A Computational Investigation of the Equilibrium Constants for the Fluorescent and Chemiluminescent States of Coelenteramide.

In spite of recent advances in understanding the mechanism of coelenterate bioluminescence, there is no consensus about which coelenteramide specie and/or state are the light emitter. In this study, a systematic investigation of the geometries and spectra of all possible light emitters has been performed at the TD ωB97XD/6-31+G(d) level of theory, including various fluorescent and chemiluminescent states in vacuum, in a hydrophobic environment and in aqueous solution. To deduce the most probable form of the fluorescent and chemiluminescent coelenteramide emitter, the equilibrium constants for the fluorescent and chemiluminescent states connecting the various species have been calculated. ωB97XD gives a qualitatively good description of fluorescent and chemiluminescent structures. Coelenteramide is formed in a "dark" chemiluminescent state and must evolve to a bright fluorescent state. Moreover, the photoacidity of the phenol group is significantly higher in the fluorescent state than in the chemiluminescent state, which allows the formation of phenolate coelenteramide and clarifies its role as the bioluminescent emitter.