title: Echinenone vibrational properties: From solvents to the orange carotenoid protein.
abstract: Orange carotenoid protein (OCP) is a cyanobacterial photoactive protein which binds echinenone as a chromophore; it is involved in photoprotection of these photosynthetic organisms against intense illumination. In its resting state, OCP appears orange (OCPo), and turns into a red form (OCPr) when exposed to blue-green light. Here we have combined resonance Raman spectroscopy and molecular modeling to investigate the mechanisms underlying the electronic absorption properties of the different forms of OCP. Our results show that there are at least two carotenoid configurations in the OCPo, suggesting that it is quite flexible, and that the OCPo to OCPr transition must involve an increase of the apparent conjugation length of the bound echinenone. Resonance Raman indicates that this chromophore must be in an all-trans configuration in OCPo. Density functional theory (DFT) calculations, in agreement with the Raman spectra of both OCP forms, show that the OCPo to OCPr transition must involve either an echinenone s-cis to s-trans isomerization which would affect the position of its conjugated end-chain rings, or a bending of the echinenone rings which would bring them from out of the plane of the CC conjugated plane in the OCPo form into the CC plane in the OCPr form.
authors.affiliation: Institut de Biologie et de Technologies de Saclay, CEA, UMR 8221 CNRS, Université Paris Sud, CEA Saclay, 91191 Gif-sur-Yvette France.
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Kish
authors.affiliation: Institut de Biologie et de Technologies de Saclay, CEA, UMR 8221 CNRS, Université Paris Sud, CEA Saclay, 91191 Gif-sur-Yvette France.
authors.forename: Maria Manuela Mendes
authors.initials: MM
authors.lastname: Pinto
authors.affiliation: Institut de Biologie et de Technologies de Saclay, CEA, UMR 8221 CNRS, Université Paris Sud, CEA Saclay, 91191 Gif-sur-Yvette France.
authors.forename: Diana
authors.initials: D
authors.lastname: Kirilovsky
authors.affiliation: CNRS, Laboratoire Analyse et Modélisation pour la Biologie et l'Environnement, UMR 8587, Université d'Evry-Val-d'Essonne, 91025 Evry Cedex, France.
authors.forename: Riccardo
authors.initials: R
authors.lastname: Spezia
authors.affiliation: Institut de Biologie et de Technologies de Saclay, CEA, UMR 8221 CNRS, Université Paris Sud, CEA Saclay, 91191 Gif-sur-Yvette France. Electronic address: bruno.robert@cea.fr.
authors.forename: Bruno
authors.initials: B
authors.lastname: Robert
keywords: Biophysics; Carotenoid; Density functional theory; Orange carotenoid protein (OCP); Photoprotection; Raman spectroscopy