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

Sun Hee Kim, Roshan Perera, Lowell P. Hager, John H. Dawson, Brian M. Hoffman*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

56 Scopus citations


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, ρS ≤ ρSmax ≈ 0.23. We further suggest that the same answer applies to Cpd-I of cytochromes P450.

Original languageEnglish (US)
Pages (from-to)5598-5599
Number of pages2
JournalJournal of the American Chemical Society
Issue number17
StatePublished - May 3 2006

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry


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