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Structural signatures of the complex formed between 3‐nitro‐4‐hydroxybenzoate and the Zn(II)‐substituted R6 insulin hexamer

3‐Nitro‐4‐hydroxybenzoate (3N4H) is a probe of the structure and dynamics of the metal‐centered His B10 assembly sites of the insulin hexamer. Each His B10 site consists of a ∼12 Å‐long cavity situated on the threefold symmetry axis. These sites play an important role in the storage and release of insulin in vivo. The allosteric behavior of the insulin hexamer is modulated by ligand binding to the His B10 zinc sites and to the phenolic pockets. Binding to these sites drives transitions among three allosteric states, designated T6, T3R3, and R6. Although a wide variety of mono anions bind to the His B10 zinc sites of R3, X‐ray structures of ligands complexed to this site exist only for H2O, Cl–, and SCN–. This work combines one‐ and two‐dimensional 1H NMR and UV‐Vis absorbance studies of the structure and dynamics of the 3N4H complex, which establish the following: (1) relative to the NMR time scale, 3N4H exchange between free and bound states is slow, while flipping among three equivalent orientations about the site threefold axis is fast; (2) binding of 3N4H perturbs resonances within the His B10 zinc site and generates NOEs between ligand resonances and the insulin C‐α and side chain resonances of ValB2, AsnB3, LeuB6, and CysB7; and (3) 3N4H exchange for other ligands is limited by a protein conformational transition. These results are consistent with coordination of the 3N4H carboxylate to the His B10 zinc ion and van der Waals interactions with Val B2, Asn B3, Leu B6, and Cys A7.