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 pKa 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 changed in pKa 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 pKa 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 pKa 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 pKa of the proton equilibrium.
Das
D K
DK
Department of Chemistry, Gauhati University, Assam, India.
Medhi
O K
OK
D000077286:Cetrimonium; D002593:Cetrimonium Compounds; D006427:Hemin / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008823:Micelles; D015394:Molecular Structure; D010084:Oxidation-Reduction; D010777:Photochemistry / Q000379:methods; D013501:Surface-Active Agents
D002593:Cetrimonium Compounds; D008823:Micelles; D013501:Surface-Active Agents; D006427:Hemin; D000077286:Cetrimonium