Interplay between locally excited and charge transfer states governs the photoswitching mechanism in fluorescent protein Dreiklang
We present the results of high-level electronic structure and dynamics simulations of the photoactive protein Dreiklang. With the goal to understand the details of Dreiklang's photocycle, we carefully characterize the excited states of the ON- and OFF-forms of Dreiklang. The key finding of our study is the existence of a low-lying excited state of a charge-transfer (CT) character in the neutral ON form and that population of this state, which is nearly isoenergetic with the locally excited (LE) bright state, initiates series of steps ultimately leading to the formation of the hydrated dark chromophore (OFF state). These results allow us to refine the mechanistic picture of the Dreiklang's photocycle and photoactivation.