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Science 337 (6099): 1225-1228

Copyright © 2012 by the American Association for the Advancement of Science

The Molecular Mechanism of Thermal Noise in Rod Photoreceptors

Samer Gozem,1 Igor Schapiro,1 Nicolas Ferré,2 Massimo Olivucci1,3,*

Abstract: Spontaneous electrical signals in the retina's photoreceptors impose a limit on visual sensitivity. Their origin is attributed to a thermal, rather than photochemical, activation of the transduction cascade. Although the mechanism of such a process is under debate, the observation of a relationship between the maximum absorption wavelength ({lambda}max) and the thermal activation kinetic constant (k) of different visual pigments (the Barlow correlation) indicates that the thermal and photochemical activations are related. Here we show that a quantum chemical model of the bovine rod pigment provides a molecular-level understanding of the Barlow correlation. The transition state mediating thermal activation has the same electronic structure as the photoreceptor excited state, thus creating a direct link between {lambda}max and k. Such a link appears to be the manifestation of intrinsic chromophore features associated with the existence of a conical intersection between its ground and excited states.

1 Department of Chemistry, Bowling Green State University, Bowling Green, OH 43403, USA.
2 Institut de Chimie Radicalaire, UMR 7273–Université d'Aix-Marseille, 13397 Marseille Cedex 20, France.
3 Dipartimento di Chimica, Università di Siena, via De Gasperi 2, I-53100 Siena, Italy.

* To whom correspondence should be addressed. E-mail: molivuc{at}bgsu.edu, olivucci{at}unisi.it

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