In summary, the density observed between BU72 and His54 is not consistent with noncovalent interactions or a metal coordination complex, and must instead represent covalent bonds to a non-metal atom, approximately isoelectronic with Mg 2+ . The density firmly establishes the presence of this atom and two covalent bonds, and suggests a polar contact with Tyr148. While this evidence does not unambiguously identify the atom, it does establish that the published model is incorrect. The use of conditions known to generate ROS, along with adducts reportedly previously in the presence of ROS, suggest a tentative structure and mechanism for the formation of an oxygen-bridged adduct. All features examined are consistent with this proposal.