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Toward an Understanding of the Retinal Chromophore in Rhodopsin Mimics B

Huntress, Mark M., Gozem, Samer, Malley, Konstantin R., Jailaubekov, Askat E., Vasileiou, Chrysoula, Vengris, Mikas, Geiger, James H., Borhan, Babak, Schapiro, Igor, Larsen, Delmar S., Olivucci, Massimo
The Journal of physical chemistry 2013 v.117 no.35 pp. 10053-10070
absorption, actuators, binding proteins, fluorescence, fluorescence emission spectroscopy, isomerization, models, mutants, quantum mechanics, retinoic acid, rhodopsin
Recently, a rhodopsin protein mimic was constructed by combining mutants of the cellular retinoic acid binding protein II (CRABPII) with an all-trans retinal chromophore. Here, we present a combined computational quantum mechanics/molecular mechanics (QM/MM) and experimental ultrafast kinetic study of CRABPII. We employ the QM/MM models to study the absorption (λᵃₘₐₓ), fluorescence (λᶠₘₐₓ), and reactivity of a CRABPII triple mutant incorporating the all-trans protonated chromophore (PSB-KLE-CRABPII). We also study the spectroscopy of the same mutant incorporating the unprotonated chromophore and of another double mutant incorporating the neutral unbound retinal molecule held inside the pocket. Finally, for PSB-KLE-CRABPII, stationary fluorescence spectroscopy and ultrafast transient absorption spectroscopy resolved two different evolving excited state populations which were computationally assigned to distinct locally excited and charge-transfer species. This last species is shown to evolve along reaction paths describing a facile isomerization of the biologically relevant 11-cis and 13-cis double bonds. This work represents a first exploratory attempt to model and study these artificial protein systems. It also indicates directions for improving the QM/MM models so that they could be more effectively used to assist the bottom-up design of genetically encodable probes and actuators employing the retinal chromophore.