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.
Francesch
Andrés
A
Departamento de Química Orgánica, Universidade de Santiago de Compostela, 15706 Santiago de Compostela, Spain.
Alvarez
Rosana
R
López
Susana
S
de Lera
Angel R.
AR