Quantitative Orientation of α-Helical Polypeptides by Attenuated Total Reflection Infrared Spectroscopy

Abstract

Spectral simulations of polarized attenuated total reflection (ATR) spectra have been used to quantitatively determine the orientation of α-helical polypeptides. Transmittance and polarized ATR spectra of five monolayers of poly-γ-benzyl-L-glutamate (PBG) transferred by the Langmuir−Blodgett techniques on germanium crystal have been recorded, and the dichroic ratios of the amide I and amide II bands have been calculated. Simulations were performed using anisotropic optical constants of PBG in the molecular coordinate system and using the 4 × 4 matrix formalism of Berreman. Assuming that the α-helices are parallel to the plane of the ATR crystal, an average angle of 38° ± 1° between the helix axes and the withdrawing direction has been found. Simulated spectra for various orientations of the α-helices are then given and influence of the azimuthal and tilt angles on the dichroic ratios of the amide bands has been investigated.