Absorption Spectrum of the Green Fluorescent Protein Chromophore AnionIn Vacuo

A sensitive photoabsorption technique for studies of gas-phase biomolecules has been used at the ELISA electrostatic heavy-ion storage ring. We show that the anion form of the chromophore of the green fluorescent protein in vacuo has an absorption maximum at 479 nm, which coincides with one of the two absorption peaks of the protein. Its absorption characteristics are therefore ascribed to intrinsic chemical properties of the chromophore. Evidently, the special $\beta$-can structure of the protein provides shielding of the chromophore from the surroundings without significantly changing the electronic structure of the chromophore through interactions with amino acid side chains.