eSpectroscopic and Structural Properties of Valine Gramicidin A in Monolayers at the Air-Water Interface

Monomolecular films of valine gramicidin A (VGA) were investigated in situ at the air-water interface by x-ray reflectivity and x-ray grazing incidence diffraction as well as polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). These techniques were combined to obtain information on the secondary structure and the orientation of VGA and to characterize the shoulder observed in its π-A isotherm. The thickness of the film was obtained by x-ray reflectivity, and the secondary structure of VGA was monitored using the frequency position of the amide I band. The PM-IRRAS spectra were compared with the simulated ones to identify the conformation adopted by VGA in monolayer. At large molecular area, VGA shows a disordered secondary structure, whereas at smaller molecular areas, VGA adopts an anti-parallel double-strand intertwined β5.6 helical conformation with 30° orientation with respect to the normal with a thickness of 25Å. The interface between bulk water and the VGA monolayer was investigated by x-ray reflectivity as well as by comparing the experimental and the simulated PM-IRRAS spectra on D2O and H2O, which suggested the presence of oriented water molecules between the bulk and the monolayer.