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  • Title: Tuning of retinal twisting in bacteriorhodopsin controls the directionality of the early photocycle steps.
    Author: Bondar AN, Fischer S, Suhai S, Smith JC.
    Journal: J Phys Chem B; 2005 Aug 11; 109(31):14786-8. PubMed ID: 16852870.
    Abstract:
    Productive proton pumping by bacteriorhodopsin requires that, after the all-trans to 13-cis photoisomerization of the retinal chromophore, the photocycle proceeds with proton transfer and not with thermal back-isomerization. The question of how the protein controls these events in the active site is addressed here using quantum mechanical/molecular mechanical reaction-path calculations. The results indicate that, while retinal twisting significantly contributes to lowering the barrier for the thermal cis-trans back-isomerization, the rate-limiting barrier for this isomerization is still 5-6 kcal/mol larger than that for the first proton-transfer step. In this way, the retinal twisting is finely tuned so as to store energy to drive the subsequent photocycle while preventing wasteful back-isomerization.
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