# Can ferric-oxyl excited states explain elongated iron-oxygen bonds in heme peroxidase catalytic intermediates?

## TL;DR

This study reveals that ferric-oxyl excited states, not typical ferryl intermediates, explain elongated iron-oxygen bonds in peroxidase enzymes.

## Contribution

The study introduces the concept of ferric-oxyl excited states as an explanation for observed bond characteristics in heme peroxidases.

## Key findings

- Ferryl intermediates in peroxidases have single, not double, iron-oxygen bond character.
- Ferric-oxyl (FeIII–O•–) excited states explain the observed bond elongation in catalytic intermediates.
- Quantum calculations and X-ray data support the existence of these excited states.

## Abstract

The use of X-ray structures to determine and interpret the ferryl iron-oxygen bond order in molecular oxygen-activating heme enzymes has, in the past, been controversial. This has mainly stemmed from the susceptibility of ferryl species to X-ray-induced electronic state changes. In this work we establishe using time-resolved serial femtosecond X-ray crystallography (tr-SFX) on a dye-decolourising peroxidase that the ferryl intermediate species (Compounds I and II) captured following in situ mixing of microcrystals with H2O2 have single, rather than the double bond character expected. X-ray emission validated tr-SFX data with quantum refinement, time-dependent-DFT calculations and QM/MM geometry optimizations together support the concept that the single iron-oxygen bond character is not an indication of ferryl reduction or a protonated form (FeIV-OH) but is instead attributed to the existence of accessible excited states possessing ferric-oxyl (FeIII–O•–) character. Such states offer insight into the nature of ferryl heme.

Peroxidases have long been considered to use ferryl heme intermediates (Fe(IV) = O) to catalyze oxidative chemistries. Here, Williams et al. find that ferric-oxyl (FeIII–O•–) excited states exist along the catalytic reaction coordinate of a peroxidase.