Knowledge Bank

Fourier transform infrared vibrational circular dichroism spectra in the amide I' region of poly(L-lysine) in $D2O$ solutions have confirmed the existence of three distinct conformational states and an unordered state in this homo-polypeptide, each with a unique, characteristic VCD spectrum. The right-handed $\alpha$-helix gives rise to a (-+-) VCD pattern, the antiparallel $\beta$-sheet to two strong negative features, and the extended helix (formerly denoted the random coil) to a (+-) VCD couplet, for the amide I' vibrations. In addition, a completely unordered conformation can be obtained which is characterized by the absence of any amide I' VCD. The negative VCD in the $\beta$-sheet conformation is interpreted in terms of vibrationally generated ring currents.