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Title: 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
Authors: Predieri, R
Beltrami, D
Pattacini, R
Parisi, Maria Laura 
Sinicropi, Adalgisa 
Valensin, Daniela 
Basosi, Riccardo 
Keywords: Zinc complex; X-ray structure; DFT calculations; Methionine hydroxy-analogue; Metal chelates for animal feeding
Issue Date: 2009
Project: None 
The bis-chelate complex of Zn2+ with 2-hydroxy-(4-methylthio)butanoate (MHA-H the anion derivedfrom the so-called methionine hydroxy-analogue, MHA) is an effective, bioavailable mineral supplementfor animal feeding. It can be obtained in two solid forms: the anhydrous [Zn(OC(O)CH2-CH(OH)CH2CH2SCH3)2] and the corresponding dihydrate species, both well distinguishable by IR spectroscopyand powder X-ray diffraction. The crystal and molecular structure of the dihydrate form hasbeen solved by single-crystal X-ray diffraction. It consists of dinuclear bis-chelate species with a bridgingcarboxylate group, both zinc atoms displaying hexacoordination involving all the hydroxyl and carboxylgroups from the four MHA-H anions and three oxygens from different water molecules. The fourth watermolecule 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 carriedout by density functional theory (DFT) on three possible MHA-H zinc chelates, i.e. the dinuclear bischelateobserved in the solid state, the mononuclear bis-chelate diaquo-complex, and the monochelatetetraaquo-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 shiftsand relaxation rates) and 13C NMR data provide further evidence for the formation of Zn/MHA-H chelatesin solution.
ISSN: 0020-1693
DOI: 10.1016/j.ica.2008.05.027
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