Substituting adduct 6 for His54 and BU72 gave an excellent fit, with no excess or unexplained density even at 2σ (Figure ; data in Additional files 3-6). Both bonds to oxygen were of typical length (1.5 Å), and were resolved up to 4.2σ -that is, higher density than most of the ligand itself and surrounding side-chains. Unlike Mg 2+ , the oxygen atom was well centered in the density. Oxygen also gave a superior B-factor to Mg 2+ , both lower and consistent with its bonding partners, making this a much more plausible candidate element (Figure ) . The lower B-factor for oxygen results in a more precise fit (DPI 0.14 vs 0.22 Å). Indeed, it is among the most precisely-resolved atoms in the entire structure, which is itself the highest-resolution structure of μOR to date. The bridging oxygen and modified histidine moiety make favorable polar contacts with Tyr148 3x33 , which are close to the length of a weak hydrogen bond. The adduct is highly strained