Potentiometric and Multi-NMR Studies of Aluminum(III) Complex with L-Glutamate in Acidic Aqueous Solutions

Abstract

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.