PHOTOISOMERIZATION MECHANISM OF THE RHODOPSIN CHROMOPHORE - PICOSECOND PHOTOLYSIS OF PIGMENT CONTAINING 11-CIS-LOCKED 8-MEMBERED RING RETINAL

The primary photochemical event in rhodopsin is an 11-cis to 11-trans photoisomerization of its retinylidene chromophore to form the primary intermediate photorhodopsin. Earlier picosecond studies have shown th at no intermediate is formed when the retinal 11-ene is fixed through a bridging five-membered ring, whereas a photorhodopsin-like intermedi ate is formed when it is fixed through a flexible seven-membered ring. Results from a rhodopsin analog formed from a retinal with locked 11- ene structure through the more flexible eight-membered ring (Ret8) are described. Incubation of bovine opsin with Ret8 formed two pigments a bsorbing at 425 nm (P425) and 500 nm (P500). P425, however, is an arti fact because it formed from thermally denatured opsin or other protein s and Ret8. Excitation of P500 with a picosecond green pulse led to fo rmation of two intermediates corresponding to photo- and bathorhodopsi ns. These results demonstrate that an appearance of early intermediate s is dependent on the flexibility of the 11-ene and that the photoisom erization of P500 proceeds by stepwise changes of chromophore-protein interaction, which in turn leads to a relaxation of the highly twisted all-trans-retinylidene chromophore in photorhodopsin.