Effect of surfactant and pH on the redox potential of microperoxidase 11 in aqueous micellar solutions

Abstract

The redox potential of heme undecapeptide from cytochrome c (microperoxidase 11) in aqueous sodium dodecyl sulfate (sds), hexadecyltrimethylammonium bromide, and Triton X-100 surfactant micelles varied from +2 mV at pH 3.0 to –222 mV at pH 9.0. The potentials at pH 7.0 were –114, –122, and –166 mV vs. the normal hydrogen electrode in the three surfactants. The nature of the axial ligands, spin state of iron, apolar nature of the local heme environment, and pH influence the potential in the micelles. Binding of histidine (HisH) of the peptide chain gave a negative shift of –60 mV, and deprotonation of co-ordinated HisH to histidinate gave a –100 mV shift of the potential in aqueous sds. At pH 5.0–6.0 the axial ligands to iron are H₂O and HisH; deprotonation of co-ordinated H₂O gave –65 mV shift of the potential. Interaction of hemin with surfactant gave a positive shift of the potential with respect to that in water. The diffusion coefficient of the undecapeptide (2.4 × 10^–7 cm² s^–1) at pH 7.0 in the micelles is an order of magnitude smaller than that in water, indicating solubilisation of heme in surfactant solutions. The potential is strongly dependent on pH and is controlled by the uptake/release of protons at three sites: the unco-ordinated HisH of the peptide chain, the axially co-ordinated H₂O and HisH ligands. The pK_a values of these redox state-dependent ionisations in the iron(III) state are ca. 4.3, 6.3, and 8.3. The change in potential per unit change of pH (ΔE/ΔpH) was ca. –59 mV, which indicates proton-coupled electron transfer involving one electron and one proton.