The effect of oxidation on the electronic structure of the green fluorescent protein chromophore
Electronic structure calculations of the singly and doubly ionized states of deprotonated 4'-hydroxybenzylidene-2,3-dimethylimidazolinone (HBDI anion) are presented. One-electron oxidation produces a doublet radical that has a blue-shifted absorption, whereas the detachment of two electrons yields a closed-shell cation with strongly red-shifted (by about 0.6 eV) absorption relative to the HBDI anion. The results suggest that doubly oxidized species may be responsible for oxidative redding of GFP. The proposed mechanism involves two-step oxidation via electronically excited states and is consistent with the available experimental information [Bogdanov et.~al, Nature Chem. Biol. 5, 459 (2009)].