Raman spectroscopic study of mechanically deformed poly‐L‐alanine

Abstract 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 3 1 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.