The role of heme propionate in controlling the redox potential of heme: Square wave voltammetry of protoporphyrinato IX iron(III) in aqueous surfactant micelles

The proton equilibrium in aqueous surfactant solutions of hemin, (PPIX)Fe(H2O)(2)(+), involving the propionic acid groups is reported (PPIX = protoporphyrinato IX). The surfactant used are sodium dodecyl sulphate (SDS), triton X-100 (TX-100) and hexadecyl trimethylammonium bromide (CTAB). The pK(a) values, determined spectroscopically from the pH variations of the Soret absorption, are found at ca. 3.5 in SDS and TX-100 micelles. In the cationic surfactant (CTAB) the absorbance of hemin is independent of the hydrogen ion concentration in solution and this surfactant form a salt-link with the heme propionate group. The dependence of mid-point potential on pH indicates that the heme propionate undergoes a redox-linked change in pK(a) from 3.2-3.5 in the ferric form to 4.4-4.5 in the ferrous form (27 degrees C, mu=0.2 M). The change in the mid-point potential per unit change in pH, Delta E/Delta pH, is ca. -59 mV. Replacement of the water molecules in hemin by tetrahydrofuran, (PPIX)Fe(thf)(2)(+), induces a considerable shift of pK(a) of the heme propionate: 6.2-6.8 in the ferric form and 7.5-7.8 in the ferrous form. The lower value of pK(a) in diaquo hemin is attributed to stabilisation of the propionate by a hydrogen bond with coordinated water molecules. All these results may be interpreted in terms of the involvement of a heme propionate group in redox linked uptake of protons and the influence of hydrogen bonding and salt-link formation on the pK(a) of the proton equilibrium. (C) 1998 Elsevier Science Inc. All rights reserved.