REVERSIBLE PHOTOCHEMISTRY IN THE α‐SUBUNIT OF PHYCOERYTHROCYANIN: CHARACTERIZATION OF CHROMOPHORE AND PROTEIN BY MOLECULAR DYNAMICS AND QUANTUM CHEMICAL CALCULATIONS

The ground and excited state properties of two regions in the Δ15‐configurational space of the phycoviolobilin chromophore in the α‐subunit of phycoerythrocyanin are analyzed. Molecular dynamics calculations reveal that the chromophore geometry determines the active‐site dynamics. The excited state torsional potential surface shows a negative barrier for isomerization and trapping of an activated complex. Strong coupling of excited states localized in the chromophore and charge transfer states from the surrounding polar residues provides favorable prerequisites for fast excited state surface crossing in competition with other deactivation processes. The formation of a photoreduced intermediate following the photoinduced charge transfer may trigger subsequent chemical reactions.