Competitive Mechanistic Pathways for Green-to-Red Photoconversion in the Fluorescent Protein Kaede: A Computational Study

In Kaede, a new class of fluorescent protein, dramatic changes of photophysical and chemical properties by UV illumination have been observed in which the color of fluorescence is irreversibly altered from green to red. Unusual photoinduced peptide backbone cleavage resulting in extending pi-conjugation of the chromophore takes place. Two mechanistic pathways (El and E2 mechanisms) involving the N-C-alpha bond cleavage at His62 and deprotonation at C-beta by Glu212 have been proposed. Here several possible pathways are explored with explicit consideration of protein environment by ONIOM(B3LYP:AMBER) calculations. The results reject the concerted E2 pathway. Instead, the stepwise E1 and new E1cb mechanisms are suggested to occur and may compete with each other in the electronic ground state. Absorption for the green- and red-type chromophore in vacuum and within the Kaede protein matrix was studied.