Proton NMR study of the thermodynamics and kinetics of the acid in equilibrium base transitions in reconstituted metmyoglobins.
Optical and proton NMR pH titrations of sperm whale metmyoglobin (metMb) in its native form and reconstituted with chemically modified hemes reveal that the pKa for the acid in equilibrium base transition decreases as the heme 2,4-substituents are made more electron withdrawing. The proton NMR spectra yields resonances which are averaged over the acidic and basic forms of the protein, but still exhibit significant exchange line broadening. Analysis of this exchange contribution to the linewidth is consistent with the simple kinetic scheme metMb+H2O + OH- k2 in equilibrium k1 metMbOH + H2O with k2 = 1.3 +/- 0.5 . 10(10) M-1 . s-1 and k1 = 1.6 +/- 0.6 . 10(5) s-1 for the native protein. The effect of electron-withdrawing substituents on the heme increase k2 and decrease k1.
Proton NMR study of the thermodynamics and kinetics of the acid in equilibrium base transitions in reconstituted metmyoglobins. Optical and proton NMR pH titrations of sperm whale metmyoglobin (metMb) in its native form and reconstituted with chemically modified hemes reveal that the pKa for the acid in equilibrium base transition decreases as the heme 2,4-substituents are made more electron withdrawing. The proton NMR spectra yields resonances which are averaged over the acidic and basic forms of the protein, but still exhibit significant exchange line broadening. Analysis of this exchange contribution to the linewidth is consistent with the simple kinetic scheme metMb+H2O + OH- k2 in equilibrium k1 metMbOH + H2O with k2 = 1.3 +/- 0.5 . 10(10) M-1 . s-1 and k1 = 1.6 +/- 0.6 . 10(5) s-1 for the native protein. The effect of electron-withdrawing substituents on the heme increase k2 and decrease k1.