Non-Bonded Interactions Drive the Sub-Picosecond Bilin Photoisomerization in the Pfr State of Phytochrome Cph1

Phytochromes are protein-based photoreceptors harboring a bilin-based photoswitch in the active site. The timescale of photosignaling via C-15=C-16 E-to-Z photoisomerization has been ambiguous in the far-red-absorbing P-fr state. Here we present a unified view of the structural events in phytochrome Cph1 post excitation with femtosecond precision, obtained via stimulated Raman and polarization-resolved transient IR spectroscopy. We demonstrate that photoproduct formation occurs within 700fs, determined by a two-step partitioning process initiated by a planarization on the electronic excited state with a 300fs time scale. The ultrafast isomerization timescale for P-fr-to-P-r conversion highlights the active role of the nonbonding methyl-methyl clash initiating the reaction in the excited state. We envision that our results will motivate the synthesis of new artificial photoswitches with precisely tuned non-bonded interactions for ultrafast response.