Integrated structural dynamics uncover a new B12 photoreceptor activation mode
Article Date: 04 February 2026
Article URL: https://www.nature.com/articles/s41586-025-10074-2
Article Image: (not provided)
Summary
This study maps, with high temporal and structural resolution, how a B12-based photoreceptor (the CarH cobalamin-binding domain, Tt CBD) is activated by light. Using time-resolved serial crystallography at XFELs (SACLA, SwissFEL), time-resolved X-ray solution scattering at ESRF, cryo- and time-resolved spectroscopy, plus QM/MM and DFT calculations, the authors capture intermediates from 10 ns to tens of milliseconds and propose a revised photoactivation pathway.
Key findings include evidence for homolytic Co–C bond photolysis producing diradical intermediates, the possibility of a reversible Co–C4′ adduct, formation and/or release of a modified adenosyl photoproduct (4′,5′-anhydroadenosine), and a subsequent ligand switch to a water-ligated cobalt that, together with histidine coordination changes, drives tetramer dissociation to the light-adapted monomeric state.
Key Points
- Time-resolved experiments capture structural changes in Tt CarH across 10 ns to 10 ms, revealing multiple intermediates on the photoreaction pathway.
- Data support a homolytic photolysis mechanism producing diradical Co(II)/C• pairs as early intermediates rather than an exclusively ionic pathway.
- QM/MM and DFT cluster calculations identify plausible photoproducts, a reversible Co–C4′ adduct route and a pathway leading to 4′,5′-anhydroadenosine release and a water-ligated Co(III) species.
- Cryotrapping and spectroscopic titrations corroborate the time-resolved crystallographic intermediates and help assign spectroscopic signatures to structural states.
- Photochemical changes in the chromophore pocket drive larger rearrangements: adenosyl departure, histidine recoordination and tetramer dissociation to the light-adapted monomer.
- Extensive datasets and models are deposited (multiple PDB entries and CXIDB accession 237) to support reproducibility and further analysis.
Content summary
The team combined serial femtosecond crystallography (SFX) at XFELs, room-temperature serial synchrotron crystallography (SSX), time-resolved X-ray solution scattering (TR-XSS), UV–Vis and EPR spectroscopy, cryo-temperature trapping and computational chemistry to map CarH photoactivation.
They collected Fourier difference maps at many time delays and pump fluences, performed kinetic analysis of spectroscopic transients, cryotrapped intermediate states and built QM/MM and DFT models to interpret electron density and energetic plausibility. The integrated approach supports a mechanism where light breaks the Co–C5′ bond, forms short-lived diradical intermediates and can follow branching routes — reversible adduct formation or release of a modified adenosyl product followed by cobalt ligand exchange — ultimately producing the characterised bis-histidine light-adapted state and oligomer dissociation.
Context and relevance
This work revises mechanistic understanding of cobalamin (vitamin B12) photochemistry in biological photoreceptors. CarH-type sensors are increasingly used in optogenetics and light-responsive materials; knowing whether photolysis proceeds through diradicals, reversible adducts or product release affects how these systems are engineered for kinetics, robustness and spectral tuning. The high-quality, time-resolved structural series and deposited datasets make this a rich resource for structural biologists, spectroscopists and synthetic biology groups working on light-responsive proteins and materials.
Why should I read this?
If you fiddle with optogenetic tools, design light-switchable materials or just love how molecules move when hit by light — this one’s for you. The paper gives a rare, step-by-step structural movie of a B12 photoreceptor doing its thing, shows alternative chemical outcomes (adduct vs product release) and ties spectroscopy to atomic models. It saves you hours of digging through separate papers and datasets by putting the structural and spectroscopic story together — plus the authors dumped the raw data and PDBs so you can dig deeper if you want to tinker.