title: Hybrid QM/MM simulations of the obelin bioluminescence and fluorescence reveal an unexpected light emitter.
abstract: 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.
authors.affiliation: Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University , Beijing 100875, P.R. China.
authors.forename: Shufeng
authors.initials: S
authors.lastname: Chen
authors.forename: Isabelle
authors.initials: I
authors.lastname: Navizet
authors.forename: Roland
authors.initials: R
authors.lastname: Lindh
authors.forename: Yajun
authors.initials: Y
authors.lastname: Liu
authors.forename: Nicolas
authors.initials: N
authors.lastname: Ferré
mesh_terms: D000818:Animals; D003198:Computer Simulation; D005453:Fluorescence; D037402:Hydrozoa / Q000737:chemistry*; D008164:Luminescent Proteins / Q000737:chemistry*; D008958:Models, Molecular
chemical_list: D008164:Luminescent Proteins; C010497:obelin