The photoisomerization of retinal in bacteriorhodospin: 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 approximately 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.
Hasson
K C
KC
Department of Physics, Harvard University, Cambridge, MA 02138, USA.
Gai
F
F
Anfinrud
P A
PA
D001436:Bacteriorhodopsins / Q000737:chemistry* / Q000528:radiation effects; D007536:Isomerism; D007700:Kinetics; D008956:Models, Chemical; D010777:Photochemistry; D012172:Retinaldehyde / Q000737:chemistry* / Q000528:radiation effects; D013816:Thermodynamics; D013997:Time Factors
D001436:Bacteriorhodopsins; D012172:Retinaldehyde