NMR studies of fluorinated visual pigment analogs

Synthesis of Retinals Fluorinated at Odd-Numbered Side-Chain Positions and of the Corresponding Fluorobacteriorhodopsins

Conventional Horner-Wadsworth-Emmons and Wittig condensations were used to fluorinate the odd-numbered positions of the retinal side chain past C(7). The stereochemically labile cis-fluororetinals were easily converted into the most stable trans-fluororetinals, which were incubated with bacterio-opsin. Contrary to expectations, the fluorinated retinals provided artificial pigments with near normal absorption properties, showing that any electrostatic interactions between the fluorine atoms and protein groups were insufficient to prevent normal binding. The new artificial pigments had smaller opsin shifts than did native bacteriorhodopsin, which is interpreted as due either to greater electrostatic interaction between the protonated imine and its counterion, or to local interactions between the fluorine substituents and nearby polar protein groups.

pKa of the protonated Schiff base of bovine rhodopsin. A study with artificial pigments

Artificial bovine rhodopsin pigments derived from synthetic retinal analogues carrying electron-withdrawing substituents (fluorine and chlorine) were prepared. The effects of the electron withdrawing substituents on the pKa values of the pigments and on the corresponding Schiff bases in solution were analyzed. The data suggest that the apparent pKa of the protonated Schiff base is above 16. However, the alternative possibility that the retinal Schiff base linkage in bovine rhodopsin is not accessible for titration from the aqueous bulk medium cannot be definitely ruled out.

Modeling rhodopsin, a member of G-protein coupled receptors, by computer graphics. Interpretation of chemical shifts of fluorinated rhodopsins

An attempt has been made to construct a 3-D model of rhodopsin, a member of G-protein coupled receptors. Sequence homology of rhodopsin with the latter was a factor considered in the modeling procedure. The constructed model has been used to compare currently available specific protein/substrate interaction information, the shape of the binding cavity derived from shape of binding retinal isomers and analogs and challenged to explain recently available results from a series of fluorinated rhodopsins.

9,11-Dicis-12-fluororhodopsin. Chromophore properties from 19F-NMR studies

From a 19F-NMR study of 9,11-dicis-12-fluororhodopsin and its photobleached product, we concluded that the initially formed chromophore retained its configuration and the photoproduct corresponded to the two-bond isomerized all-trans. Upon standing, it slowly isomerized to the 9-cis isomer. The method represents a direct, non-destructive procedure for determining configuration purity of the pigment formed. Its unique fluorine opsin shift value is consistent with the expected different orientation of the fluoro-substituent in a dicis pigment.