Selective Adsorption of Aspartate Facilitated by Calcium on Brucite [Mg(OH)(2)]

Dissolved ions present in an aqueous environment may significantly improve biomolecule attachment at mineral surfaces through the formation of cooperative surface complexes. To test whether this phenomenon results in the selective adsorption of an organic species, we conducted batch adsorption experiments with an equimolar mixture of the amino acids aspartate, glycine, lysine, leucine, and phenylalanine onto powdered brucite [Mg(OH)(2)] at pH 10.2. We performed the batch experiments in triplicate both without and with 4.1 mM CaCl2. In experiments without CaCl2, we observed that up to 0.7 mu mol m(-2) of aspartate and about 0.4 mu mol m(-2) each of the remaining four amino acids adsorbed onto brucite. When we added CaCl2, we found that up to 1.6 mu mol m(-2) of aspartate selectively adsorbed onto the brucite surface relative to between 0.2 and 0.3 mu mol m(-2) of the other amino acids. We measured the brucite particle surface charge to be slightly positive without added CaCl2, but the surface charge becomes significantly more positive in the presence of CaCl2. Our results suggest that negatively charged molecules selectively and cooperatively adsorb onto brucite when CaCl2 is added to the system. This study emphasizes the importance of the dissolved ionic profile of a geochemical environment when evaluating the role of mineral surfaces in the evolution of prebiotic chemistry. The presence of dissolved ions at a mineral-water interface can selectively enhance the adsorption and concentration of specific molecules, which may serve as a key process in molecular self-organization and the assembly of proteins that are composed of metal-ligand complexes.