Anisotropic Optical Constants of α-Helix and β-Sheet Secondary Structures in the Infrared

Pure α-helices and antiparallel β-sheets were obtained using poly-γ-benzyl-l-glutamate (PBG) and model synthetic peptide K(LK)7, respectively. Monolayers of these polypeptides were transferred by the Langmuir−Blodgett technique on calcium fluoride plates and gold mirrors. The optical constants (index of refraction and extinction coefficient) were determined in the space coordinate system from normalized polarized transmittance and reflectance spectra. Considering the symmetry of the two secondary structures, the anisotropic optical constants were calculated in the molecular coordinate system of the α-helix and the β-sheet. From anisotropic components of the extinction coefficients, oscillator strengths of amide I and amide II modes have been calculated for the two secondary structures. These data give important information on the relative intensities of the amide I and amide II modes; moreover, for PBG the angle between the transition moment of the amide I mode and the helix axis has been estimated to be 34−38°. Finally, on the basis of these anisotropic optical constants, PM-IRRAS spectra of a single monolayer at the air/water interface were simulated for the two secondary structures.