Characterization of Gramicidin A in a Lecithin Bilayer during Ethanol-Induced Interdigitation

Abstract

Differential scanning calorimetry, intrinsic fluorescence, and ³¹P-NMR were used to study an interdigitated 1,2-Dimyristoyl- sn -Glycero-3-Phosphocholine (DMPC) membrane containing a gramicidin-A integral protein. DMPC, a biologically important 14-carbon diacyl lecithin, has previously been studied in interdigitated ethanol solutions. It is known to exhibit the biphasic effect, hysteresis, and distinctive endothermic peaks, all of which undergo notable changes in the presence of integral proteins. The transition from the interdigitated to the liquid crystalline phase was shown to involve multiple endothermic peaks, indicating the formation of laterally segregated microdomains. The threshold concentration of ethanol required to induce interdigitation was found to increase in the presence of Gramicidin A. Red-edge excitation shift fluorescence spectroscopy monitored the conformation of gramicidin before and after interdigitation, effectively measuring the effects of the interdigitated phase on the protein?s conformation in the membrane. The overall effects of DMPC?s main transition from the interdigitated state to the liquid crystalline were thus assessed with respect to both the protein and membrane components of the system.