Probing Key Vision Event

Scientists for the first time have followed the photoisomerization of 11-cis-rhodopsin to its all-trans photoproduct from start to finish. In the process of studying this central step in vision, they have gathered the first experimental support of a phenomenon thought to be ubiquitous in photochemistry (Nature 2010, 467, 440).

Photoisomerization of rhodopsin's chromophore—the chemical reaction that initiates vision—clocks in at less than 200 femtoseconds. A team led by Marco Garavelli of the University of Bologna and Giulio Cerullo of the Polytechnic University of Milan used ultrafast spectroscopy to show that this reaction's extreme speed results from a "conical intersection" of the first electronic excited state and the ground state. Such conical intersections, which represent vanishing energy gaps between the two electronic states, can be seen as the photochemical counterpart of transition states in thermal reactions, Garavelli says.

"The conical intersection can be seen as a 'funnel' that attracts and accelerates photoexcited molecules, triggering ultrafast chemical reactions and their decay to the ground state," Garavelli says. "This physical entity represents the cornerstone for understanding the paradoxically fast and efficient photoinduced process initiating vision."

The researchers initiated the photoisomerization of rhodopsin by exciting the chromophore with 10-fs pulses of 500-nm laser light. They then probed the reaction dynamics with time-delayed pulses in the visible and near-infrared spectral regions. At the earliest times, the reaction is dominated by stimulated emission from the first electronic excited state. At about 75 fs, the signal rapidly switches to photoinduced absorption from the ground state of the isomerized molecule. The abrupt transition provides experimental evidence that the molecule is indeed moving through a conical intersection.

The extreme efficiency of rhodopsin isomerization made the experiments technically challenging, Cerullo says. "The experiment needs to be performed in absolute darkness to avoid accidental isomerization of rhodopsin molecules by ambient light," he says.

The researchers also simulated the photoinduced process and were able to closely reproduce the experimental data. "The simulations show the chemical reaction responsible for the recorded spectroscopic signatures," Garavelli says. "Taken together, those experimental and computational data provide the most compelling evidence for the existence of a conical intersection."

"This is an impressive combination of ultrafast spectroscopy and computational simulations that directly support and almost prove the long-evolving picture of the primary event in vision," says Arieh Warshel, a chemistry professor at the University of Southern California. In 1976,Warshel first predicted—by molecular dynamics simulations and quantum mechanical/molecular mechanical calculations—the 200-fs timescale and small energy gap now being seen experimentally. "The observed time-dependent red shift of the energy gap is very convincing, in particular with the attempt to combine the time of stimulated emission from the excited state and absorption from the ground state."

Although they are seeing more of the rhodopsin isomerization than has ever been seen, some aspects remain out of reach. For example, the experiment can't resolve the coherent vibrations of C–C backbone stretching in rhodopsin's chromophore, Cerullo says.