The DFsc and DFscE11D de novo designed protein scaffolds support biomimetic diiron cofactor sites that react with dioxygen forming a 520 nm 'intermediate' species with an apparent pseudo-first-order formation rate constant of 2.2 and 4.8 s(-1), respectively. Resonance Raman spectroscopy shows that this absorption feature is due to a phenolate-to-ferric charge transfer transition arising from a single tyrosine residue coordinating terminally to one of the ferric ions in the site. Phenol coordination could provide a proton to promote rapid loss of a putative peroxo species.