High-resolution iron X-ray absorption spectroscopic and computational studies of non-heme diiron peroxo intermediates

Ferritin-like carboxylate-bridged non-heme diiron enzymes activate O-2 for a variety of difficult reactions throughout nature. These reactions often begin by abstraction of hydrogen from strong C-H bonds. The enzymes activate O-2 at their diferrous cofactors to form canonical diferric peroxo intermediates, with a range of possible coordination modes. Herein, we explore the ability of high-energy resolution fluorescence detected X-ray absorption spectroscopy (HERFD XAS) to provide insight into the nature of peroxo level intermediates in non-heme diiron proteins. Freeze quenched (FQ) peroxo intermediates from p-aminobenzoate N-oxygenase (AurF), aldehyde-deformylating oxygenase (ADO), and the beta subunit of class Ia ribonucleotide reductase from Escherichia coli (Ec beta) are investigated. All three intermediates are proposed to adopt different peroxo binding modes, and each exhibit different Fe K alpha HERFD XAS pre-edge features and intensities. As these FQ-trapped samples consist of multiple species, deconvolution of HERFD XAS spectra based on speciation, as determined by Mossbauer spectroscopy, is also necessitated - yielding 'pure' diferric peroxo HERFD XAS spectra from dilute protein samples. Finally, the impact of a given peroxo coordination mode on the HERFD XAS pre-edge energy and intensity is evaluated through time-dependent density functional theory (TDDFT) calculations of the XAS spectra on a series of hypothetical model complexes, which span a full range of possible peroxo coordination modes to a diferric core. The utility of HERFD XAS for future studies of enzymatic intermediates is discussed.