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

摘要

The ground-state structure and excited-state isomerization dynamics of the Pr and Pfr forms of phytochrome Cph1 are investigated using resonance Raman intensity analysis. Electronic absorption and stimulated resonance Raman spectra of Pr and Pfr 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 C15–H HOOP modes that appear because of symmetry-reducing distortions about the C14–C15 and C15=C16 bonds. Pfr is especially well primed for ultrafast isomerization and torsional Franck-Condon analysis predicts a <200 fs Pfr → Pr isomerization. This time is significantly faster than the observed 700 fs reaction time, indicating that the Pfr S1 surface has a D-ring rotational barrier caused by steric interactions with the protein.