Nonisomerizable non-retinal chromophores initiate light-induced conformational alterations in bacterioopsin

The photoactivation of retinal proteins is usually interpreted in terms of C=C photoisomerization of the retinal moiety, which triggers appropriate co nformational changes in the protein. In this work several dye molecules. ch aracterized by a completely rigid structure in which no double-bond isomeri zation is possible, were incorporated into the binding site of bacteriorhod opsin (bR). Using a light-induced chemical reaction of a labeled EPR probe, it was observed that specific conformational alterations in the protein ar e induced following light absorption by the dye molecules occupying the bin ding site. The exact nature of these changes and their relationship to thos e occurring in the bR photocycle are still unclear. Nevertheless, their occ urrence proves that C=C or C=NH+ isomerization is not a prerequisite for pr otein conformational changes in a retinal protein. More generally, we show that conformational changes, leading to changes in reactivity, may be induc ed in proteins by optical excitation of simple nonisomerizable dyes located in the macromolecular matrix.