Kaede,
an analogue of green fluorescent protein (GFP), is a green-to-red
photoconvertible fluorescent protein used as an in vivo “optical highlighter” in bioimaging. The fluorescence
quantum yield of the red Kaede protein is lower than that of GFP,
suggesting that increasing the conjugation modifies the electronic
relaxation pathway. Using a combination of anion photoelectron spectroscopy
and electronic structure calculations, we find that the isolated red
Kaede protein chromophore in the gas phase is deprotonated at the
imidazole ring, unlike the GFP chromophore that is deprotonated at
the phenol ring. We find evidence of an efficient electronic relaxation
pathway from higher-lying electronically excited states to the S1 state of the red Kaede chromophore that is not accessible
in the GFP chromophore. Rapid autodetachment from high-lying vibrational
states of S1 is found to compete efficiently with internal
conversion to the ground electronic state.
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