Models of cytochrome b: Mössbauer studies on bis-ligated complexes of (protoporphyrinato IX)iron(III) with histidine and its substituted derivatives

Abstract

Mössbauer spectroscopic studies have been carried out on a series of complexes of the type [FeL(L′)₂]⁺, where H₂L = 3,7,12,17-tetramethyl-8,13-divinylporphyrin-2,18-dipropionic acid, L′= histidine, N^α-acetylhistidine, histamine, or pilocarpate. Measurements were made at various pH values in the range 7.5–12.0 in 20%(v/v) ethanol–water solution frozen at 80 K. When the pH is 8.0–8.5, the major species are low-spin bis complexes [δ= 0.25(2) mm s^–1 and ΔE_Q= 2.0–2.2 mm s^–1]. These are rapidly converted into a molecular complex of histidine and [(FeL)₂O] at high pH (10.1–12.0). The results for the bis(histidine) and related complexes show that histidine binds as a sterically hindered imidazole and that the iron–imidazole bonds are weak. The ΔE_Q value of 2.14 mm s^–1 and large linewidths (0.48–1.22 mm s^–1) of the complexes indicate that the two imidazole planes in the bis(histidine) complex are non-parallel with a large angle between the planes. The observation of asymmetric quadrupole doublets and broad lines are typical of slow spin–lattice relaxation of iron similar to that observed for cytochrome C. The present Mössbauer spectroscopic results for the bis(histidine) complexes are similar to those found for low-spin iron(III)-cytochromes and cytochrome b₅. Steric strain due to the histidine side-chains and electrostatic interactions between the charged groups and the porphyrin propionate carboxylates are found to influence the iron electronic structure and the imidazole plane orientations.