Light emission colour modulation study of oxyluciferin synthetic analogues via QM and QM/MM approaches.

Firefly oxyluciferin is the chemical product of bioluminescence responsible for light emission. Experiments have already shown that different analogues of natural oxyluciferin, exhibit different emission colours. In particular, the structure of natural oxyluciferin has been modified by atom or group substitutions. However, a rationalization of the origin of the bioluminescence emission colour modulation of these analogues has still not been reported. For these reasons, the aim of this study is to explain the influence of structural modifications within the natural oxyluciferin on the colour modulation of bioluminescence. To do this, natural firefly oxyluciferin and three synthetic analogues whose experimental bioluminescence spectra are red- and blue-shifted compared to the natural one were studied. The absorption and emission transition energies have been calculated at the Time-dependent density functional theory (TD-DFT) level using both quantum mechanics (QM) and quantum mechanics/molecular mechanics (QM/MM) methods. Moreover, the solvent (water using the PCM model) and the protein surrounding effect have also been considered. The predicted emission spectra are in quite good agreement with the available experimental spectra, validating the methodology followed in this study. In particular, it was demonstrated that using the QM/MM approach, and considering explicitly the protein environment, the experimental bioluminescence spectra can be reproduced. Furthermore, this study shows that the substitution within the oxyluciferin structure causes a change of its electronic distribution and energies of the HOMO and LUMO orbitals involved in the vertical transitions, leading to different light emission colours. This work will promote future studies focused on luciferin mutations guided by the prediction of their bioluminescence emission spectra.