Aluminum speciation in biological environments. The deprotonation of free and aluminum bound citrate in aqueous solution

Abstract

Citrate is the main low mass molecule chelator of aluminum in serum, and knowledge of the interaction mode of this organic molecule with this cation is necessary to understand aluminum speciation in biosystems. However, the 1 ∶ 1 complexation of citric acid to Al(III) is a complex process due to the myriad of coordination sites and protonation states of this molecule. Moreover, due to the acidic character of the complex, its entire experimental characterization is elusive. The system is also challenging from a computational point of view, due to the difficulties in getting a balanced estimation of the large range of solvation free energies encountered for the different protonation states of a multiprotic acid in both situations, complexed and uncomplexed with a trivalent cation. Herein, the deprotonation process of the free citric acid in solution and that interacting with Al(III) have been investigated considering all possible coordination modes and protonation states of the citric acid. All the structures were optimized in solution combining the B3LYP density function method with the polarizable continuum IEFPCM model. In addition, different schemes have been employed to obtain reliable solvation energies. Taking into account the most stable isomer of each protonation state, the pK_a values were computationally estimated for the free citric acid and that interacting with Al(III), showing a good agreement with the experimental data. All these results shed light on how the deprotonation process of the citric acid takes place, and show that Al(III) not only increases the acidity of the molecule, but also changes qualitatively the deprotonation pattern of the citric acid. This information is highly relevant to understand aluminum speciation in biological environments, for which citrate is the main low molecular weight chelator, and responsible for its cellular in-take.