A proton-nuclear-magnetic-resonance study at 500 MHz on Megasphaera elsdenii flavodoxin. A study on the stability, proton exchange and the assignment of some resonance lines

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Abstract
1H NMR studies were performed on the 3 redox states of M. elsdenii flavodoxin. The protein is remarkably stable, as concluded from amide proton exchange studies. Some amide protons are still present in the 1H NMR spectrum even after 1 mo. in 2H2O at 33.degree. C (pH 8.3). The reactivity of the exchangeable protons can be grouped into 3 categories, i.e., t1/2 [half-life] .mchgt. 5 min, 10 s .ltorsim. t1/2 .ltorsim. 5 min, and t1/2 .mchlt. 10 s. The amide proton exchange reactions are hardly dependent on the redox state. Optimal resolution of 1H NMR spectra is obtained at 33.degree. C, independent of the redox state. No conformational change of the protein is observed in the pH range between 6.0-8.5. Assignments of resonances to protons of flavin and of some amino acid residues are established in the oxidized and the hydroquinine state using chemically and isotopically substituted flavins and the driven nuclear Overhauser technique. Preliminary 2-dimensional 1H-1H correlated spectra show that the protein is amenable to 2-dimensional NMR techniques. Previous assignments are confirmed by this technique.