Theoretical study on the absorption maxima of real GFPs

The excited-state calculations for the real green fluorescent protein (GFP) and its mutants, including the wild-type GFP, Y66F, blue fluorescent protein, and cyan fluorescent protein, were carried out at the INDO/S-CIS//ONIOM (B3LYP/6-31G∗∗:AMBER) level. The numbers of singly excited configuration state functions at INDO/S-CIS were over 20 million. The calculated absorption maxima were in good agreement with the experimental data with a computational error of ⩽15 nm. For the wild-type GFP, INDO/S-CIS yielded a bathochromic shift of 38 nm with respect to its model chromophore in vacuo, which was also in good agreement with the extrapolated experimental shift of 45 nm.