DEFINING SPECIATION PROFILES OF AL3-LIGANDS - THE AL3+-HEIDI SYSTEM( COMPLEXED WITH SMALL ORGANIC)

One of the outstanding chemical problems associated with the biologica l importance of aluminium is the definition of speciation profiles for Al3+ in biological fluids. By speciation profiles is meant the distri bution of Al3+ among its different complexes, and their structures, st ability constants and rates of formation and breakdown. As is often th e case with metal speciation, it is the complexes formed with small li gands that present most experimental difficulties. A procedure is desc ribed for characterizing speciation profiles for Al3+ complexed to sma ll 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 exa ct structures of the major species are uncertain in some cases. The ma in 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 solut ion structures is shown to be problematic, even with the comparative v ibrational spectroscopies and solid-state NMR.