Polarized Raman scattering from a helical biopolymer

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Abstract
Polarized Raman spectra are reported for thin films of oriented α−helices of poly−γ−benzyl−l−glutamate. All four independent spectra were measured using 180° scattering at near−normal incidence and 90° scattering at grazing incidence. Polarization ratios are analyzed in terms of scattering from a helical array of units with a Raman tensor of axial symmetry and definite orientation depending on the mode involved. The method is applied to three stretching modes: the amide I, the ester carbonyl stretch, and the N−H stretch. Orientation angles of the principal axis relative to the helix axis are found to be 39°±2°, 55°±3°, and 28°±3°, respectively, in good agreement with the results of infrared dichroism.