While the element is evidently misidentified, the fit of the Mg 2+ ion to the density does firmly establish a non-hydrogen atom in this approximate position. As noted above, this missing atom is likely nearer to both His54 and BU72 than the modelled position of Mg 2+ ; that is, < 1.9 Å from each (Figure ). This is much too close for non-covalent interactions (≥ 2.4 Å) , which would also not result in strong, uninterrupted electron density connecting the three atoms. For instance, the protonated tertiary amine of BU72 forms a charge-assisted hydrogen bond (salt bridge) to aspartate Asp147 3x32 (Figure ); these are among the shortest of all noncovalent interactions .