On photoabsorption of the neutral form of the green fluorescent protein chromophore.

We present results of theoretical studies of the photoabsorption band corresponding to the vertical electronic transition S(0)-S(1) between first two singlet states of the model chromophore from the green fluorescent protein (GFP) in its neutral form. Predictions of quantum chemical approaches including ab initio and semi-empirical approximations are compared for the model systems which mimic the GFP chromophore in different environments. We provide evidences that the protein matrix in GFP accounts for a fairly large shift of about 40 nm in the S(0)-S(1) absorption band as compared to the gas phase.