Knowledge Bank

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-$ϵ$-CBZ-lysine, poly-O-CBZ-tyrosine and poly-methionine in chloroform solutions and for poly-glutamic acid in $D2O$ solution $(PD\approx 4)$ 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 ($PD\approx 9$)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 $prediction1$ and with the recent $measurement2$ 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.