Vibrational CD of polypeptides. XII. Reevaluation of the fourier transform vibrational CD of poly(γ‐benzyl‐L‐glutamate)

Using a newly constructed Fourier transform ir (FTIR) based vibrational CD (VCD) instrument, we have found that elimination of the ellipsoidal collection mirror before the detector and its replacement by a lens leads to a significant improvement in the absorption artifact problem often seen previously in FTIR‐VCD. Reduction in artifact level brings the FTIR‐VCD band shape of the amide 1 band in poly(γ‐benzyl‐ L ‐glutamate) into better agreement with that found with dispersive instruments. These results indicate that the difference between the spectra obtained with the two methods is not primarily a factor of resolution but is more dependent on artifact level. Thus the previously reported deconvolution of the amide I VCD results and their subsequent interpretation in terms of the helical vs. exciton mechanisms overemphasize weak features in the spectrum. Results based on deconvolution of the absorption spectrum remain valid. Further new data on the amide II and several lower energy transitions that encompass the amide III region show broad single‐signed features—but of the opposite sense—in the two regions, implying that their VCD arises through mutual coupling.