Resonance Raman study of the dark-adapted form of the purple membrane protein.

The resonance Raman spectrum of the dark-adapted form of the purple membrane protein (bacteriorhodopsin) has been obtained and is compared to the light-adapted pigment and model chromophore spectra. As in the light-adapted form, the chromophore-protein linkage is found to be a protonated Schiff base. Electron delocalization appears to play the dominant role in color regulation. The dark-adapted spectrum indicates a conformation closer to 13-cis than the light-adapted spectrum.