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Rhodopsins are light-responsive proteins forming two vast and evolutionary distinct superfamilies whose functions are invariably triggered by the photoisomerization of a single retinal chromophore. In 2018 a third widespread superfamily of rhodopsins called heliorhodopsins was discovered using functional metagenomics. Heliorhodopsins, with their markedly different structural features with respect to the animal and microbial superfamilies, offer an opportunity to study how evolution has manipulated the chromophore photoisomerization to achieve adaptation. One question is related to the mechanism of such a reaction and how it differs from that of animal and microbial rhodopsins. To address this question, we use hundreds of quantum-classical trajectories to simulate the spectroscopically documented picosecond light-induced dynamics of a heliorhodopsin from the archaea thermoplasmatales archaeon (TaHeR). We show that, consistently with the observations, the trajectories reveal two excited state decay channels. However, inconsistently with previous hypotheses, only one channel is associated with the –C13QC14– rotation of microbial rhodopsins while the second channel is characterized by the –C11QC12– rotation typical of animal rhodopsins. The fact that such –C11QC12– rotation is aborted upon decay and ground state relaxation, explains why illumination of TaHeR only produces the 13-cis isomer with a low quantum efficiency. We argue that the documented lack of regioselectivity in double-bond excited state twisting motion is the result of an ‘‘adaptation’’ that could be completely lost via specific residue substitutions modulating the steric hindrance experienced along the isomerization motion.
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This content will become publicly available on August 21, 2026
Quantum Dynamics Predicts Coherent Oscillations in the Early Times of a Biological Photoisomerization
In this work, we study the quantum dynamics of the isomerization associated with the primary event in vision employing a model for the 2-cis-penta-2,4-dieniminium cation (cis-PSB3). We aim to demonstrate that the observed relationship between a specific wag mode and the reaction quantum yield emerges naturally from wavepacket propagation. To do so, we address two previously undetected methodological issues related to (i) establishing the appropriate level of convergence of the quantum dynamics calculations, and (ii) describing the emergence of distinct oscillatory behaviors during the formation of the cis and trans isomers in the ground state following cis-PSB3 photoexcitation. The two issues are strictly related, since only upon reliable convergence, the simulated dynamics is able to capture the large amplitude motion associated with the torsional and wag deformations in the region of the reactive bond.
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- Award ID(s):
- 2102619
- PAR ID:
- 10647477
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- The Journal of Physical Chemistry Letters
- Volume:
- 16
- Issue:
- 33
- ISSN:
- 1948-7185
- Page Range / eLocation ID:
- 8486 to 8494
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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