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PHYSICAL SCIENCES / BIOLOGICAL SCIENCES / CHEMISTRY / BIOPHYSICS
Tracking the excited-state time evolution of the visual pigment with multiconfigurational quantum chemistry
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Dipartimento di Chimica, Università di Siena, via Aldo Moro 2, I-53100 Siena, Italy; ||Department of Chemistry, Bowling Green State University, Bowling Green, OH 43403; Institute of Physical and Theoretical Chemistry, Department of Chemistry, Wroclaw University of Technology, 27 Wyb. Wyspianskiego, 50-370, Wroclaw, Poland; Istituto per i Processi Chimico-Fisici, Consiglio Nazionale delle Ricerche, Via Moruzzi 1, I-56124 Pisa, Italy; and ¶Laboratoire de Chimie Théorique et de Modélisation Moléculaire, Unité Mixte de Recherche 6517, Centre National de la Recherche Scientifique, Université de Provence, Case 521, Faculté de Saint-Jérôme, Avenue Esc. Normandie Niemen, 13397 Marseille Cedex 20, France
Edited by Ernest R. Davidson, University of Washington, Seattle, WA, and approved March 8, 2007 (received for review February 25, 2007)
The primary event that initiates vision is the photoinduced isomerization of retinal in the visual pigment rhodopsin (Rh). Here, we use a scaled quantum mechanics/molecular mechanics potential that reproduces the isomerization path determined with multiconfigurational perturbation theory to follow the excited-state evolution of bovine Rh. The analysis of a 140-fs trajectory provides a description of the electronic and geometrical changes that prepare the system for decay to the ground state. The data uncover a complex change of the retinal backbone that, at 
60-fs delay, initiates a space saving "asynchronous bicycle-pedal or crankshaft" motion, leading to a conical intersection on a 110-fs time scale. It is shown that the twisted structure achieved at decay features a momentum that provides a natural route toward the photoRh structure recently resolved by using femtosecond-stimulated Raman spectroscopy.
photoisomerization | rhodopsin | vision
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/cgi/content/full/0701732104/DC1.
To whom correspondence should be addressed. E-mail: olivucci{at}unisi.it
© 2007 by The National Academy of Sciences of the USA
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