Monolayers of gramicidin A, pure and in mixts. with dimyristoylphosphatidylcholine (DMPC), were studied in situ at the air/H2O and air/D2O interfaces by polarization-modulated IR 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 calcns. The structure of gramicidin A in the DMPC monolayer could clearly be assigned to a b6.3 helix. Quant. anal. of the amide I bands revealed that film pressures of up to 25-30 mN/m the helix tilt angle from the vertical in the pure gramicidin A layer exceeded 60 Deg. A marked dependence of the peptide orientation on the applied surface pressure was obsd. 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. [on SciFinder (R)]