14-Fluorobacteriorhodopsin and other fluorinated and 14-substituted analogs. An extra, unusually red-shifted pigment formed during dark adaptation

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Abstract
Five vinyl-substituted fluororetinal analogues (8-F, 10-F, 12-F, 14-F, and 13,14-F2) were found to give bacteriorhodopsin analogues with properties similar to those of the parent system. Of these, only 14-fluororetinal was found to give an extra red-shifted BR analogue (.lambda.max .ltoreq. 680 nm) in equilibrium with the normal 587-nm pigment. The 680-nm pigment was enriched upon irradiation. It rearranged to the 587-nm pigment at room temperature (.DELTA.E* = 20.8 kcal/mol). Chromophore extraction experiments revealed the all-trans geometry for the 680-nm pigment. 14-Chlororetinal gave a similarly red-shifted pigment while 14-methylretinal did not. A scheme for dark adaptation of the 14-halogenated bacteriorhodopsins has been proposed in which the new red-shifted pigment was assigned the all-trans, 15-syn geometry.