(Trifluoromethyl)lumazine derivatives as 19F NMR probes for lumazine protein

Lumazine protein acts as an electronic excited state transducer in bioluminescence of Photobacterium species. The protein binds 6,7-dimethyl-8-(D-ribityl)lumazine (1) which serves as the fluorophore. This compound also serves as a biosynthetic precursor of riboflavin and is the substrate of the enzyme riboflavin synthase. This enzyme and lumazine protein show considerable sequence homology. The interaction of lumazine apoprotein with several trifluoromethyl analogs of 6,7-dimethyl-8-ribityllumazine was investigated by 19F NMR spectroscopy. Upon binding to the protein, the 19F NMR resonances of the ligand shift to lower field with broadened line widths to around 30 Hz. By comparison, all ligands studied show more complex NMR spectra when bound to riboflavin synthase. Only one position 7 epimer (designated epimer A) of 6,7-bis(trifluoromethyl)-7-hydroxy-8-(D-ribityl)lumazine binds to lumazine apoprotein and riboflavin synthase. The apoprotein can also bind lumazine derivatives with a quarternary C-7. It is suggested that the binding site of lumazine protein corresponds to the donor binding site of riboflavin synthase.