Hemes and hemoproteins. Part 7. Co-ordination of ammonia, aniline and pyridine by the iron(III) porphyrin microperoxidase-8

Abstract

The equilibrium constant for the substitution of co-ordinated H₂O by NH₃ in the monomeric iron(III) haem octapeptide microperoxidase-8 (MP-8) has been determined by UV/VIS spectrophotometry in 20% aqueous MeOH (l= 0.1 mol dm^–3) at 25 °C from the pH dependence of the apparent binding constant over the range pH 7–10 as (2.21 ± 0.15)× 10³ dm³ mol^–1(log₁₀K 3.34 ± 0.03); this appears to be the first such equilibrium constant for the co-ordination of NH₃ reported for any iron(III) porphyrin or haemoprotein. Variation of the spectrum of the NH₃ complex in aqueous solution over the range pH 9–13 revealed a second equilibrium with pK 11.9 involving one proton, which is ascribed to ionisation of the imidazole ring of the trans-histidine ligand to give the co-ordinated imidazolate. The octapeptide binds pyridine and aniline in 20% aqueous MeOH at 25 °C with equilibrium constants of log₁₀K= 2.73 ± 0.02 and 2.67 ± 0.01 respectively to give products which also exhibit further pH-dependent equilibria (pK at 11.2 and 11.8 respectively) and show spectra very similar to those of the NH₃ complexes; this indicates that they both bind through co-ordination to the metal and not π–π interaction with the porphyrin ring.