Protonation, photobleaching, and photoactivation of yellow fluorescent protein (YFP 10C): a unifying mechanism.

Yellow fluorescent protein (YFP 10C) is widely used as a probe in biology, but its complex photochemistry gives rise to unusual behavior that requires fuller definition. Here we characterize the kinetics of protonation and reversible bleaching over time scales of picoseconds to hours. Stopped-flow and pressure-jump techniques showed that protonation of the fluorescent YFP(-) anion state is two-step with a slow transition that accounts for blinking of 527 nm emission at the single molecule level on the seconds time scale. Femtosecond spectroscopy revealed that the protonated excited-state (YFPH*) decayed predominantly by a radiationless mechanism, but emission at 460 nm was detected within the first picosecond. Limited excited-state proton transfer leads to 527 nm emission characteristic of the YFP(-*) anion. Prolonged continuous wave illumination at the peak of YFP(-) absorbance (514 nm) yields, irreversibly, a weakly fluorescent product that absorbs at 390 nm. This "photobleaching" process also gives a different species (YFPHrb) that absorbs at 350/430 nm and spontaneously regenerates YFP(-) in the dark on the time scale of hours but can be photoactivated by UV light to regenerate YFP(-) within seconds, via a ground-state protonated intermediate. Using a pulsed laser for photobleaching resulted in decarboxylation of YFP as indicated by the mass spectrum. These observations are accounted for in a unifying kinetic scheme.