Defining speciation profiles of Al3+ complexed with small organic ligands: the Al3+-heidi system

One of the outstanding chem. problems assocd. with the biol. importance of aluminum is the definition of speciation profiles for Al3+ in biol. fluids. By speciation profiles is meant the distribution of Al3+ among its different complexes, and their structures, stability consts. and rates of formation and breakdown. As is often the case with metal speciation, it is the complexes formed with small ligands that present most exptl. difficulties. A procedure is described for characterizing speciation profiles for Al3+ complexed to small org. ligands, the approach being illustrated with the Al3+-heidi system. It is shown that comparative soln. NMR and potentiometric studies are in reasonable agreement concerning the no. and identity of the major Al3+-heidi components present in a soln., but the exact structures of the major species are uncertain in some cases. The main approach to detg. such structures involves x-ray diffraction studies of crystals obtained from the given solns., but whether the cryst. forms represent a thermodynamically preferred species in soln. is not always clear. Bridging the gap between solid and soln. structures is shown to be problematic, even with the comparative vibrational spectroscopies and solid-state NMR. [on SciFinder(R)]