Mineral surfaces, geochemical complexities, and the origins of life

Abstract

Crystalline surfaces of common rock-forming minerals are likely to have played several important roles in life's geochemical origins. Transition metal sulfides and oxides promote a variety of organic reactions, including nitrogen reduction, hydroformylation, amination, and Fischer-Tropsch-type synthesis. Fine-grained clay minerals and hydroxides facilitate lipid self-organization and condensation polymerization reactions, notably of RNA monomers. Surfaces of common rock-forming oxides, silicates, and carbonates select and concentrate specific amino acids, sugars, and other molecular species, while potentially enhancing their thermal stabilities. Chiral surfaces of these minerals also have been shown to separate left- and right-handed molecules. Thus, mineral surfaces may have contributed centrally to the linked prebiotic problems of containment and organization by promoting the transition from a dilute prebiotic "soup" to highly ordered local domains of key biomolecules.

Figure 1.

Crystal surfaces display a variety of atomic surface features. (A) The surface of an ideal crystal may be represented as a periodic two-dimensional arrangement of atoms; these atoms may be coplanar or they may occur at slightly different heights. Real crystals feature surfaces that are typically stepped (B) or kinked (C). Kink sites provide chiral (left- or right-handed) centers (X). Experimental and theoretical studies reveal that molecular adsorption is enhanced at such surface irregularities.

Figure 2.

The common calcite form of CaCO₃ often displays chiral surfaces. (A) The structure of the (21–34) face of calcite features a chiral arrangement of positive (+) and negative (X) charge centers near the crystal termination. Ca, C, and O atoms are turquoise, blue, and red, respectively. In this 20 × 20 Å view the (01–8) axis is vertical—an orientation that provides a useful image of the surface structure. (B) A view of this surface tilted 3° from horizontal (projected almost down the [01–8] axis) reveals the irregular surface topology, including 2-Å-deep steps (yellow arrow) that result from the oblique intersection of layers of Ca and rigid CO₃ groups with the surface (yellow line).

Figure 3.

Amino acids bind in different ways to mineral surfaces. Numerous possible modes of attachment exist for glutamate adsorbed to rutile (TiO₂) surface sites, consistent with surface complexation calculations (Sverjensky et al. 2008; Jonsson et al. 2009). Large red spheres indicate oxygen atoms, small black spheres carbon, small pink or blue spheres hydrogen or nitrogen, respectively, and the lowermost blue spheres titanium at the rutile surface. (A) Bridging-bidentate species with four points of attachment involving one inner-sphere Ti-O-C bond and one Ti-OH…O=C hydrogen bond for each carboxylate. (B) Chelating species with two points of attachment involving one inner-sphere Ti-O-C bond and one Ti-OH₂⁺…O=C to a single titanium. (C) Alternative to the bridging-bidentate species in (A). This bridging-bidentate species has four points of attachment involving one inner-sphere Ti-O-C bond and one Ti-OH…O=C hydrogen bond of the a-carboxylate, and one Ti-OH…⁻O-C hydrogen bond and one Ti-OH…O=C hydrogen bond of the g-carboxylate (stabilized through resonance). (D) Alternative to the chelating species in (B), outer-sphere or hydrogen bonded to the surface. After Jonsson et al. 2009.

Figure 4.

Predicted surface speciation of glutamate on rutile as a function of environmental conditions. The species names refer to the pictures in Figure 3. After Jonsson et al. 2009.

Figure 5.

The most stable configurations for l- and d-aspartate on the calcite (21–34) surface (A and B, respectively). The d enantiomer, which requires significantly less calcite surface relaxation and aspartate distortion, is favored by 8 Kcal/mol—the largest known enantiospecific effect.