Ultrafast dynamics of epicocconone, a second generation fluorescent protein stain.

Femtosecond upconversion experiment has been carried out for epicocconone and its butylamine adduct in acetonitrile and tert-butanol. An ultrafast component is found to dominate the decay of fluorescence of epicocconone in acetonitrile solution. Upon reacting with butylamine, a model for the epicocconone-protein adduct, this ultrafast component remains almost unaffected but an additional rise time occurs, indicating the formation of a highly emissive species from the locally excited state. This phenomenon is central to the extraordinary applications of epicocconone in biotechnology. The magnitude of the rise time of the butylamine adduct is similar to that of the longer component of the decay of epicocconone in acetonitrile, suggesting that the dynamics of epicocconone and its butylamine adduct are similar. The ultrafast component is slowed upon increasing the viscosity of the solvent. This results in a marked increase in quantum yield and suggests that it corresponds to rapid bond isomerization, leading to a nonradiative decay. Surprisingly, in water/sucrose mixtures, the ultrafast component remains unaffected but there is still an increase in quantum yield, suggesting that there are at least two nonradiative pathways, one involving bond isomerization and another involving proton transfer. The correct interpretation of these data will allow the design of second generation protein stains based on the epicocconone scaffold with increased quantum yields and photostability.