Anisotropic optical constants of alpha-helix and beta-sheet secondary structures in the infrared

Pure a-helices and antiparallel beta-sheets were obtained using poly-gamma- benzyl-L-glutamate (PBG) and model synthetic peptide K(LK)(7), respectively . Monolayers of these polypeptides were transferred by the Langmuir-Blodget t technique on calcium fluoride plates and,gold mirrors. The optical consta nts (index of refraction and extinction coefficient) were determined in the space coordinate system from normalized polarized transmittance and reflec tance spectra. Considering the symmetry of the two secondary structures, th e anisotropic optical constants were calculated in the molecular coordinate system of the alpha-helix and the beta-sheet. From anisotropic components of the extinction coefficients, oscillator strengths of amide I and amide I I modes have been calculated for the two secondary structures. These data g ive important information on the relative intensities of the amide I and am ide II modes; moreover, for PEG the angle between the transition moment of the amide I mode and the helix axis has been estimated to be 34-38 degrees. Finally, on the basis of these anisotropic optical constants, PM-IRRAS spe ctra of a single monolayer at the air/water interface were simulated for th e two secondary structures.