| Summary: | Among all bioluminescent organisms, the firefly is the most famous, with a high luminescent efficiency of 41%, which is widely used in the fields of biotechnology, biomedicine and so on. The entire bioluminescence (BL) process involves a series of complicated in-vivo chemical reactions. The BL is initiated by the enzymatic oxidation of luciferin (LH<sub>2</sub>). However, the mechanism of the efficient spin-forbidden oxygenation is far from being totally understood. Via MD simulation and QM/MM calculations, this article describes the complete process of oxygenation in real protein. The oxygenation of luciferin is initiated by a single electron transfer from the trivalent anionic LH<sub>2</sub> (<b>L</b><sup>3−</sup>) to O<sub>2</sub> to form <sup>1</sup>[<b>L</b><sup>•2−</sup>…O<sub>2</sub><sup>•−</sup>]; the entire reaction is carried out along the ground-state potential energy surface to produce the dioxetanone (FDO<sup>−</sup>) via three transition states and two intermediates. The low energy barriers of the oxygenation reaction and biradical annihilation involved in the reaction explain this spin-forbidden reaction with high efficiency. This study is helpful for understanding the BL initiation of fireflies and the other oxygen-dependent bioluminescent organisms.
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