SYNTHESIS OF RETINALS FLUORINATED AT ODD-NUMBERED SIDE-CHAIN POSITIONS AND OF THE CORRESPONDING FLUOROBACTERIORHODOPSINS

Conventional Horner-Wadsworth-Emmons and Wittig condensations were use d to fluorinate the odd-numbered positions of the retinal side chain p ast C-7. The stereochemically labile cis-fluororetinals were easily co nverted into the most stable trans-flouretinals, which were incubated with bacterio-opsin. Contrary to expectations, the fluorinated retinal s provided artificial pigments with near normal absorption properties, showing that any electrostatic interactions between the fluorine atom s and protein groups were insufficient to prevent normal binding. The new artificial pigments had smaller opsin shifts than did native bacte riorhodopsin, which is interpreted as due either to greater electrosta tic interaction between the protonated imine and its counterion, or to local interactions between the fluorine substituents and nearby polar protein groups.