Mössbauer identification of a protonated ferryl species in catalase from Proteus mirabilis: Density functional calculations on related models
Résumé
The Proteus mirabilis catalase is one of the most efficient heme-containing catalase and forms a relatively stable compound II. Samples of compound II were prepared from PMC enriched in 57Fe. For the first time, two different forms of compound II, namely low pH compound II (LpH II) (43%) and high pH compound II (HpH II) (25%), have been characterized by Mössbauer spectroscopy at pH 8.3. The ratio LpH II/HpH II increases irreversibly with decreasing pH. The large quadrupole splitting value of LpH II (ΔEQ = 2.29 (2) mm/s, with δ/Fe = 0.03 (2) mm/s), compared to that of HpH II (ΔEQ = 1.47 (2) mm/s, with δ/Fe = 0.07 (2) mm/s), reflects the protonation of the ferryl group. Quadrupole splitting values of 1.46 and 2.15 mm/s have been computed by DFT for optimized models of the ferryl compound II (model 1) and the protonated ferryl compound II (model 2), respectively, starting from the FeIVO model initially published by Rovira and Fita [C. Rovira, I. Fita, J. Phys. Chem. B 107 (2003) 5300–5305]. Therefore, we attribute the LpH II compound to a protonated ferryl FeIV–OH complex, whereas the HpH II compound corresponds to the classical ferryl FeIVO complex.