title: Unusual kinetics of thermal decay of dim-light photoreceptors in vertebrate vision.
abstract: We present measurements of rate constants for thermal-induced reactions of the 11-cis retinyl chromophore in vertebrate visual pigment rhodopsin, a process that produces noise and limits the sensitivity of vision in dim light. At temperatures of 52.0-64.6 °C, the rate constants fit well to an Arrhenius straight line with, however, an unexpectedly large activation energy of 114 ± 8 kcal/mol, which is much larger than the 60-kcal/mol photoactivation energy at 500 nm. Moreover, we obtain an unprecedentedly large prefactor of 10(72±5) s(-1), which is roughly 60 orders of magnitude larger than typical frequencies of molecular motions! At lower temperatures, the measured Arrhenius parameters become more normal: Ea = 22 ± 2 kcal/mol and Apref = 10(9±1) s(-1) in the range of 37.0-44.5 °C. We present a theoretical framework and supporting calculations that attribute this unusual temperature-dependent kinetics of rhodopsin to a lowering of the reaction barrier at higher temperatures due to entropy-driven partial breakup of the rigid hydrogen-bonding network that hinders the reaction at lower temperatures.
authors.affiliation: Department of Chemistry, Yale University, New Haven, CT 06520.
authors.forename: Ying
authors.initials: Y
authors.lastname: Guo
authors.affiliation: Department of Chemistry, Yale University, New Haven, CT 06520.
authors.forename: Sivakumar
authors.initials: S
authors.lastname: Sekharan
authors.affiliation: Department of Chemistry, Yale University, New Haven, CT 06520.
authors.forename: Jian
authors.initials: J
authors.lastname: Liu
authors.affiliation: Department of Chemistry, Yale University, New Haven, CT 06520.
authors.forename: Victor S
authors.initials: VS
authors.lastname: Batista
authors.affiliation: Department of Chemistry, Yale University, New Haven, CT 06520 john.tully@yale.edu elsa.yan@yale.edu.
authors.forename: John C
authors.initials: JC
authors.lastname: Tully
authors.affiliation: Department of Chemistry, Yale University, New Haven, CT 06520 john.tully@yale.edu elsa.yan@yale.edu.
authors.forename: Elsa C Y
authors.initials: EC
authors.lastname: Yan
mesh_terms: D000818:Animals; D002417:Cattle; D057809:HEK293 Cells; D006801:Humans; D007700:Kinetics; D008027:Light; D008958:Models, Molecular; D020419:Photoreceptor Cells, Vertebrate / Q000378:metabolism* / Q000528:radiation effects*; D012243:Rhodopsin / Q000378:metabolism; D013056:Spectrophotometry, Ultraviolet; D013696:Temperature; D014714:Vertebrates / Q000502:physiology*; D014785:Vision, Ocular / Q000502:physiology* / Q000528:radiation effects*; D014867:Water / Q000737:chemistry
chemical_list: D014867:Water; D012243:Rhodopsin
keywords: dim-light vision; isomerization rate; non-Arrhenius; transition state theory