title: Towards an interpretation of 13C chemical shifts in bathorhodopsin, a functional intermediate of a G-protein coupled receptor.
abstract: Photoisomerization of the membrane-bound light receptor protein rhodopsin leads to an energy-rich photostate called bathorhodopsin, which may be trapped at temperatures of 120 K or lower. We recently studied bathorhodopsin by low-temperature solid-state NMR, using in situ illumination of the sample in a purpose-built NMR probe. In this way we acquired (13)C chemical shifts along the retinylidene chain of the chromophore. Here we compare these results with the chemical shifts of the dark state chromophore in rhodopsin, as well as with the chemical shifts of retinylidene model compounds in solution. An earlier solid-state NMR study of bathorhodopsin found only small changes in the (13)C chemical shifts upon isomerization, suggesting only minor perturbations of the electronic structure in the isomerized retinylidene chain. This is at variance with our recent measurements which show much larger perturbations of the (13)C chemical shifts. Here we present a tentative interpretation of our NMR results involving an increased charge delocalization inside the polyene chain of the bathorhodopsin chromophore. Our results suggest that the bathochromic shift of bathorhodopsin is due to modified electrostatic interactions between the chromophore and the binding pocket, whereas both electrostatic interactions and torsional strain are involved in the energy storage mechanism of bathorhodopsin.
authors.affiliation: School of Chemistry, University of Southampton, SO17 1BJ Southampton, England, UK.
authors.forename: Axel
authors.initials: A
authors.lastname: Gansmüller
authors.forename: Maria
authors.initials: M
authors.lastname: Concistrè
authors.forename: Neville
authors.initials: N
authors.lastname: McLean
authors.forename: Ole G
authors.initials: OG
authors.lastname: Johannessen
authors.forename: Ildefonso
authors.initials: I
authors.lastname: Marín-Montesinos
authors.forename: Petra H M
authors.initials: PH
authors.lastname: Bovee-Geurts
authors.forename: Peter
authors.initials: P
authors.lastname: Verdegem
authors.forename: Johan
authors.initials: J
authors.lastname: Lugtenburg
authors.forename: Richard C D
authors.initials: RC
authors.lastname: Brown
authors.forename: Willem J
authors.initials: WJ
authors.lastname: Degrip
authors.forename: Malcolm H
authors.initials: MH
authors.lastname: Levitt
mesh_terms: D002247:Carbon Isotopes; D018360:Crystallography, X-Ray; D007553:Isotope Labeling / Q000379:methods; D008024:Ligands; D008027:Light; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D043562:Receptors, G-Protein-Coupled / Q000096:biosynthesis / Q000737:chemistry*; D012176:Retinoids / Q000737:chemistry; D012243:Rhodopsin / Q000737:chemistry* / Q000378:metabolism / Q000528:radiation effects
chemical_list: D002247:Carbon Isotopes; D008024:Ligands; D043562:Receptors, G-Protein-Coupled; D012176:Retinoids; C121835:retinylidene chromophore; C015017:bathorhodopsin; D012243:Rhodopsin