Primary Events of Photodynamics in Reversible Photoswitching Fluorescent Protein Dronpa

Reversible photoswitching fluorescent protein Dronpa can reversibly switch between fluorescent on-state and nonfluorescent off-state by two radiations. The primary events of photodynamics in Dronpa were elucidated by nonadiabatic ONIOM (CASSCF:AMBER) molecular dynamics simulations. All radiationless decay processes are found mainly to result from one bond flip of a bridge C−C bond of the chromophore in the protein, regardless of its protonation state or conformation, rather than hula twisting. In the off-state protein, trans−cis photoisomerization of the neutral trans chromophore takes place via a rotation around the imidazolinone ring. In the wild-type on-state protein, the anionic cis chromophore mostly remains planar for at least 20 ps. In contrast, in the H193T mutant on-state, faster decay via a rotation of the phenoxy ring or imidazolinone ring of the anionic cis chromophore was found, suggesting that flexibility of the chromophore and its immediate protein environment is the key to radiationless decays.