Rapid freeze-quench ENDOR study of chloroperoxidase compound I: The site of the radical

The classical heme-monooxygenase active intermediate, compound I (Cpd-I), incorporates a heme which is oxidized by two equivalents above the resting ferric state, one equivalent associated with a ferryl center, [Fe=O]²⁺ (^FeS = 1), and the other with an active-site radical (^RS = 1/2). Theoretical calculations on models of a Cpd-I with a thiolato axial ligand have presented divergent views about its electronic structure. In one picture, the radical is on the porphyrin; in the other, it is on the sulfur. In this report, ENDOR spectroscopy answers the question, does Cpd-I of the enzyme chloroperoxidase contain a porphyrin π-cation radical or an iron-bound cysteinyl radical: the radical is predominantly on the porphyrin, with spin density on sulfur having an upper bound, ρ_S ≤ ρS_max ≈ 0.23. We further suggest that the same answer applies to Cpd-I of cytochromes P450.