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

    Year: 2001

    Journal: J. Phys. Chem. B 2001, 105, 1464-1471, 20111221

    Authors: T. Buffeteau,* E. Le Calvez, B. Desbat, I. Pelletier, and M. Pezolet

    Organizations: Laboratoire de Physico-Chimie Moleculaire, UMR 5803 du CNRS, Universite Bordeaux I, 33405 Talence, France and Centre de Recherche en Sciences et Ingenierie des Macromolecules, Departement de Chimie, Universite Laval, Quebec, G1K 7P4, Canada

    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 x 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.