Laser‐Raman spectroscopy of biomolecules. XI. Conformational study of poly( L ‐valine) and copolymers of L ‐valine and L ‐alanine

Raman spectroscopic studies have been carried out on polymers of L ‐valine ranging in degree of polymerization ( DP ) from 2 to 930. The spectrum of the hexapeptide ( DP = 6) is closely similar over the entire range 40–1750 cm −1 to those of polymers with much higher DP , and the structure is clearly shown to be that of the antiparallel pleated sheet (β‐structure) by the amide I and III frequencies. The formation of a little α‐helical structure occurs in polymers with DP above 500, although the amount does not appear to be a linear function of DP . The α‐helical structure is unstable and readily destroyed in samples cast from trifluoroacetic acid solution. It is stabilized by the incorporation of L ‐alanine, a strong helix‐former; polymers of the latter may in turn be forced into a α‐structure in copolymers sufficiently rich in L ‐valine.