Interaction between l-aspartate and the brucite [Mg(OH)2]–water interface

Abstract

The interaction of biomolecules at the mineral-water interface could have played a prominent role in the emergence of more complex organic species in life's origins. Serpentinite-hosted hydrothermal vents may have acted as a suitable environment for this process to occur, although little is known about biomolecule-mineral interactions in this system. We used batch adsorption experiments and surface complexation modeling to study the interaction of L-aspartate onto a thermodynamically stable product of serpentinization, brucite [Mg(OH) 2 ], over a wide range of initial aspartate concentrations at four ionic strengths governed by [Mg 2+ ] and [Ca 2+ ]. We observed that up to 1.0 µmol of aspartate adsorbed per m 2 of brucite at pH~10.2 and low Mg 2+ concentrations (0.7x10-3 M), but surface adsorption decreased at high Mg 2+ concentrations (5.8x10-3 M). At high Ca 2+ concentrations (4.0x10-3 M), aspartate surface adsorption doubled (to 2.0 µmol•m-2), with Ca 2+ adsorption at 29.6 µmol•m-2. We used the extended triple-layer model (ETLM) to construct a quantitative thermodynamic model of the adsorption data. We proposed three surface reactions involving the adsorption of aspartate (HAsp-) and/or Ca 2+ onto brucite: 2>SOH + H + + HAsp-= >SOH 2 + >SAsp-+ H 2 O, >SOH + HAsp-+ Ca 2+ = >SO-_Ca(HAsp) + + H + , and >SOH + Ca 2+ + 2H 2 O = >SOH 2 + _ Ca(OH) 2 + H +. We used the ETLM to predict that brucite particle surface charge becomes more negative with increasing [Mg 2+ ], creating an unfavorable electrostatic environment for a negativelycharged aspartate molecule to adsorb. In contrast, our addition of Ca 2+ to the system resulted in Ca 2+ adsorption and development of positive surface charge. Our prediction of surface speciation of aspartate on brucite with Ca 2+ revealed that the calcium-aspartate complex is the predominant surface aspartate species, which suggests that the increase in aspartate adsorption with Ca 2+ is primarily driven by calcium adsorption. The cooperative effect of Ca 2+ and the inhibitive effect of Mg 2+ on aspartate adsorption onto brucite indicate that serpentinite-hosted hydrothermal fluids provide an ideal environment for these interactions to take place.

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

“…The authors attributed the increase in adsorption to the formation of a cation–ligand complex, where the M 2+ cation acted as a bridge between the clay surface and the nucleic acid. Similarly, our previous investigations of brucite [Mg­(OH) 2 ] revealed that the surface adsorption of aspartate and ribose was enhanced in the presence of Ca 2+ . , We used a surface complexation model to predict that a cooperative calcium–aspartate complex adsorbs at the brucite–water interface via a “cation-bridge” configuration …”

“…In this study, we tested this prediction by investigating the adsorption of aqueous solutions of the five amino acids aspartate, glycine, lysine, leucine, and phenylalanine in equimolar mixtures in the presence of brucite with and without CaCl 2 . These five amino acids were chosen because they have a wide range of pI values (between 2.98 and 9.74) relative to the estimated PZC of brucite of 10.5 . Furthermore, we tested the accuracy of the calculations made by the previously established surface complexation model by making electrophoretic mobility measurements of the brucite surface.…”

Chance, necessity and the origins of life: a physical sciences perspective

“…Recent studies of molecular adsorption/desorption reactions on mineral surfaces, while not directly measuring TOFs, also shed light on these reaction rates. For example, we find that amino acids, sugars and nucleosides exposed to rutile (TiO 2 ), brucite (Mg(OH) 2 ) or clay minerals typically approach equilibrium concentrations in 1-6 h [78,[101][102][103]-values that imply turnover rates less than 10 3 s per active site. Accordingly, we suggest a plausible average time, t, for prebiotic organic reactions on mineral surfaces: t = 10 3 s/reaction.…”

Hydrothermal vents and prebiotic chemistry: a review

“…Pyr Pyrrhotite, troilite, arsenopyrite, pyrite, marcasite, sphalerite, chalcopyrite Novikov and Copley (2013). Estrada et al . (2015).…”