Absorption spectra of the GFP chromophore in solution: comparison of theoretical and experimental results Original Research Article

Spectroscopic characteristics of the green fluorescent protein (GFP) and its mutants are controlled through protonation states of the chromophore and the polarity of its environment. Using the semiempirical method NDDO-G, absorption spectra of all possible protonation states of a model chromophore in the gas phase and in polar solution (ethanol) were calculated. Very good agreement between the simulated and experimental spectra has been found. We discuss the following protonation states of the model GFP chromophore: the anion (OY, N, OX)−, the neutral forms (HOY, N, OX) and (OY, N, HOX), the zwitter ion (OY−, HN+, OX), the cations (HOY, HN, OX)+, (OY, HN, HOX)+ and (HOY, N, HOX)+, and the dication (HOY, HN, HOX)2+. Our calculations show that solvatochromic effects on the electronic spectrum of the GFP chromophore can induce red or blue shifts of the absorption, depending on the protonation state. The different spectroscopic characteristics found for the various protonation states may be useful for the interpretation of spectra of intact GFPs.