Surface enhanced resonance Raman scattering (SERRS) as a probe of the structural differences between the Pr and Pfr forms of phytochrome

Surface enhanced resonance Raman scattering (SERRS) spectra have been obtained from the active, far-red light absorbing (Pfr) and biologically inactive (Pr) forms of phytochrome adsorbed on silver colloids. Substantial differences between the SERRS spectra of the two forms in the low and high wavenumber regions are observed using 406.7 nm wavelength excitation. These differences reinforce those seen with 413.1 nm wavelength excitation in the high wavenumber region. Simultaneously, extensive differences are observed in the SERRS obtained from the same form in the low wavenumber region using 406.7 nm, as compared with 413.1 nm wavelength excitation. The relative intensity differences observed for the two forms, and those obtained using two slightly different excitation wavelengths to illuminate the same form, suggest that some type of subtle, protein-controlled structural variation is responsible for the spectroscopic differences. AZ----E isomerization during the Pr----Pfr phototransformation is consistent with the SERRS data, although the overall chromophore conformations are most likely conserved for the native Pr- and Pfr-phytochrome species. Slight out-of-plane ring twisting, accompanying the Pr----Pfr photoisomerization, may be responsible for the large difference in the spectroscopic properties of the native Pr and Pfr chromophores.