[NMR spectroscopic characterization of deuterohemin complexes in aqueous media]

1H-NMR measurement and NMR susceptibility measurements were carried out with deuterohemin complexes in aqueous solution. The hydroxyaquo complex of deuterohemin, which is dimeric in weakly alkaline medium, turns into a low-spin dicyano complex on addition of cyanide. This reaction proceeds in two steps: whereas in the first, slow step a dimeric low-spin hydroxycyano complex is formed, in the second step the latter is converted quickly into the dimeric dicyano complex. The pK values for the overall reaction, the first and the second step were measured to be 23.25; 0.39 and 22.86, respectively. The values of free energy variation obtained therefrom suggest that conformational changes are decisive in the liganding of methemoglobin.