Influence of Heterogeneity on the Ultrafast Photoisomerization Dynamics of Pfr in Cph1 Phytochrome

Photoisomerization of a protein-bound chromophore is the basis of light sensing and signaling in many photoreceptors. Phytochrome photoreceptors can be photoconverted reversibly between the Pr and Pfr states through photoisomerization of the methine bridge between rings C and D. Ground-state heterogeneity of the chromophore has been reported for both Pr and Pfr. Here, we report ultrafast visible (Vis) pump-probe and femtosecond polarization-resolved Vis pump-infrared (IR) probe studies of the Pfr photoreaction in native and C-13/N-15-labeled Cph1 phytochrome with unlabeled PCB chromophore, demonstrating different S-0 substates, Pfr-I and Pfr-II, with distinct IR absorptions, orientations and dynamics of the carbonyl vibration of ring D. We derived time constants of 0.24ps, 0.7ps and 6ps, describing the complete initial photoreaction. We identified an isomerizing pathway with 0.7ps for Pfr-I, and silent dynamics with 6ps for Pfr-II. We discuss different origins of the Pfr substates, and favor different facial orientations of ring D. The model provides a quantum yield for Pfr-I of 38%, in line with similar to 35% ring D rotation in the electronic excited state. We tentatively assign the silent form Pfr-II to a dark-adapted state that can convert to Pfr-I upon light absorption.