The photoisomerization of retinal in bacteriorhodopsin: Experimental evidence for a three-state model

The primary events in the all-trans to 13-cis photoisomerization of retinal in bacteriorhodopsin have been investigated with femtosecond time-resolved absorbance spectroscopy. Spectra measured over a broad range extending from 7000 to 22,400 cm(-1) reveal features whose dynamics are inconsistent with a model proposed earlier to account for the highly efficient photoisomerization process. Emerging from this work is a new three-state model, Photoexcitation of retinal with visible light accesses a shallow well on the excited state potential energy surface, This well is bounded by a small barrier, arising from an avoided crossing that separates the Franck-Condon region from the nearby reactive region of the photoisomerization coordinate, At ambient temperatures, the reactive region is accessed with a time constant of approximate to 500 fs, whereupon the retinal rapidly twists and encounters a second avoided crossing region, The protein mediates the passage into the second avoided crossing region and thereby exerts control over the quantum yield for forming 13-cis retinal, The driving force for photoisomerization resides in the retinal, not in the surrounding protein. This view contrasts with an earlier model where photoexcitation was thought to access directly a reactive region of the excited-state potential and thereby drive the retinal to a twisted conformation within 100-200 fs.