QM/MM Calculations on Excited-State Proton Transfer and Photoisomerization of a Red Fluorescent Protein mKeima with Large Stokes Shift

Large Stokes shift red fluorescent proteins (LSS-RFPs) are of growing interest for multicolor bioimaging applications. However, their photochemical mechanisms are not fully understood. Here, we employed the QM(XDW-CASPT2//CASSCF)/MM method to investigate the excited-state proton transfer and photoisomerization processes of the LSS-RFP mKeima starting from its cis neutral isomer. Upon excitation to the bright S₁ state in the Franck-Condon region, mKeima relaxes to a metastable minimum-energy state. From this short-lived species, two competing deactivation pathways are available: the excited-state proton transfer in the S₁ state, and the S₁ decay via the S₁/S₀ conical intersection as a result of the cis-trans photoisomerization. In comparison, the former is a dominant excited-state relaxation pathway, leading to the cis anionic isomer of mKeima in the S₁ state. This anionic intermediate then undergoes cis-trans photoisomerization after overcoming a barrier of approximately 10 kcal/mol in the S₁ state, which is followed by an excited-state decay via the S₁/S₀ conical intersection region. The efficient nonadiabatic decay of the cis anionic isomer of mKeima in the S₁ state inhibits the radiative process, leading to a weak emission around 520 nm observed experimentally. These findings shed important mechanistic light on the experimental observations and provide valuable insights that could help in the design of LSS-RFPs with superior fluorescence properties.