Photoinduced Distance-Dependent Electron Transfer of Fluorescent Proteins

Fluorescent proteins are capable of photoinduced electron transfer, and this property has vast application potentials for chemistry, green catalysis, sustainable energy supply, etc. However, the detailed mechanism, the boundaries, and the maximum possible output reduction potentials remain elusive. Herein, three different metal ions of various reduction potentials are applied to examine their interaction with fluorescent protein PsmOrange through X-ray crystallography and spectrometry. PsmOrange is an irreversible photo-switchable fluorescent protein that's easily oxidized, causing obvious alterations including cleavage of peptide chain, red-shift of fluorescence, and color change. It's shown here that electron transfer from the chromophore to bound metal ions leads to quenching of fluorescence, which causes neither redox damage to the chromophore, nor breakage of peptide chain, nor red-shift of fluorescence, as suggested by structures and spectrophotometry. The perseverant nature of fluorescent proteins promises limitless possibilities to exploit. The reducing power decreases as the distance from the chromophore increases, setting up natural boundaries for the utilization of electrons transferred from fluorescent proteins. The mechanism uncovered here has profound implications for the employment of fluorescent proteins for practical application purposes.