Pulse radiolysis study of protoferrihaem IX intercalated in sodium dodecyl sulphate micelles

Abstract

Haemin intercalated in sodium dodecyl sulphate (SDS) micelles exists as monomer in an organic sphere of dimensions similar to that of cytochrome-c and thus provides a model which is closer to the biological molecule than any which has been studied hitherto. Hydrated electrons reduce intercalated haemin at pH 9.2 with k= 5.2 × 10⁹ dm³ mol^–1 s^–1. CH₃CHOH radicals reduce it at pH 9.2 with k= 5.6 × 10⁸ dm³ mol^–1 s^–1 and at pH 4 with k= 1.6 × 10⁹ dm³ mol^–1 s^–1. Reduction is accompanied by spectroscopic changes in the haemin molecule at both pH 9.2 and 4, but at pH 9.2 additional changes take place afterwards over periods of several milliseconds. At pH 4.0 there are no additional changes. The final spectrum attained in both cases corresponds to the formation of reduced haemin in 100 % yield.

The additional changes taking place after reduction in mildly alkaline solution are found to have an activation energy of 66.6 kJ mol^–1. They are independent of pH in the range 9.2–10.7, and of added electrolyte in the range 0.05–0.2 mol dm^–3. They are attributed to conversion of OH^– in reduced haemin into H₂O, perhaps by protonation by water: [OH–Fe^III(por)–H₂O]°+ e^–_aq→[graphic omitted]. Corresponding changes are not seen at pH 4 because the haemin exists as [H₂O–Fe^III(por)–H₂O]⁺ at that pH.