| Research Abstract |
Elucidation of interactions between carbohydrates and metal ions is of importance in bioinorganic chemistry, since many sugar-metabolizing enzymes and some sugar-binding proteins function with a variety of divalent metal ions such as Mg^<2+>, Mn^<2+>, Co^<2+>, Zn^<2+>, and Ca^<2+>. In some cases, carboxylate-bridged divalent dimetal centers, a ubiquitous motif in many hydrolytic metalloenzymes, also play important roles via interactions with carbohydrates.
Reactions of MCl_2-nH_2O with N,N'-bis (D-glucopyranosyl)-1, 4, 7-triazacyclononane (D-Glc)_2-tacn), which was formed from D-glucose and 1, 4, 7-triazacyclononane (tacn) in situ, afforded a series of mononuclear divalent metal complexes with two β-D-glucopyranosyl moieties, [M{(D-Glc)_2-tacn}Cl]Cl (M=Zn (11), Cu (12), Ni (13), Co (14)). Complexes 11-14 were characterized by analytical and spectroscopic measurements and X-ray crystallography to have a distorted octahedral M(II) center ligated by the pentacoordinate N-glycoside ligand,
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(b-D-glucopyranosyl) 2-tacn, and a chloride anion. Each D-glucose moiety is tethered to the metal center through the b-N-glycosidic bond with tacn and additionally coordinated via the C-2 hydroxyl group, resulting in a λ-gauche five-membered chelate ring. Reactions of 11 (M=Zn) with [Zn (XDK)(H_2O)] (21) or [Cu(XDK)(py)_2] (22) (H_2XDK=m-xylylenediamine bis (Kemp's triacid imide)) yielded homo-and heterotrimetallic complexes formulated as [Zn_2M'{(D-Glc)_2-tacn}_2(XDK)] Cl_2 (M'=Zn (31), Cu (32)). The similar reactions of 12 (M=Cu) with complex 21 or 22 afforded [Cu_2M'{(D-Glc)_2-tacn}_2(XDK)]-Cl_22 (M'=Cu (33), Zn (34)). X-ray crystallographic study revealed that complexes 31 and 34 have a Zn^<II>_3 or Cu^<II>Zn^<II>Cu^<II> trimetallic centers bridged by two carboxylate groups of XDK and two D-glucopyranosyl residues. The M…M' separations are 3.418 (3)-3.462 (3) Å (31) and 3.414 (1)-3.460 (1) Å (34) and the M…M'…M angles are 155.18 (8) ° (31) and 161.56 (6)°(34). The terminal metal ions are octahedrally coordinated by the (D-Glc)2-tacn ligand through three nitrogen atoms of tacn, two oxygen atoms of C-2 hydroxyl groups of the carbohydrates, and a carboxylate oxygen atom of XDK ligand. The central metal ions sit in a distorted octahedral environment ligated by four oxygen atoms of the carbohydrate residues in the (D-Glc)_2-tacn ligands and two carboxylate oxygen atoms of XDK.The deprotonated β-D-glucopyranosyl unit at C-2 hydroxyl group bridges the terminal and central ions with the C-2 μ-alkoxo group and with the C-1 N-glycosidic amino and the C-3 hydroxyl groups coordinating to each metal center. Complexes 31-34 are the first examples of metal complexes in which D-glucose units act as bridging ligands. These structures could be very useful substrate binding models of xylose or glucose isomerases, which promote D-glucose <-> D-fructose isomerization by using divalent dimetallic centers bridged by a glutamate residue. Less
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