| Abstract
| - Conventional Horner−Wadsworth−Emmons and Wittig condensationswere used to fluorinate theodd-numbered positions of the retinal side chain past C7.The stereochemically labile cis-fluororetinals were easily converted into the most stabletrans-fluororetinals, which were incubatedwith bacterio-opsin. Contrary to expectations, the fluorinatedretinals provided artificial pigmentswith near normal absorption properties, showing that any electrostaticinteractions between thefluorine atoms and protein groups were insufficient to prevent normalbinding. The new artificialpigments had smaller opsin shifts than did native bacteriorhodopsin,which is interpreted as dueeither to greater electrostatic interaction between the protonatedimine and its counterion, or tolocal interactions between the fluorine substituents and nearby polarprotein groups.
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