EFFECT OF SURFACTANT AND PH ON THE REDOX POTENTIAL OF MICROPEROXIDASE11 IN AQUEOUS MICELLAR SOLUTIONS

The redox potential of heme undecapeptide from cytochrome c (micropero xidase 11) in aqueous sodium dodecyl sulfate (sds), hexadecyltrimethyl ammonium 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 surf actants. The nature of the axial ligands, spin state of iron, apolar n ature of the local heme environment., and pH influence the potential i n 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 p H 5.0-6.0 the axial ligands to iron are H2O and HisH; deprotonation of coordinated H2O gave -65 mV shift of the potential. Interaction of he min with surfactant gave a positive shift of the potential with respec t to that in water. The diffusion coefficient of the undecapeptide (2. 4 x 10(-7) cm(2) s(-1)) at pH 7.0 in the micelles is an order of magni tude 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 ionisati ons in the iron(III) state are ca. 4.3, 6.3, and 8.3. The change in po tential per unit change of pH (Delta E/Delta pH) was ca. -59 mV, which indicates proton-coupled electron transfer involving one electron and one proton.