Structure of the retinal chromophore in halorhodopsin

Resonance Raman (RR) spectra of the light‐driven chloride pump halorhodopsin (HR) were recorded after isolation of the protein from cell membranes of Halobacterium halobium . The spectra of the unphotolyzed state HR 578 were compared with those of two unphotolyzed species of the light‐driven proton pump bacteriorhodopsin, BR 570 and BR 548 , which were obtained from light‐ and dark‐adapted purple membranes. Identical structural components in the retinal chromophores of HR 578 and BR 570 are found, both having the all‐ trans configuration of the retinal chain and a protonated Schiff base linkage to the protein with anti ‐position of the two hydrogens. Only minor conformational differences between the chromophoric structures in the two proteins could be inferred from the vibrational structure of the RR spectra. The function of the two chromophores as triggers of light‐induced chloride or proton transport emphasizes the role of the protein part in the ion specificity of the pumps.