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

One of the outstanding chemical problems associated with the biological importance of aluminium is the definition of speciation profiles for Al3+ in biological fluids. By speciation profiles is meant the distribution of Al3+ among its different complexes, and their structures, stability constants 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 experimental difficulties. A procedure is described for characterizing speciation profiles for Al3+ complexed to small organic ligands, the approach being illustrated with the Al3+-heidi system. It is shown that comparative solution NMR and potentiometric studies are in reasonable agreement concerning the number and identity of the major Al3+-heidi components present in a solution, but the exact structures of the major species are uncertain in some cases. The main approach to determining such structures involves X-ray diffraction studies of crystals obtained from the given solutions, but whether the crystalline forms represent a thermodynamically preferred species in solution is not always clear. Bridging the gap between solid and solution structures is shown to be problematic, even with the comparative vibrational spectroscopies and solid-state NMR.