An obstacle to isolation of the adduct may be instability. Previously-reported adducts 4 and 5 were not isolated, but detected only by mass spectrometry as reaction intermediates . However, the tethered conformation of the N-terminus separates Gly52 from His54, rendering a nickel complex between the two residues impossible (Figure ). Thus, adduct formation would liberate the ion and end the catalytic cycle. Moreover, the 'lid' formed by the N-terminus almost entirely occludes the binding pocket , leaving only a narrow tunnel filled with structured water molecules. Thus, the adduct bonds are sterically shielded, which may inhibit further reactions.