Interactions of aluminum(III) with the biologically relevant ligand d-ribose

Abstract

Aluminum(III) can be absorbed when it is appropriately complexed. There are several plasma components which can bind weakly Al(III). Many proteins bind Al(III) in solution quite strongly. Carbohydrates bearing an abundance of electronegative functional groups can interact with metal cations. In solution, d-ribose exists as a mixture at equilibrium of many isomers and only a few of them bear a ‘complexing’ sequence of the hydroxyl groups. The presence of d-ribose in an Al(III) solution experiences a decrease of its Brönsted-acid sites. The lowering of the Brönsted acidity of an Al(III)– d-ribose mixture suggests the existence of attractive interactions (‘association’) between Al(III) ion and the complexing sequence of the hydroxyls of d-ribose. There is enhancement in the stability of the interaction complexes between Al(III) and d-ribose through strong intramolecular hydrogen bonding, which offers the possibility to investigate the kinetics of the subsequent proton release reactions. On the basis of the kinetic results, it may be concluded that proton release reactions, which are associated with the complexation reactions, are associatively activated. The complexes (Al(H 2O) 6− n ( d-ribose − nH ) (3− n)+ ) resulting from the various ‘complexing’ forms of d-ribose are formed at mainly acidic pH. As the pH increases, the values of the activation enthalpy, Δ H ≠ , are changing, because of the formation of mixed hydroxo-complexes (Al(H 2O) 6− n− m (OH) m ( d-ribose − nH ) (3− n− m)+ ); finally, OH − displaces d-ribose from the coordination sphere of Al(III) in a rather slow process, i.e. with high values of Δ H ≠ ; the activation enthalpy values, Δ H ≠ , decrease with the progression of the displacement, becoming finally very small due to the formation of a precipitate. Chelate coordination of d-ribose with some divalent and trivalent metal ions has been also reported.