Formation of Schiff-base for photoreaction mechanism of red shift of GFP spectra

We have proposed the formation of Schiff-base between R96 and chromophore (CRO) to elucidate the reaction mechanism for the irreversible red shift of green fluorescent protein (GFP) spectra under the absence of oxygen. The difference between absorption energies of reactant and product for our GFP models with CIS(D)/6-31G* level is 0.21 eV, which is in reasonable agreement with the corresponding experimental value of 0.25 eV. We have suggested the irreversible photoreaction mechanism, where the CRO excited from ground (S0) state to first excited singlet (S1) state immediately turns to the first excited triplet (T1) state, and the nucleophilic addition reaction occurs on the T1 state.