Mechanistic Insight into Cypridina Bioluminescence with a Combined Experimental and Theoretical Chemiluminescent Approach.

The bioluminescent reaction of the "sea firefly" Cypridina hilgendorfii is a prototypical system for marine bioluminescence, as its substrate possesses an imidazopyrazinone core that is a common link among organisms of eight phyla. The elucidation of the mechanism behind Cypridina bioluminescence is essential for future applications in bioimaging, biomedicine, and bioanalysis. In this study we have investigated the key step of chemiexcitation with a combined experimental and theoretical approach. The obtained results indicate that neutral dioxetanone is responsible for efficient chemiexcitation, as the thermolysis of this species gives access to a long region of the potential energy surface (PES), where the ground and excited singlet states are degenerated. Contrary to expected, neither chemically induced electron-exchange luminescence (CIEEL) nor charge transfer-initiated luminescence (CTIL) can be used to explain imidazopyrazinone-based bioluminescence, as there is no clear relationship between electron (ET)/charge (CT) transfer (occurring between the electron-rich moiety and dioxetanone) and chemiexcitation. Attractive electrostatic interactions between the CO and oxyluciferin moieties allow neutral dioxetanone to spend time in the PES region of degeneracy, while repulsive interactions for anionic dioxetanone lead to a quicker CO detachment.