Quantum Chemical Benchmark Studies of the Electronic Properties of the Green Fluorescent Protein Chromophore.

J. Chem. Theory Comput. 2009, 5, 1895–1906 1895Quantum Chemical Benchmark Studies of the ElectronicProperties of the Green Fluorescent ProteinChromophore. 1. Electronically Excited and IonizedStates of the Anionic Chromophore in the Gas Phase,† ‡ ‡ ‡,§Evgeny Epifanovsky,* Igor Polyakov, Bella Grigorenko, Alexander Nemukhin,,†and Anna I. Krylov*Department of Chemistry, UniVersity of Southern California, Los Angeles, California90089, Department of Chemistry, M.V. LomonosoV Moscow State UniVersity, Moscow119991, Russia, and Institute of Biochemical Physics, Russian Academy of Sciences,Moscow 119334, RussiaReceived March 25, 2009Abstract: We present the results of quantum chemical calculations of the electronic propertiesof the anionic form of the green fluorescent protein chromophore in the gas phase. The verticaldetachment energy of the chromophore is found to be 2.4-2.5 eV, which is below the stronglyabsorbing ππ* state at 2.6 eV. The vertical excitation of the lowest triplet state is around 1.9 eV,which is below the photodetachment continuum. Thus, the lowest bright singlet state is aresonance state embedded in the photodetachment continuum, whereas the lowest triplet stateis a regular bound state. Based on our estimation of the vertical detachment energy, we attributeaminorfeatureintheactionspectrumasduetothephotodetachmenttransition.Thebenchmarkresults for the bright ππ* state demonstrated that the scaled opposite-spin method yields ...