Ground-state proton transfer in the photoswitching reactions of the fluorescent protein Dronpa

The reversible photoswitching between the 'on' and 'off' states of the fluorescent protein Dronpa involves photoisomerization as well as protein side-chain rearrangements, but the process of interconversion remains poorly characterized. Here we use time-resolved infrared measurements to monitor the sequence of these structural changes, but also of proton transfer events, which are crucial to the development of spectroscopic contrast. Light-induced deprotonation of the chromophore phenolic oxygen in the off state is a thermal ground-state process, which follows ultrafast (9 ps) trans-cis photoisomerization, and so does not involve excited-state proton transfer. Steady-state infrared difference measurements exclude protonation of the imidazolinone nitrogen in both the on and off states. Pump-probe infrared measurements of the on state reveal a weakening of the hydrogen bonding between Arg66 and the chromophore C=O, which could be central to initiating structural rearrangement of Arg66 and His193 coinciding with the low quantum yield cis-trans photoisomerization.