Transient 2D IR spectroscopy and multiscale simulations reveal vibrational couplings in the Cyanobacteriochrome Slr1393-g3

TL;DR

This study combines transient 2D IR spectroscopy with multiscale simulations to uncover vibrational couplings and intermediates in the photocycle of the cyanobacteriochrome Slr1393-g3, advancing understanding of its structural dynamics.

Contribution

It introduces a novel combination of advanced IR spectroscopy and QM/MM calculations to identify intermediates and vibrational couplings in cyanobacteriochrome photocycles.

Findings

Discovered a new intermediate preceding Pr formation.

Measured vibrational couplings during intermediate states.

Achieved close-to-quantitative agreement between simulations and experiments.

Abstract

Cyanobacteriochromes are bi-stable photoreceptor proteins with desirable photochemical properties for biotechnological applications such as optogenetics or fluorescence microscopy. Here, we investigated Slr1393-g3, a cyanobacteriochrome that reversibly photo-switches between a red-absorbing (Pr) and green-absorbing (Pg) form. We applied advanced IR spectroscopic methods to track the sequence of intermediates during the photocycle over many orders in magnitude in time. In the conversion from Pg to Pr, we have revealed a new intermediate which precedes the Pr formation by using transient IR spectroscopy. In addition, stationary and transient 2D~IR experiments measured the vibrational couplings between different groups of the chromophore and the protein during these intermediate states. Anharmonic QM/MM calculations predict spectra in close-to-quantitative agreement with experimental 2D~IR spectra of the initial and the final state of the photocycle. They facilitate the assignment of the IR spectra and provide an atomistic insight into the coupling mechanism. This serves as a basis for the interpretation of the spectroscopic results and suggests structural changes of the intermediates along the photocycle.