A MOLECULAR MECHANICS STUDY OF THE COMPLEXATION OF METAL-IONS BY INOSITOLS

Molecular mechanics calculations have been used to rationalize a previous observation that metal ions of ionic radius less than 0.8 angstrom adopt the ax-ax-ax mode while those of ionic radius greater than 0.8 angstrom adopt the ax-eq-ax mode with cis-inositol (cyclohexane-1,2,3,4,5,6-hexol). It is found that the different modes adopted are controlled by the need for large metal ions to maximize the number of five-membered chelate rings of which they form a part, while smaller metal ions need to maximize the number of six-membered chelate rings. Molecular mechanics also indicates that the previous observation that metal ions of ionic radius less than 0.6 angstrom do not co-ordinate with neutral cis-inositol is due to van der Waals repulsion between the donor groups at these shorter M-O bond lengths.