Coordination studies of inositols with aluminium and related cations

Abstract

In this work cis-inositol, epi-inositol and myo-inositol carbonate were successfully synthesised and used for coordination studies. The preparation of cis-inositol was achieved by reduction of tetrahydroxybenzoquinone via hydrogenation with palladium hydroxide as the catalyst and was purified by chromatographic separation using Dowex resin. The synthesis of epi-inositol was achieved by the nitric acid oxidation of myo-inositol to form epi-inosose which was subsequently reduced by hydrogenation using palladium hydroxide as the catalyst. myo-Inositol was converted into its mono-orthoformate derivative and the equatorial hydroxy group was then protected as a tertiary-butyldimethylsilyl ether. The carbonate group was introduced onto this protected inositol and then the protecting groups were removed by acid hydrolysis. The coordination characteristics of four inositols, viz cis-inositol, epi-inositol, myo-inositol and myo-inositol carbonate with calcium, aluminium, gallium, lanthanum and samarium ions have been investigated. Interactions of the aluminate anion with epi-inositol and myo-inositol in deuterated sodium hydroxide were also investigated. Three methods were used in the study of complexation behaviour of these systems. namely, [superscript]13C NMR spectroscopy, HPLC and ion exchange chromatography. [superscript]13C NMR spectroscopy was found to be most useful for determining possible complexation behaviour of the inositols. Chemical shift changes of the resonance signals in the [superscript]13C NMR spectra on sequential addition of cations to solutions of the inositols at near neutral pH, have led to determination of possible coordination sites of the inositols. In general, large induced chemical shift changes have been interpreted to signify strong cation-inositol interaction at specific hydroxy groups.Triaxial sites of the inositols have shown a preference to coordinate small ions with ionic size of at least 60 pin, smaller ions than this displayed very weak interactions. Likewise large ions (90-100 pm) imparted weak interactions on triaxial sites of the inositols. These large ions coordinated well with the axial-equatorial-axial sites of the inositols although it was observed that calcium ions appeared to form a 2:1 ligand:cation complex with cis-inositol at the triaxial site despite being a large cation (100 pm). The detection of complex formation by HPLC showed a possible formation of very stable complexes of epi-inositol complexes with calcium ions. However, a change of refractive index of the solution on sequential addition of the cation may have caused an interference in the results such that direct interpretation was not possible. Ion exchange chromatography provided the quickest guide on how strongly the inositols interact with a particular cation. However, determination of complex stoichiometry and or structure was not possible using this technique.