Multireference theoretical studies on the solvent effect of firefly multicolor bioluminescence

In concert with the recent spectroscopic studies of the light-color modulation mechanism of firefly (Hirano et al., J Am Chem Soc 2009, 131, 2385), quantum chemical calculations using complete active space SCF (CASSCF), multistate complete active space second order perturbation (MS-CASPT2) theory as well as a time-dependent density functional theory (TD-DFT) approach with the Coulomb attenuated hybrid exchange-correlation functional (CAM-B3LYP) were performed on the excited state (S₁) of the keto-form oxyluciferin (keto-OxyLH₂). Benzene, DMSO, CH₃CN, and H₂O were chosen as polar solvents. The polarization effect of less polar solvent was considered by a simple model, complex of keto-OxyLH_2, and NH ₃ with different covalent character. The calculated results supported the experimental conclusion: (1) the light emitter of bioluminescence is the S₁ state of keto-OxyLH₂ anion [(keto-1)*], and (2) light emission from (keto-1)* is modulated by the polarity of surrounding environment and the degree of covalent character of hydrogen bond between (keto-1)* and a protonated basic moiety. The mechanism of the multicolor bioluminescence was discussed from the theoretical viewpoint.