Ultrafast photo-induced reaction dynamics in bacteriorhodopsin and its Trp mutants

This review paper presents the recent advances made in observing and understanding the ultrafast photo-reaction dynamics in retinal proteins, in particular bacteriorhodopsin (bR), with a special emphasis on the retinal-protein interactions and the mechanisms of protein activation on a sub-picosecond timescale. We review our latest results obtained on wild-type (wt) bR, and on two tryptophan mutants W86F and W182F, obtained by femtosecond pump-probe experiments. It was shown that light-induced charge translocations and modifications of the protein electrostatics can be monitored by the near-UV differential absorption of Trp86, which experiences a linear intra-protein Stark effect. In the same spectral region, non-exponential wavepacket-like dynamics was found for the formation of the 13-cis retinal isomer in wt-bR. The present paper highlights how this finding is underpinned by the experiments on the mutants. New results are presented regarding the reaction dynamics in the Trp mutants, studied in the vis/near-IR by transient absorption. Interestingly, W86F displays a faster excited state decay and photoproduct formation than wt-bR. This is tentatively attributed to an all-trans/13-cis ground state mixture known to occur in the light-adapted state in this special mutant, due to increased conformational flexibility of the retinal chromophore.