The infrared dichroism of transmembrane helical polypeptides

Polarized attenuated total internal reflectance techniques were applied to study the infrared dichroism of the amide I transition moment in two membrane-bound peptides that are known to form oriented transmembrane helices: gramicidin A in a supported phospholipid monolayer and Ac-Lys2-Leu24-Lys2-amide (L24) in oriented multibilayers. These studies were performed to test the ability of these techniques to determine the orientation of these peptides, to verify the value of optical parameters used to calculate electric field strengths, to examine the common assumptions regarding the amide I transition moment orientation, and to ascertain the effect of surface imperfections on molecular disorder. The two peptides exhibit marked differences in the shape and frequency of their amide I absorption bands. Yet both peptides are highly ordered and oriented with their helical axes perpendicular to the membrane surface. In the alpha-helix formed by L24, there is evidence for a mode with type E1 symmetry contributing to amide I, and the amide I transition moment must be more closely aligned with the peptide C=O (< 34 degrees) than earlier studies have suggested. These results indicate that long-standing assumptions about the orientation of amide I in a peptide require some revision, but that in general, infrared spectroscopy yields reliable information about the orientation of membrane-bound helical peptides.