Protein photochromism observed by ultrafast vibrational spectroscopy

Photochromic proteins, such as Dronpa, are of particular importance in bioimaging and form the basis of ultraresolution fluorescence microscopy. The photochromic reaction involves switching between a weakly emissive neutral trans form of the chromophore (A) and its emissive cis anion (B). Controlling the rates of switching has the potential to significantly enhance the spatial and temporal resolution in microscopy. However, the mechanism of the switching reaction has yet to be established. Here we report a high signal-to-noise ultrafast transient infrared investigation of the photochromic reaction in the mutant Dronpa2, which exhibits facile switching behavior. In these measurements we excite both the A and B forms and observe the evolution in the IR difference spectra over hundreds of picoseconds. Electronic excitation leads to bleaching of the ground electronic state and instantaneous (subpicosecond) changes in the vibrational spectrum of the protein. The chromophore and protein modes evolve with different kinetics. The chromophore ground state recovers in a fast nonsingle-exponential relaxation, while in a competing reaction the protein undergoes a structural change. This results in formation of a metastable form of the protein in its ground electronic state (A′), which subsequently evolves on the time scale of hundreds of picoseconds. The changes in the vibrational spectrum that occur on the subnanosecond time scale do not show unambiguous evidence for either proton transfer or isomerization, suggesting that these low-yield processes occur from the metastable state on a longer time scale and are thus not the primary photoreaction. Formation of A′, and further relaxation of this state to the cis anion B, are relatively rare events, thus accounting for the overall low yield of the photochemical reaction.