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\caption{Three-dimensional structure of carboxybiotin. \textit{(left)} N1$^\prime$ -carboxybiotin is composed of a planar imidazolidone ring fused to a thiophane ring, existing primarily in the endo conformation. \textit{(right)} The space filling model shows how the N1$^\prime$ of biotin is partially by the steric bulk of the thiophane ring. Adapted from a structure of the streptavidin:biotin complex (PDB ID: 3RY2)}