Structural studies in solution and in the solid state on the zinc chelate of 2-hydroxy-(4-methylthio)butanoic acid, an effective mineral supplement in animal feeding

The bis-chelate complex of Zn2+ with 2-hydroxy-(4-methylthio)butanoate (MHA-H the anion derived from the so-called methionine hydroxy-analogue, MHA) is an effective, bioavailable mineral supplement for animal feeding. It can be obtained in two solid forms: the anhydrous [Zn(OC(O)CH2CH(OH)CH2CH2SCH3)2] and the corresponding dihydrate species, both well distinguishable by IR spectroscopy and powder X-ray diffraction. The crystal and molecular structure of the dihydrate form has been solved by single-crystal X-ray diffraction. It consists of dinuclear bis-chelate species with a bridging carboxylate group, both zinc atoms displaying hexacoordination involving all the hydroxyl and carboxyl groups from the four MHA-H anions and three oxygens from different water molecules. The fourth water molecule does not participate in coordination. Therefore, the dihydrate complex must be formulated as [Zn2(OC(O)CH2CH(OH)CH2CH2SCH3)4(H2O)3] · H2O (1). A molecular computational analysis has been carried out by density functional theory (DFT) on three possible MHA-H zinc chelates, i.e. the dinuclear bis-chelate observed in the solid state, the mononuclear bis-chelate diaquo-complex, and the monochelate tetraaquo-complex. Calculations have suggested that between the dinuclear and mononuclear bis-chelates, the preferred form in aqueous solution may be the second one. Moreover, both 1H (chemical shifts and relaxation rates) and 13C NMR data provide further evidence for the formation of Zn/MHA-H chelates in solution.