We studied an analogue of bacteriorhodopsin whose chromophore is based on all-trans retinal. A five-membered ring was built around the 13–14 double bond so as to prohibit trans to 13-cis isomerization. No light-induced photochemical changes were seen, other than those due to a small amount (approximately 5%) of unbleached bacteriorhodopsin remaining in the apomembrane used for regeneration. The techniques used included flash photolysis at room and liquid nitrogen temperatures and Fourier-transform infrared difference spectroscopy. When the trans-fixed pigment was incorporated into phospholipid vesicles, no evidence of light-initiated proton pumping could be found. The results indicate that trans to 13-cis isomerization is essential for the photochemical transformation and function of bacteriorhodopsin. © 1985, The Biophysical Society. All rights reserved.
Chang, C. H., Govindjee, R., Ebrey, T., Bagley, K. A., Dollinger, G., Eisenstein, L., … Fang, J. M. (1985). Trans/13-cis isomerization is essential for both the photocycle and proton pumping of bacteriorhodopsin. Biophysical Journal, 47(4), 509–512. https://doi.org/10.1016/S0006-3495(85)83944-8