Hybrid QM/MM simulations of the obelin bioluminescence and fluorescence reveal an unexpected light emitter.

Obelia longissima, a tiny hydrozoan living in temperate and cold seas, features the Obelin photoprotein, which emits blue light. The Obelin bioluminescence and the Ca(2+)-discharged Obelin fluorescence spectra show multimodal characteristics that are currently interpreted by the concomitant participation of several light emitters. Up to now, the coelenteramide luminophore is thought to exist in different protonation states, one of them engaged in an ion-pair with the nearby residue, His22. Using hybrid quantum mechanics/molecular mechanics (QM/MM) calculations, we demonstrate that such an ion-pair cannot exist as a stable light emitter. However, when His22 electric neutrality is maintained by means of another proton transfer, the phenolate state of coelenteramide exhibits emission properties in agreement with experiment. Finally, an alternative nonradiative decay pathway, involving the formation of a diradical excited state, is postulated for the first time.