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SynopsisVibrational CD (VCD) spectra have been obtained in the amide I region of poly(ybenzyl-L-glutamate), using a Fourier transform ir VCD spectrometer operating at 4-cm-l resolution. These results reveal a new negative VCD band at 1650 cm-l in addition to the previously observed negative positive VCD couplet centered at 1660 cm-l . Fourier selfdeconvolution (FSD) of the VCD spectrum reveals three strong VCD component bands, rather than just two, as originally supposed. FSD also resolves the absorbance spectrum into two components corresponding to the parallel polarized A mode at lower frequency, and an unresolved perpendicular polarized El mode at higher frequency. The VCD spectrum is reassigned on the basis of this new spectral data and the FSD-VCD results are analyzed for the Moffitt contribution and the so-called helical contribution.Comparison with theory shows the VCD to be more intense than expected, particularly for the helical term. Charge flow between subunits is suggested as an additional source of VCD intensity that is presently not included in theoretical exciton descriptions. The splitting of the El mode and the large VCD contribution attributed to the helical term implies a deviation of the polypeptide structure from perfect helical symmetry and, as a result, a mixing of the A and El vibrational modes.

We report FT-IR-VCD recorded under improved conditions of signal-to-noise ratio, resolution, and spectral range (to 800 cm−1). Detailed descriptions are provided in the areas of artifact suppression, phase correction procedures, and comparison of FT-IR to dispersive VCD measurements. Spectral evidence supports the proposition that VCD absorbance artifacts are a surface effect originating at the optical window-solution interface.

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