The main objective of the proposal is to investigate the structural role of Zn in human insulin (HI) polymorphs (hexamers adopting structures described as T6, R6, T3R3) using XAFS spectroscopy which provides the local bonding environment of this ion (tetrahedral / octahedral coordination, binding sites, distances to the neighbouring atoms) complementing the crystallographic data obtained by the group that will prepare the samples. The samples under study will be solutions of HI polymorphs co-crystallized with Zn in the presence of different salts, small organic molecules (phenolic or benzoic acid derivatives), and at solutions with different pH values. Samples at different synthesis stages (supersaturated solution, precipitates, crystals) will be also studied. The synthesis conditions are expected to affect the HI polymorphs to be formed. RT Zn-K-edge XAFS spectra will be used for the identification of the polymorphs and their relative content in case of their coexistence.