One-electron reduction of iron(II) porphyrin and characterization of iron(I) porphyrin in aqueous medium. Steady-state and pulse radiolysis studies

EPR and ENDOR studies of Fe(II) hemoproteins reduced and oxidized at 77 K

gamma-irradiation of frozen solutions of Fe(II) hemoproteins at 77 K generates both electron paramagnetic resonance (EPR) active singly reduced and oxidized heme centers trapped in the conformation of the Fe(II) precursors. The reduction products of pentacoordinate (S = 2) Fe(II) globins, peroxidases and cytochrome P450cam show EPR and electron-nuclear double resonance (ENDOR) spectra characteristic of (3d 7) Fe(I) species. In addition, cryoreduced Fe(II) alpha-chains of hemoglobin and myoglobin exhibit an S = 3/2 spin state produced by antiferromagnetic coupling between a porphyrin anion radical and pentacoordinate (S = 2) Fe(II). The spectra of cryoreduced forms of Fe(II) hemoglobin alpha-chains and deoxymyoglobin reveal that the Fe(II) precursors adopt multiple conformational substates. Reduction of hexacoordinate Fe(II) cytochrome c and cytochrome b5 as well as carboxy complexes of deoxyglobins produces only Fe(II) porphyrin pi-anion radical species. The low-valent hemoprotein intermediates produced by cryoreduction convert to the Fe(II) states at T > 200 K. Cryogenerated Fe(III) cytochrome c and cytochrome b5 have spectra similar to these for the resting Fe(III) states, whereas the spectra of the products of cryooxidation of pentacoordinate Fe(II) globins and peroxidases are different. Cryooxidation of CO-Fe(II) globins generates Fe(III) hemes with quantum-mechanically admixed S = 3/2, 5/2 ground states. The trapped Fe(III) species relax to the equilibrium ferric states upon annealing at T > 190 K. Both cryooxidized and reduced centers provide very sensitive EPR/ENDOR structure probes of the EPR-silent Fe(II) state.

Parameters controlling the kinetics of ferric and ferrous hemeproteins reduction by hydrated electrons

To clarify the processes of hemeproteins reduction, three classes of these proteins (ferric, ferrous and desFe) were reduced by hydrated electrons generated by pulse radiolysis. Spectral and kinetic investigations were made on alpha hemoglobin chain and myoglobin. Human alpha chain has been chosen to avoid all ferric contaminations and horse ferric myoglobin to eliminate all ferrous protein fractions. We have successively studied the influences of: the iron presence, its oxidation state (II and III), the protein charge and the iron-ligand nature (H2O, OH-, N3- and CN-). For alpha human hemoglobin chain without metallic ion or with ferrous iron, the reduction rates are the same: 1.1 +/- 0.2.10(10) M-1.s-1. In the case of horse ferric myoglobin, the reduction rates depend principally on the protein charge (from pH 6.3 to pH 9.5, the reduction rate of Mb(FeIII)N3- decreases from 2.5 +/- 0.5.10(10) M-1.s-1 to 1.2 +/- 0.2.10(10) M-1.s-1) and are also modulated by the equilibrium constant of the hemeprotein-ligand association (1.2 +/- 0.2.10(10) M-1.s-1 for Mb(FeIII)N3- and 0.8 +/- 0.2.10(10) M-1.s-1 for Mb(FeIII)CN-, at pH 9.8).