Understanding the mechanism of spectral tuning of biological chromophores is a major challenge in photobiology. We show here using large-scale full quantum chemical calculations of the green fluorescent protein that state-of-the-art coupled-cluster calculations provide accurate excitation energies and detailed insight about specific environmental effects. We obtain vertical excitation energies of 3.13 eV (396 nm) and 2.68 eV (463 nm), which are in quantitative agreement with the experimental absorption energies of 3.12 eV (397 nm) and 2.61 eV (475 nm) for the A- and B-forms of the protein. We find that the protein environment redshifts the absorption spectra by ![[similar]](http://www.rsc.org/images/entities/char_223c.gif)
0.56 eV and 0.22 eV for the two states, which can be attributed to 80% electrostatic effects and 20% steric effects.
Physical Chemistry Chemical Physics
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