Quantum-mechanical simulation of the structure and stability of starch - metal cation complexes

Molecular mechanics computations and simulations involving the Mopac assisted SCF LCAO MO ZINDO1 method were performed for complexes of alpha-D-glucose with Co(II), Cu(II), Fe(III), Mn(II) and Ni(II) acetates, chlorides and nitrates. The computations resulted in the following findings: Usually the saccharide ligand completely repulsed water molecules from the inner coordination sphere of the central metal atoms. The complexes with Fe(III) acetate and chloride and nitrates of Co(II) and Cu(II) constituted random exceptions having some water molecules in that sphere. The oxygen atom of the 3-hydroxyl group in alpha-D-glucose (the O3 atom) was the preferred coordination site. The stability of complexes declined as the primary saccharide chain composed of one alpha-D-glucose was extended by subsequently 1 -> 4 bound alpha-D-glucose mers. Also such chains coordinated to the central atoms through the O3 oxygen atom. Analysis of the increment of increase in the heat formation with each subsequent alpha-D-glucose mer in the saccharide chain might suggest that such chain forms chelates with the central metal atom. With CuCl2 first and third alpha-D-glucose mers were involved in the chelate formation, whereas in case of CoCl2 the first and fourth alpha-D-glucose mers were involved.