Pro-oxidant Activity of Aluminum: Stabilization of the Aluminum Superoxide Radical Ion

The pro-oxidant activity of aluminum, a nonredox metal, through superoxide formation is studied by theoretical methods, determining the ESR g-tensor values of O-2(center dot-) with a variety of metals and the reaction energies for Al3+ superoxide affinity in solution. First, the intrinsic ability of aluminum to induce a splitting of the pi(g) levels is compared to that of other significant biological metals, such as Na+, K+, Mg2+, and Ca2+. Additional properties such as bond lengths, ionization potentials, and electron affinities are also determined, and the coherency with the trends observed from ESR g-tensor values is analyzed. As it corresponds to the high charge and its small size, there is a strong interaction between Al3+ and the superoxide. We predict that this strong inherent interaction remains when aluminum is microsolvated. Finally, we analyze the possibility of Al3+ superoxide formation in solution, leading to the conclusion that substitution of the first coordination shell water molecules is plausible, but not of hydroxides. This points to the possibility of Al3+ superoxide formation in solution, which would be pH-dependent. Taking into account the earlier established linear relationship between metal superoxide interactions and promoting effects in electron-transfer reactions, our work reinforces the idea that the presence of aluminum in biological systems could lead to an important pro-oxidant activity through a superoxide formation mechanism.