The effect of a phase transition on penetration of phospholipid monolayers by melittin and glucagon

The penetration of melittin and glucagon into phospholipid monolayers was studied by measuring compression isotherms of phospholipids in the absence and presence of various concentrations of protein in the subphase. Differences in molecular area were calculated as a function of protein concentration at constant pressure. Area change as a function of surface pressure at constant protein concentration was also calculated. Melittin showed greater affinity for penetration into phosphatidylglycerol (PG) than into phosphatidylcholine (PC) monolayers. The cutoff pressure for melittin penetration was 45 mN/m with PC and 60 mN/m (extrapolated) with PG. Dipalmitoyl PC and PG monolayers show phase transitions upon compression at 25 degrees C. Both melittin and glucagon showed increased penetration as measured by area change within the region of the phase transition with both lipids. Glucagon showed a cutoff pressure of 25 mN/m for penetration into dimyristoyl PC. The preference of glucagon for interaction with lipid bilayers in the gel phase is discussed with respect to monolayer penetration as a function of surface pressure.