SPECTRAL TUNING IN BACTERIORHODOPSIN IN THE ABSENCE OF COUNTERION ANDCOPLANARIZATION EFFECTS

The basis for wavelength regulation in bacteriorhodopsin (ER) and reti nylidene proteins in general has been studied for decades but is still only partially understood, Here are report the preparation and spectr oscopic characterization of BR analogs aimed at investigating the exis tence of spectral tuning mechanisms other than the two widely accepted mechanisms, weakened counterion interactions and ring/chain coplanari zation. We synthesized two novel retinal analogs containing a saturate d 13-14 bond, which interrupts the interaction of the protein counteri ons with the chromophore conjugation system, Furthermore, one of the a nalogs has a planar polyene system so that the contribution to the red shift of BR by retinal ring/chain coplanarization is also absent. We incorporated these analogs into bacterioopsin and discovered a sizable amount of red shift, which can be accounted for by interactions betwe en the polar or polarizable groups of the protein and the retinal poly ene chain. Our results suggest that the wavelength regulation in BR is achieved by synergistic chromophore/protein interactions including ri ng/ chain coplanarization, excited state stabilization by polar or pol arizable protein side chains located along the polyene chain, and weak ened counterion interactions near the Schiff base positive charge.