Abstract

¹⁹F nuclei are useful labels in solid-​state NMR studies, since their chemical shift and tensor elements are very sensitive to the electrostatic and space-​filling properties of their local environment. In this study we have exploited a fluorine substituent, strategically placed at the C-12-position of 11-cis retinal, the chromophore of visual rhodopsins. This label was used to explore the local environment of the chromophore in the ground state of bovine rhodopsin and its active photo-​intermediate Meta II. In addition, the chemical shift and tensor elements of the chromophore in the free state in a membrane environment and the bound state in the protein were determined. Upon binding of the chromophore into rhodopsin and Meta II, the isotropic chemical shift changes in opposite direction by +9.7 and -8.4 ppm, respectively. An unusually large isotropic shift difference of 35.9 ppm was observed between rhodopsin and Meta II. This partly originates in the light-triggered 11-cis to all-trans isomerization of the chromophore. The other part reflects the local conformational rearrangements in the chromophore and the binding pocket. These NMR data were correlated with the available X-ray structures of rhodopsin and Meta II using bond polarization theory. For this purpose hydrogen atoms have to be inserted and hereto a family of structures was derived that best correlated with the well-​established ¹³C chemical shifts. Based upon these structures, a 12-F derivative was obtained that best corresponded with the experimentally determined ¹⁹F chemical shifts and tensor elements. The combined data indicate strong changes in the local environment of the C-12 position and a substantially different interaction pattern with the protein in Meta II as compared to rhodopsin.