Lipid discrimination in phospholipid monolayers by the antimicrobial frog skin peptide PGLa. A synchrotron X-ray grazing incidence and reflectivity study.

We present a first study using synchrotron grazing incidence diffraction and X-ray reflectivity measurements on mixed phospholipid/peptide monolayers at the air/water interface. The thermodynamic properties of the pure and mixed monolayers were characterized using the classical film balance technique. Surface pressure/potential-area isotherms showed that the antimicrobial frog skin peptide PGLa formed a very stable monolayer with two PGLa molecules per kinetic unit and a collapse pressure of ~22 mN/m. X-ray grazing incidence diffraction indicated that the peptide-dimer formation did not lead to self-aggregation with subsequent crystallite formation. However, the scattering length density profiles derived from X-ray reflectivity measurements yield information on the PGLa monolayer that protrudes into the air phase by about 0.8 nm, suggesting that the peptide is aligned parallel to the air/water interface. The monolayers, composed of disaturated phosphatidylcholines or phosphatidylglycerols, were stable up to 60 mN/m and exhibited a first-order transition from a liquid-expanded to a liquid-condensed state around 10 mN/m. Structural details of the phospholipid monolayers in the presence and absence of PGLa were obtained from synchrotron experiments. Thereby, the X-ray data of distearoylphosphatidylcholine/PGLa can be analyzed by being composed of the individual components, while the peptide strongly perturbed the lipid acyl chain order of distearoylphosphatidylglycerol. These results are in agreement that PGLa mixes at a molecular level with negatively charged lipids, but forms separate islands in zwitterionic phosphatidylcholine monolayers and demonstrates that antimicrobial peptides can discriminate between the major phospholipid components of bacterial and mammalian cytoplasmic membranes.