Polarization-Modulated FTIR Spectroscopy of Lipid/Gramicidin Monolayers at the Air/Water Interface

AbstractMonolayers of gramicidin A, pure and in mixtures with dimyristoylphosphatidylcholine (DMPC), were studied in situ at the air/H2O and air/D2O interfaces by polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS). Simulations of the entire set of amide I absorption modes were also performed, using complete parameter sets for different conformations based on published normal mode calculations. The structure of gramicidin A in the DMPC monolayer could clearly be assigned to a β6.3 helix. Quantitative analysis of the amide I bands revealed that film pressures of up to 25–30mN/m the helix tilt angle from the vertical in the pure gramicidin A layer exceeded 60°. A marked dependence of the peptide orientation on the applied surface pressure was observed for the mixed lipid-peptide monolayers. At low pressure the helix lay flat on the surface, whereas at high pressures the helix was oriented almost parallel to the surface normal