Raman spectroscopy has been used in investigating the conformational transitions of poly-L-alanine (PLA) induced by mechanical deformation. We see evidence of the alpha-helical, antiparallel beta-sheet, and a disordered conformation in PLA. The disordered conformation has not been discussed in previous infrared and X-ray diffraction investigations and may have local order similar to the left-handed 31 poly glycine helix. The amide III mode in the Raman spectrum of PLA is more sensitive than the amide I and II modes to changes in secondary structure of the polypeptide chain. Several lines below 1200 cm−1 are conformationally sensitive and may generally be useful in the analysis of Raman spectra of proteins. A line at 909 cm−1 decreases in intensity after deformation of PLA. In general only weak scattering is observed around 900 cm−1 in the Raman spectra of antiparallel beta-sheet polypeptides.

The Raman spectra of the amide N–H deuterated PLA and poly-L-leucine (PLL) in the alpha-helical conformation and poly-L-valine (PLV) in the beta-sheet conformation are presented. Splitting is observed in the amide III mode of PLV and the components of this mode are assigned. The Raman spectrum of an alpha-helical random copolymer of L-leucine and L-glutamic acid is shown to be consistent with the spectra of other alphahelical polypeptides.