Carbon-13 and nitrogen-15 NMR studies on the interaction between 6,7-dimethyl-8-ribityllumazine and lumazine protein

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Abstract
The interaction between the prosthetic group 6,7-dimethyl-8-(1''-D-ribityL)lumazine and the lumazine apoproteins from two marine bioluminescent bacteria, one from a relatively thermophilic species, Photobacterium leiognathi, and the other from a psychrophilic species, Photobacterium phosphoruem, was studied by 13C and 14N NMR using various selectively enriched derivatives. It is shown that the electron distribution in the protein-bound 6,7-dimethyl-8-ribityllumazine differs from that of free 6,7-dimethyl-8-ribityllumazine in buffer. The 13C and 15N chemical shifts indicate that the protein-bound 6,7-dimethyl-8-ribityllumazine is embedded in a polar environment and that the ring system is strongly polarized. It is concluded that the two carbonyl groups play an important role in the polarization of the molecule. The N(3)-H group is not accessible to bulk solvent. The N(8) atom is sp2 hybridized and has .delta.+ character. Nuclear Overhauser effect studies indicated that the 6,7-dimethyl-8-ribityllumazine ring is rigidly bound with no internal mobility. The NMR indicate that the interaction between ring system and the two apoproteins is almost the same.
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