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

Molecular mechanics calculations have been used to rationalize a previ ous 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 control led by the need for large metal ions to maximize the number of five-me mbered chelate rings of which they form a part, while smaller metal io ns need to maximize the number of six-membered chelate rings. Molecula r mechanics also indicates that the previous observation that metal io ns of ionic radius less than 0.6 angstrom do not co-ordinate with neut ral cis-inositol is due to van der Waals repulsion between the donor g roups at these shorter M-O bond lengths.