Zinc Interactions with a Soluble Mutated Rat Amylin to Mimic Whole Human Amylin: An Experimental and Simulation Approach to Understand Stoichiometry, Speciation and Coordination of the Metal Complexes

Islet amyloid polypeptide (IAPP) is a hormone co‐secreted with insulin and zinc from pancreatic β‐cells. To overcome the low solubility of human IAPP, we characterized zinc complexes species formed with 1) a mutated form of rat‐IAPP(1–37; R18 H) able to mimic the human IAPP, 2) the r‐IAPP(1–37) and the IAPP(1–8) fragment. Stoichiometry, speciation and coordination features of zinc(II) complexes were unveiled by ESI‐MS, potentiometry and NMR measurements combined with DFT and free‐energy simulations. Mononuclear species start to form around pH 6; Zn2+ binds both His18 and N‐amino terminus in rat‐IAPP(1–37; R18 H). The in silico study allows us to assess not only a structured turn compact domain in r‐IAPP(1–37) and r‐IAPP(1–37; R18 H) featured by a different free energy barrier for the transition from the compact to elongated conformation upon the coordination of Zn2+, but also to bring into light a coordination shell further stabilized by noncovalent interactions. Stoichiometry, speciation and coordination features of zinc(II) complexes with islet amyloid polypeptide (IAPPS) were unveiled by a combined ESI‐MS, potentiometric and NMR study. Mononuclear species start to form around pH 6; Zn2+ binds both His18 and N‐amino terminus in rat‐IAPP(1‐37; R18 H). NMR and DFT findings confirm the binding and stress the role of noncovalent interactions in the metal complexes.