Phenomenological simulation and density functional theory prediction of 57 Fe Mössbauer parameters: application to magnetically coupled diiron proteins

Abstract

The use of phenomenological spin Hamiltonians and of spin density functional theory for the analysis and interpretation of Mossbauer spectra of antiferromagnetic or ferromagnetic diiron centers is briefly discussed. The spectroscopic parameters of the hydroxylase component of methane monooxygenase (MMOH), an enzyme that catalyzes the conversion of methane to methanol, have been studied. In its reduced diferrous state (MMOHRed) the enzyme displays 57Fe Mossbauer and EPR parameters characteristic of two ferromagnetically coupled high spin ferrous ions. However, Mossbauer spectra recorded for MMOHRed from two different bacteria, Methylococcus capsulatus (Bath) and Methylosinus trichosporium OB3b, display slightly different electric quadrupole splittings (ΔEQ) in apparent contradiction to their essentially identical active site crystallographic structures and biochemical functions. Herein, the Mossbauer spectral parameters of MMOHRed have been predicted and studied via spin density functional theory. The somewhat different ΔEQ recorded for the two bacteria have been traced to the relative position of an essentially unbound water molecule within their diiron active sites. It is shown that the presence or absence of the unbound water molecule mainly affects the electric field gradient at only one iron ion of the binuclear active sites.