Ultrafast Excited-State Dynamics of the Photoswitchable Protein Dronpa

Dronpa is a photoswitchable protein from the family of green fluorescent proteins (GFPs). Photoswitching involves forward and backward proton transfer between a bright deprotonated form (B) and a dark protonated form (A 2 ). We have used femtosecond transient absorption spectroscopy to determine the kinetics of the first step of the photoconversion from the dark to the bright form, which we have measured as 4 ps. The 2-fold isotope effect on the kinetics of this process shows that excited-state proton transfer (ESPT) is involved in this step. It was also demonstrated that the acid-induced protonated form A 1 and the photoconverted protonated form A 2 are two distinct spectroscopic species.