Synthesis of 13C-dehydrocoelenterazine and model studies on Symplectoteuthis squid bioluminescence

In the photoprotein of an Okinawan squid bioluminescence of Symplectoteuthis oualaniensis L a dehydrocoelenterazine has been assigned as a chromophoric precursor to its apoprotein. To prove this mechanism, we have established new synthetic route to ca. 100%-13C incorporated dehydrocoelenterazine and coelenterazine at the neighboring carbon of the 2-position of 2,3-dihydroimidazo-[1,2a]-pyrazinone skeleton. This 13C enriched dehydrocoelenterazine readily converted in equilibrium between its adduct forms as a diastereomixture with glutathione (GSH) or dithiothreitol (DTT) compounds having sulfhydryl group. Structures of such adducts were fixed under acidic conditions and then discussed by NMR spectroscopy as well as absorbance and fluorescence spectra.