ИСТИНА

Fluorescent proteins (FPs) are widely used in biology as tags to trace other proteins in living cells. Photoswitchable fluorescent proteins constitute a unique subclass as they can change their absorption/emission maxima upon laser illumination. Purified from the sea anemone Anemonia sulcata the asCp is one of the most studied FP of this type. Its ability to be reversibly switched between the nonfluorescent “off”-state and the fluorescent “on”-state upon the intensive irradiation with green light has been attributed to the Z-E isomerization of its chromophore. The present work was aimed at studying multiple de-excitation channels of both dark and bright forms of the asCp protein. We have found planar structure of the S1 excited state minimum of the Z-chromophore anion. The estimated emission wavelength (605 nm) is close to the experimental value (595 nm). Fluorescence of the “on”-state can thereby be accounted for by the anionic form of the chromophore. Such a planar structure is unstable for the E-chromophore in both anionic and zwitterionic forms. It results in a very low quantum yield of fluorescence in the “off”-state of the protein. The calculated value of the oscillator strength (0.01) of the S1-S0 transition at the S1 equilibrium geometry configuration also confirms the above statement. Extremely short excited-state lifetime of the dark form has been found to be related to the presence of fast internal conversion channels located close to the S1 equilibrium structure of the anion. Relaxation proceeds via a conical S0/S1 intersection in both cases. Minimum energy conical intersection structures are highly twisted along the phenoxy-bridge bond and lie less than 6 kcal/mol above the S1 excited state minimum.