with a variet y of single-component lipid bilayers, and with membrane polar lipid ex tracts of Acholeplasma laidlawii B and Escherichia coli, were examined by differential scanning calorimetry (DSC), P-31-nuclear magnetic res onance (NMR) spectroscopy, and X-ray diffraction. The DSC data indicat e that the effects of GS on the thermotropic phase behavior of phospha tidylcholine and phosphatidylethanolamine dispersions are compatible w ith those expected of peptides interacting primarily with the polar he adgroup and/or the polar/apolar interfaces of lipid bilayers. These DS C studies also suggest that GS exhibits stronger interactions with the more fluid bilayers, For mixtures of GS with lipids such as phosphati dylcholine, phosphatidylserine, cardiolipin, and sphingomyelin, axiall y symmetric P-31-NMR powder patterns are observed throughout the entir e temperature range examined (0-90 degrees C), and there is little evi dence for significant destabilization of the lipid bilayer with respec t to nonlamellar phases. With mixtures of GS with either phosphatidyle thanolamine, phosphatidylglycerol, or a nonlamellar phase-forming phos phatidylcholine, axially symmetric P-31-NMR powder patterns are also o bserved at low temperatures. However, at high temperatures, an isotrop ic component is observed in their P-31-NMR spectra, and the relative i ntensity of this component increases significantly with temperature an d with GS concentration. Once formed at high temperatures, this isotro pic component exhibits a marked cooling hysteresis and in most cases d isappears only when the sample is recooled to temperatures well below the lipid hydrocarbon chain-melting phase transition temperature. We a lso show that GS induces the formation of isotropic components in the P-31-NMR spectra of heterogeneous lipid mixtures such as occur in A. l aidlawii B and E. coli membranes. These observations suggest that CS i nduces the formation of cubic or other three dimensionally ordered inv erted nonlamellar phases when it interacts with some types of lipid bi layers, a suggestion strongly supported by our X-ray diffraction studi es, Our results also suggest that the capacity of GS to induce the for mation of such phases increases with the intrinsic nonlamellar phase-p referring tendencies of the lipids with which it interacts probably by producing localized increases in membrane monolayer curvature stress. The latter effect could be part of the mechanism through which this p eptide exhibits its antimicrobial and hemolytic activities.