title: The molecular mechanism of thermal noise in rod photoreceptors.
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 (λ(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 λ(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.
authors.affiliation: Department of Chemistry, Bowling Green State University, Bowling Green, OH 43403, USA.
authors.forename: Samer
authors.initials: S
authors.lastname: Gozem
authors.forename: Igor
authors.initials: I
authors.lastname: Schapiro
authors.forename: Nicolas
authors.initials: N
authors.lastname: Ferré
authors.forename: Massimo
authors.initials: M
authors.lastname: Olivucci
mesh_terms: D000818:Animals; D002417:Cattle; D007536:Isomerism; D007700:Kinetics; D008956:Models, Chemical; D055668:Photochemical Processes; D011789:Quantum Theory; D017948:Retinal Rod Photoreceptor Cells / Q000737:chemistry* / Q000502:physiology; D012243:Rhodopsin / Q000737:chemistry* / Q000502:physiology*; D017299:Rod Opsins / Q000737:chemistry / Q000502:physiology; D012545:Schiff Bases; D013696:Temperature; D013816:Thermodynamics
chemical_list: D017299:Rod Opsins; D012545:Schiff Bases; D012243:Rhodopsin