Vibrational circular dichroism (VCD), the differential absorption of circularly polarized light by vibrational transitions, is a new technique of potential use for stereochemical analysis.13 Recent instrumental developments have allowed extension of the VCD technique down to 1250 cm-l, thereby encompassing some of the characteristic amide Vibrations of polypeptides? The polarized ir absorption spectra of these bands have in the past been extensively utilized to analyze the secondary structure of such polymers.5 Use of VCD should add another dimension to their conformational utility.
Schellman6 and Snir et al.7 have previously reported predictions of the VCD that might result from the coupling of the monomer vibrations in a-helical, parallel P-sheet, and antiparallel 0-sheet polypeptide structures. Using the coupling parameters of Miyazawa and Blout,B large sigmoidally shaped VCD of opposite sense was predicted for both the amide I (-1658 cm-l) and amide I1 (-15550 cm-') bands of the c~-helix.~ However, using the parameters of Krimm: the amide I1 band was predicted to have a substantially reduced magnitude and a sigmoidal shape of the same sense as predicted for the amide I band. Both models predict essentially the same absorption spectra. In our first efforts to examine aspects of this prediction and the general feasibility of measuring the VCD of peptides, we here report the VCD of poly(y-benzyl-L-glutamate) (poly[Glu(OBzl)]), which is known to exhibit an a-helical structure in CHC13 s o l ~t i o n . ~ Solutions of both the L and D-isomers were made by dissolving 15 mg of poly(Glu[OBzl]) (Sigma, 150,000 M,) in 5 ml of CHCI3 (for amide I) or, alternatively, CDCl3 (for amide 11).
VCD and absorption measurements were made on a previously described in~trument.'.~ All spectra were taken on-line with a dedicated PDP11/03 computer and were subsequently replotted. Base lines for the VCD were taken to he the average of the L and D traces and were subtracted from the data before plotting. Since the signals were so large, only small corrections were needed. The absorption spectra were digitally generated from comparison of sequential single-beam sample and solvent transmission scans.
As predicted, the amide I band gives rise to a distinctive sigmoidally shaped band a t 1658 cm-' that has its negative lobe higher in energy than its positive l ~b e . ~, ~ (See Fig. 1.) The Soc.