VIBRATIONAL CIRCULAR DICHROISM IN AMIDE STRETCHING VIBRATIONS FOR A VARIETY OF POLYPEPTIDES

$^{1}$J. Snir, R. A. Frankel and J. A. Schellman, and Biopolymers (1975), 14, 173. $^{2}R$. D. Singh and T. A. Keiderling, Biopolymers (1981), 20, 237.Author Institution:Vibrational circular dichroism (VCD) spectra have been obtained for a variety of synthetic polypeptides to determine the effects of helical conformation on VCD observed in the amide I (carbonyl stretching) region as well as the amide N-H stretching and the C-H stretching regions. Spectra have been obtained for poly-$\gamma$-benzyl-L-glutamate, poly-$\beta$-benzyl-aspartate, poly-im-benzyl-histidine. poly-$\epsilon$-CBZ-lysine, poly-O-CBZ-tyrosine and poly-methionine in chloroform solutions and for poly-glutamic acid in $D_{2}O$ solution $(P_{D}\approx 4)$ and poly-$\gamma$-methyl-glutamate in trifluoroethanolod solution. All of these polypeptides form $\alpha $-helices. In addition, we have obtained the spectrum of poly-lysine $\bullet$ HBr ($P_{D} \approx 9$)in D O in the random coil conformation. We observe sigmoidally shaped VCD complets in the amide I region which are negative toward higher wavenumbers and positive toward lower for right handed $\alpha$-helices. This is in agreement with the earlier $prediction^{1}$ and with the recent $measurement^{2}$ of VCD of poly-$\gamma$-benzyl-L-glutamate. We find that the sign of the VCD in this region depends on conformation of $\alpha$-helix and not upon the configuration on (L or D) of the peptide units. Thus our present VCD studies of polypeptides demonstrate that the characteristic VCD couplet in the amide I region can be used as a probe of helical and random-coil conformations in polypeptides. Additional trends in these VCD spectra will also be discussed