Quantum control of population transfer in green fluorescent protein by using chirped femtosecond pulses

We demonstrate that the methods of quantum control can be applied successfully to very large molecules in room temperature liquid solution. Chirped femtosecond pulses are used to excite a green fluorescent protein mutant in both buffered aqueous solution and solid acrylamide gel. At high energy densities, the fluorescence shows a strong chirp dependence, with positively chirped pulses transferring almost 50% more population to the excited state than negatively chirped pulses. By measuring the photobleaching rate in the gel as a function of pulse chirp, we find that the data are consistent with the bleaching of the protein being due to a thermal mechanism rather than to an excited-state photoreaction.