Vibronic Reorganization Suppresses Salinixanthin-to-Retinal Energy Transfer in the Freshwater Kin4B8 Xanthorhodopsin

Salinixanthin, a 4-keto xanthophyll, acts as an efficient light-harvesting antenna by transferring excitation energy to retinal in the terrestrial xanthorhodopsin from Salinibacter ruber (SrXR). Although it also binds to the freshwater xanthorhodopsin Kin4B8, it does not transfer energy to retinal, whereas hydroxylated xanthophylls show high excitation energy transfer (EET) efficiency in this protein. Here, we combine molecular dynamics simulations with polarizable quantum mechanics/molecular mechanics (QM/MM) calculations to construct and characterize the Kin4B8–salinixanthin complex. We obtain a spectroscopically consistent binding model that reproduces the Kin4B8–salinixanthin experimental absorption and circular dichroism spectra and reveals strong electronic coupling between salinixanthin and retinal, comparable to the other xanthophylls. However, energy transfer is strongly suppressed by the red-shift of salinixanthin S2 state and its large reorganization energy, which drastically reduce donor–acceptor spectral overlap. These results demonstrate that, in this system, donor vibronic relaxation, rather than geometry or electronic coupling, is the decisive factor suppressing EET.

pubs.acs.org

Structural and Spectroscopic Basis of Excitation Energy Transfer in Microbial Rhodopsins Binding Xanthophylls

Carotenoids are essential for light harvesting in certain microbial rhodopsins, with xanthorhodopsins that bind salinaxanthin being the most well-known example. But how do carotenoids that lack the ty...

molecolab.dcci.unipi.it

MolecoLab at the 21st Congress of European Society for Photobiology, ESP-2025

MolecoLab was proudly represented at the 21st Congress of European Society for Photobiology last week in Bari (Italy), with three members contributing to key symposia on cutting-edge re...

molecolab.dcci.unipi.it

An Efficient and Robust Implementation of CASSCF Linear Response Theory

We present a robust and efficient implementation of linear response theory for a Complete Active Space─Self-Consistent Field wave function. Our approach relies on the Cholesky Decomposition of the two-electron integrals, enabling the routine treatment of large molecular systems on standard hardware. It allows for the computation of both absorption energies and transition properties, as well as frequency-dependent molecular response functions. For both classes of properties, numerically stable and efficient algorithms have been developed. The capabilities of our implementation are demonstrated through the calculation of absorption spectra and molecular response properties of large systems with extended basis sets.

doi.org

A Computational Approach to Modeling Excitation Energy Transfer and Quenching in Light-Harvesting Complexes

Light-harvesting complexes (LHCs) play a critical role in modulating energy flux within photosynthetic organisms in response to fluctuating light. Under high light conditions, they activate quenching ...

doi.org

Protein-Driven Electron-Transfer Process in a Fatty Acid Photodecarboxylase

Naturally occurring photoenzymes are rare in nature, but among them, fatty acid photodecarboxylases derived from Chlorella variabilis (CvFAPs) have emerged as promising photobiocatalysts capable of performing the redox-neutral, light-induced decarboxylation of free fatty acids (FAs) into C1-shortened n-alka(e)nes. Using a hybrid QM/MM approach combined with a polarizable embedding scheme, we identify the structural changes of the active site and determine the energetic landscape of the forward electron transfer (fET) from the FA substrate to the excited flavin adenine dinucleotide. We obtain a charge-transfer diradical structure where a water molecule rearranges spontaneously to form a H-bond interaction with the excited flavin, while the FA’s carboxylate group twists and migrates away from it. Together, these structural modifications provide the driving force necessary for the fET to proceed in a downhill direction. Moreover, by examining the R451K mutant where the FA substrate is farther from the flavin core, we show that the marked reduction of the electronic coupling is counterbalanced by an increased driving force, resulting in a fET lifetime similar to the WT, thereby suggesting a resilience of the process to this mutation. Finally, through QM/MM molecular dynamic simulations, we reveal that, following fET, the decarboxylation of the FA radical occurs within tens of picoseconds, overcoming an energy barrier of ∼0.1 eV. Overall, by providing an atomistic characterization of the photoactivation of CvFAP, this work can be used for future protein engineering.

pubs.acs.org