The fluorescent properties of coelenteramide, a substrate of aequorin and obelin

The absorption and emission spectra of six different possible light emitters of coelenteramide: neutral, phenolate anion, phenolate anion with a counter ion (phenolate−–M+), amide anion, pyrazine anion, and the dianion were investigated with the CAM-B3LYP method in the gas phase, in aqueous solution, in a hydrophobic environment and in benzene. The emission spectra of the phenolate anion can be modulated by the covalent character of an oxygen metal bond between the phenolate anion and the countercation. The phenolate anion is more easily formed than the amide anion by deprotonation of the neutral in the excited state. The phenolate anion could not be deprotonated to form the dianion. To examine the charge transfer states of coelenteramide, the charge distributions for the ground and excited states of the complexes were predicted. The emission spectra were predicted in the excited state solvent reaction field. The ground state energies were determined with non-equilibrium solvation, at the excited state geometry with static solvation from the excited state.

Graphical abstractThe structures and fluorescence properties of coelenteramides were studied by CAM-B3LYP method in the gas phase, in aqueous solution, in a hydrophobic environment and in benzene. The results indicated that the phenolate anion could be more easily formed than the amide anion by deprotonation of the neutral in the excited state. The phenolate anion could not be deprotonated to form the dianion.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Alkali metal ions were employed in modeling the ion pairs phenolate−–M+. ► The emission spectra of phenolate anion were modulated by the oxygen metal bond. ► The phenolate anion is more easily formed than the amide anion. ► The emission spectra were predicted in the excited state solvent reaction field.