Heterogeneous Photodynamics of the P(fr) State in the Cyanobacterial Phytochrome Cph1

[Image: see text] Femtosecond photodynamics of the P(fr) form of the red/far-red phytochrome N-terminal PAS-GAF-PHY photosensory core module of the cyanobacterial phytochrome Cph1 (termed Cph1Δ) from Synechocystis were resolved with visible broadband transient absorption spectroscopy. Multiphasic generation dynamics via global target analysis revealed parallel evolution of two pathways with distinct excited- and ground-state kinetics. These measurements resolved two subpopulations: a majority subpopulation with fast excited-state decay and slower ground-state dynamics, corresponding to previous descriptions of P(fr) dynamics, and a minority subpopulation with slower excited-state decay and faster ground-state primary dynamics. Both excited-state subpopulations generated the isomerized, red-shifted Lumi-F(f) photoproduct (715 nm); subsequent ground-state evolution to a blue-shifted Meta-F(r) population (635 nm) proceeded on 3 ps and 1.5 ns time scales for the two subpopulations. Meta-F(r) was spectrally similar to a recently described photoinactive fluorescent subpopulation of P(r) ((Fluor)P(r)). Thus, the reverse P(fr) to P(r) photoconversion of Cph1Δ involves minor structural deformation of Meta-F(r) to generate the fluorescent, photochemically refractory form of P(r), with slower subsequent equilibration with the photoactive P(r) subpopulation ((Photo)P(r)).