Intensity of the Polarized Raman Band at 1340−1345 cm-1 As an Indicator of Protein α-Helix Orientation:  Application to Pf1 Filamentous Virus

Raman spectra of oriented alpha-helical protein molecules exhibit a prominent band near 1340-1345 cm(-)(1), the intensity of which is highly sensitive to molecular orientation. Polarization of the 1340-1345 cm(-)(1) marker is evident in Raman spectra of alpha-helical poly-L-alanine (alphaPLA) and alpha-helical poly-gamma-benzyl-L-glutamate (alphaPBLG). Corresponding polarization is also observed in Raman spectra of the filamentous virus Pf1, which is an assembly of alpha-helical coat protein molecules. In alphaPLA and alphaPBLG, we assign the band to a normal mode of symmetry type E(2) and specifically to a vibration localized in the (O=C)-C(alpha)-H linkages of the main chain peptide group. Although strict helical symmetry does not apply to coat subunits of filamentous viruses, an approximate E(2)-type mode may be presumed to account for a corresponding Raman band of Pf1 and fd filamentous viruses. Spectroscopic studies of N-methylacetamide and isotopically-edited fd viruses support the present assignment of the 1340-1345 cm(-)(1) band. Polarization anisotropy indicates that this band may be exploited as a novel indicator of protein alpha-helix orientation. Application of this approach to the polarized Raman spectrum of Pf1 suggests that, on average, the axis of the alpha-helical coat protein subunit in the native virion structure forms an angle of 20 +/- 10 degrees with respect to the virion axis.