The transfer of a positively-charged hydrogen atom a bare proton along a reaction chain in GFP generates a green flash of light. The laser snapshots show that when the light absorber, or chromophore, nestled in the middle of the protein barrel absorbs an incoming photon of blue light, it starts vibrating, and the electrons start sloshing around the chromophore until it is aligned just right for the proton to hop via a water molecule to a nearby amino acid in the protein. From there, it continues down the reaction chain, creating a state with a negatively charged chromophore that emits green light.
"A lot of people have studied green fluorescent protein for many years and found out that proton transfer in the excited state emits a very efficient flash for every 100 blue photons going in, 80 green ones come out," said first author Chong Fang, a UC Berkeley postdoctoral fellow in the Department of Chemistry. "This experiment shows why it is so efficient with vivid atomistic details."
Previous studies had shown that after the chromophore absorbs blue light, it undergoes proton transfer, and green light is emitted. In the current study, Mathies, Fang and their colleagues could actually resolve the early stage of this proton transfer reaction, taking snapshots of the vibrational wagging of the chromophore skeleton in sync with the electron cloud in the chromophore sloshing back and forth. However, the wagging oscillation might have stopped after a few picoseconds, when the chromophore and its vicinity are aligned just right for the proton to hop off down the reaction chain, and the whole protein shines bright green which it does in its own good time, in about 3 nanoseconds.
"We don't need the wagging oscillation to persist throughout the proton transfer process, we only need it to position the chromophore
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University of California - Berkeley