Aluminium speciation in environmental samples: a review

Because of its toxic effects on living beings, Al may represent an environmental hazard, particularly under increased acidic conditions. Growing environmental concern over the presence of increased Al concentrations in soil solutions and fresh waters resulted in the development of numerous analytical techniques for the determination of Al species. Al has a very complex chemistry that is significantly influenced by pH. Different Al species are present in environmental solutions, and many of them are unstable. Contamination of samples and reagents by extraneous Al represents an additional problem in speciation of Al at trace concentrations. Due to these reasons quantitative determination of particular chemical forms of Al is still a very difficult task for analytical chemists. The most important analytical methodologies of the last decade and new trends for the speciation of Al in environmental samples are comprehensively reviewed here.

NMR spectra and potentiometry studies of aluminum(III) binding with coenzyme NAD+ in acidic aqueous solutions

Complexation and conformational studies of coenzyme NAD+ with aluminum were conducted in acidic aqueous solutions (pH 2-5) by means of potentiometry as well as multinuclear (1H, 13C, 31P, 27Al) and two-dimensional (1H, 1H-NOESY) NMR spectroscopy. These led to the following results: (1) Al could coordinate with NAD+ through the following binding sites: N7' of adenine and pyrophosphate free oxygen (O(A)1, O(N)1,O(A)2) to form various mononuclear 1:1 (AlLH23+, AlLH2+) and 2:1 (AlL2-) species, and dinuclear 2:2 (Al2L22+) species. (2) The conformations of NAD+ and Al-NAD+ depended on the solvents and different species in the complexes. The results suggest the occurrence of an Al-linked complexation, which causes structural changes at the primary recognition sites and secondary conformational alterations for coenzymes. This finding will help us to understand role of Al in biological enzyme reaction systems.

Multimethod characterization of the interaction of aluminum ion with alpha-ketoglutaric acid in acidic aqueous solutions

It has recently been reported that aluminum plays a very important role in reducing the activity of Krebs-cycle enzymes and glutamate dehydrogenase in rat brain homogenate. Therefore, it is necessary to identify the aluminum binding ability with the pivotal substrate alpha-ketoglutarate in biological systems. The interactions of aluminum with alpha-ketoglutarate were studied with pH-potentiometry, cyclic voltammetry, UV-vis, 1H, 27Al-NMR and Raman spectra multi-analytical techniques in acidic aqueous solution to measure the stoichiometries and stability constants of the complexes and its keto-enol tautomerism. The alpha-ketoglutarate was found to bind Al in a bidentate manner at the carboxylate and carbonyl moieties. The mononuclear 1:1 (AlLH(-1), AIL+, AlHL2+) and 2:1 (AlL2-, AlL2H(-2)3-) species, and dinuclear 2:1 (Al2L4+) species were found in acidic aqueous solution. Meanwhile, Al can promote alpha-KG tautomerize to its enolic-structure compounds in solutions. These findings may help to further understand the influence of Al on GDH enzyme reactions in biological systems.

Potentiometric and multi-NMR studies of aluminum(III) complex with L-glutamate in acidic aqueous solutions

Complex formation studies of L-glutamate with aluminum(III) ion were conducted in acidic aqueous solutions (pH 2.0 - 5.5) by means of pH-metric titration and multinuclear (1H, 13C and 27Al) NMR techniques. The following results were obtained: (1) Al could weakly coordinate with Glu to form various mononuclear 1:1 (AlLH2+, AlL+, AlLH(-1)) species and dinuclear 2:1 (Al2L4+) species in acidic aqueous solutions, which somewhat agreed with previous findings. (2) The multi-NMR spectra of Al-Glu and Al-Asp strongly suggest that, besides negatively charged carboxylate donors (-COO(-), -COO(-)), the amino group of Glu can participate in the binding of Al in the AlL+ and AlLH(-1) species in the case of its deprotonation, which rather agreed with the case of Al-Asp. (3) These tridentate five-+seven-membered joint chelate (-COO(-), -NH2, -COO(-)) complexes exhibit an enhanced stability, which can help to better understand the biological studies that Al-Glu could cross the erythrocyte membrane and the blood-brain barrier (BBB) and be deposited selectively in various brain regions, particularly in the cortex. It will also help to intrinsically understand the Al's role in the biological transamination system, which is a very important process in all living things.