On the Mechanism of Reduction of Porphyrins. Pulse Radiolytic Study

The mechanisms of reduction of hematoporphyrin, PH₂, and its Zn complex, ZnP, by strong single-electron reducing agents in alkaline aqueous solutions were studied. It was found that for both compounds three intermediates are formed prior to the formation of the stable products. The mechanism of reduction of both compounds seems to be the following: (a) PH₂/ZnP + R · ^- → PH₃ · or (PH₂ · ^-)/(ZnPH · (ZnP ·^-) + R (k₁), where k₁ depends on R ·^-; (b) 2PH₃./2ZnPH · → (PH₃)₂/(ZnPH)₂ (k₂ = 2.1 × 10⁸ M^-1 sec^-1), where the dimer is plausibly a π-mer; (c) (PH₃)₂/(ZnPH)₂→(PH₃)_2′/(ZnPH)_2′ (k_PH2 = 10 sec^-1; k_ZnP = 17 sec^-1) (this reaction seems to be some kind of rearrangement of the dimer); (d) (here the mechanisms differ) (PH₃)_2′-→ PH₄ or (PH₅⁺) + PH₂ (k = 0.11 sec^-1) and (ZnPH)_2′ → Zn(dihydroporphyrin) + ZnP, where PH₄ is a phlorin, whereas in the Zn(dihydroporphyrin) one of the hydrogen atoms is added on a peripheral position. This mechanism is nearly independent of pH in the 7 ≤ pH ≤ 13 range and of the solvent water, methanol, or ethanol. The absorption spectra of all intermediates are reported. The reduction of hemin by e_aq^- or CH₃CHO^- on the other hand proceeds via direct reduction of the central Fe(III) to Fe(II) in one step.