Conformational Homogeneity and Excited-State Isomerization Dynamics of the Bilin Chromophore in Phytochrome Cph1 from Resonance Raman Intensities

The ground-state structure and excited-state isomerization dynamics of the P-r and P-fr forms of phytochrome Cph1 are investigated using resonance Raman intensity analysis. Electronic absorption and stimulated resonance Raman spectra of P-r and P-fr are Presented; vibronic analysis of the Raman intensities and absorption spectra reveals that both conformers exist as a single, homogeneous population of molecules in the ground state. The homogeneous and inhomogeneous contributions to the overall electronic broadening are determined, and it is found that the broadening is largely homogeneous in nature, pointing to fast excited-state decay. Franck-Condon displacements derived from the Raman intensity analysis reveal the initial atomic motions in the excited state, including the highly displaced, nontotally symmetric torsional and C-15-H HOOP modes that appear because of symmetry-reducing distortions about the C-14-C-15 and C-15=C-16 bonds. P-fr is especially well primed for ultrafast isomerization and torsional Franck-Condon analysis predicts a <200 fs P-fr -> P-r isomerization. This time is significantly faster than the observed 700 fs reaction time, indicating that the P-fr S-1 surface has a D-ring rotational barrier caused by steric interactions with the protein.