Retinal photoisomerization versus counterion protonation in light and dark-adapted bacteriorhodopsin and its primary photoproduct. 2024

Partha Malakar, and Samira Gholami, and Mohammad Aarabi, and Ivan Rivalta, and Mordechai Sheves, and Marco Garavelli, and Sanford Ruhman
Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, 9190401, Israel.

Discovered over 50 years ago, bacteriorhodopsin is the first recognized and most widely studied microbial retinal protein. Serving as a light-activated proton pump, it represents the archetypal ion-pumping system. Here we compare the photochemical dynamics of bacteriorhodopsin light and dark-adapted forms with that of the first metastable photocycle intermediate known as "K". We observe that following thermal double isomerization of retinal in the dark from bio-active all-trans 15-anti to 13-cis, 15-syn, photochemistry proceeds even faster than the ~0.5 ps decay of the former, exhibiting ballistic wave packet curve crossing to the ground state. In contrast, photoexcitation of K containing a 13-cis, 15-anti chromophore leads to markedly multi-exponential excited state decay including much slower stages. QM/MM calculations, aimed to interpret these results, highlight the crucial role of protonation, showing that the classic quadrupole counterion model poorly reproduces spectral data and dynamics. Single protonation of ASP212 rectifies discrepancies and predicts triple ground state structural heterogeneity aligning with experimental observations. These findings prompt a reevaluation of counter ion protonation in bacteriorhodopsin and contribute to the broader understanding of its photochemical dynamics.

UI MeSH Term Description Entries
D008027 Light That portion of the electromagnetic spectrum in the visible, ultraviolet, and infrared range. Light, Visible,Photoradiation,Radiation, Visible,Visible Radiation,Photoradiations,Radiations, Visible,Visible Light,Visible Radiations
D010777 Photochemistry A branch of physical chemistry which studies chemical reactions, isomerization and physical behavior that may occur under the influence of visible and/or ultraviolet light. Photochemistries
D001436 Bacteriorhodopsins Rhodopsins found in the PURPLE MEMBRANE of halophilic archaea such as HALOBACTERIUM HALOBIUM. Bacteriorhodopsins function as an energy transducers, converting light energy into electrochemical energy via PROTON PUMPS. Bacteriorhodopsin
D017494 Proton Pumps Integral membrane proteins that transport protons across a membrane. This transport can be linked to the hydrolysis of ADENOSINE TRIPHOSPHATE. What is referred to as proton pump inhibitors frequently is about POTASSIUM HYDROGEN ATPASE. H(+) Pump,H+ Pump,Proton Pump,Pump, H+,Pump, Proton,Pumps, Proton

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