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The αII-helix (ϕ = −70.47°, ψ = −35.75°) is a structure having the same n and h as the (standard) αI-helix (ϕ = −57.37°, ψ = −47.49°). Its conformational angles are commonly found in proteins. Using an improved α-helix force field, we have compared the vibrational frequencies of these two structures. Despite the small conformational differences, there are significant predicted differences in frequencies, particularly in the amide A, amide I, and amide II bands, and in the conformation-sensitive region below 900 cm−1. This analysis indicates that αII-helices are likely to be present in bacteriorhodopsin [Krimm, S. & Dwivedi, A. M. (1982) Science216, 407–408].

Synopsis Raman and polarized ir spectra have been obtained on built-up monomolecular films of poly(a-aminoisobutyric acid), and analyzed in the context of normal mode calculations on 310-, a-, and a’-helix conformations of this molecule. The average discrepancy between observed and calculated frequencies is significantly smaller for the 3,,helix than for the other structures. This, together with the more satisfactory explanation of several special features of the spectra, indicates that this polypeptide adopts a 310-helix conformation in such thin films.

A force field has been refined for the 31-helix structure of polyglycine 11, using the polyglycine I force field plus previous CaHa. . .O force constants as a starting point. Besides force constants associated with the hydrogen bonds, which must change since the hydrogen-bond characteristics are different in the two structures, we have had to modify only 10 force constants from the polyglycine I force field to make it suitable for reproducing the polyglycine I1 frequencies. Most involve the NC" bond, which is the torsion angle that changes from the I to the I1 structure. Calculations were done for parallel chain and antiparallel chain crystal structures of polyglycine 11, the observed spectra being found to agree best with the latter structure. Since this provides strong evidence for the loss of strict threefold symmetry in the chain, our analysis strengthens the support for the existence of OHU. . .O hydrogen bonds in the structure of polyglycine 11.

Using Wilson's GF-matrix method as modified by Higgs for infinite helical polymers, dispersion curves and the frequency distribution function have been calculated for poly-L-proline II chain. Infrared spectrum is obtained and a Urey-Bradley force field, which provides best fit with the observed frequencies, is evaluated. The result are discussed from the viewpoint of the conformational characteristics of forms I and II.

A complete normal coordinate analysis, infrared absorption, and inelastic neutron scattering studies on α-glycylglycine are reported. A Urey-Bradley force field which is also valid for N-deuterated sample is obtained. Assignments in the low-frequency region are based on inelastic neutron spectrum obtained in one phonon and cubic approximation.