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Relationship between the Excited State Relaxation Paths of Rhodopsin and Isorhodopsin
journal contribution
posted on 2008-03-19, 00:00 authored by Angela Strambi, Pedro B. Coto, Luis Manuel Frutos, Nicolas Ferré, Massimo OlivucciThe pigment Isorhodopsin, an analogue of the visual pigment Rhodopsin, is investigated via
quantum-mechanics/molecular-mechanics computations based on an ab initio multiconfigurational quantum
chemical treatment. The limited <5 kcal mol-1 error found for the spectral parameters allows for a nearly
quantitative analysis of the excited-state structure and reactivity of its 9-cis-retinal chromophore. We
demonstrate that, similar to Rhodopsin, Isorhodopsin features a shallow photoisomerization path. However,
the structure of the reaction coordinate appears to be reversed. In fact, while the coordinate still corresponds
to an asynchronous crankshaft motion, the dominant isomerization component involves a counterclockwise,
rather than clockwise, twisting of the 9-cis bond. Similarly, the minor component involves a clockwise,
rather than counterclockwise, twisting of the 11-trans bond. Ultimately, these results indicate that Rhodopsin
and Isorhodopsin relax along a common excited-state potential energy valley starting from opposite ends.
The fact that the central and lowest energy region of such valley runs along a segment of the intersection
space between the ground and excited states of the protein explains why the pigments decay at distinctive
conical intersection structures.
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counterclockwiseenergy valleyenergy regionab initio multiconfigurational quantum chemical treatmentpigments decayIsorhodopsinThe pigment Isorhodopsinisomerization componentasynchronous crankshaft motionphotoisomerization pathpigment Rhodopsinconical intersection structuresIsorhodopsin featuresintersection spaceExcited State Relaxation Pathsbond
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