Non-Condon Effects in the One- and Two-Photon Absorption Spectra of the Green Fluorescent Protein

We report calculations of one and two photon absorption (OPA and TPA, respectively) spectra Of the green fluorescent protein (GFP) chromophore using the double-harmonic parallel-mode approximation and including explicit dependence of the electronic transition moments on nuclear geometry. The non-Condon effects are found to be more significant for TPA resulting in a different shape of the spectra and a blue shift of 500 cm(-1) of the TPA peak absorption relative to OPA. The computed shift is in excellent agreement with the experimentally observed 700 cm(-1).