Structures of Scn bound to FeIIICMB. The carboxylate tail is found bound in the lower portion of pocket #2 in both the n = 6 (FeIIICMB-6, middle) and n = 7 (FeIIICMB-7, bottom) structures. In contrast, the tail adapts both a “tail in” and “tail out” conformation in the case of n = 5 (FeIIICMB-5, top). To accommodate the “tail in” binding conformation, the hydroxamate in pocket #3 shifts 2.0 Å down into the calyx, disrupting ideal iron coordination.