Selective adsorption and chiral amplification of amino acids in vermiculite clay-implications for the origin of biochirality

Abstract

Smectite clays are hydrated layer silicates that, like micas, occur naturally in abundance. Importantly, they have readily modifiable interlayer spaces that provide excellent sites for nanochemistry. Vermiculite is one such smectite clay and in the presence of small chain-length alkyl-NH 3 Cl ions, forms sensitive, 1-D ordered model clay systems with expandable nano-pore inter-layer regions. These inter-layers readily adsorb organic molecules. N-propyl NH 3 Cl vermiculite clay gels were used to determine the adsorption of alanine, lysine and histidine by chiral HPLC. The results show that during reaction with fresh vermiculite interlayers, significant chiral enrichment of either L-and D-enantiomers occurs depending on the amino acid. Chiral enrichment of the supernatant solutions is up to about 1% per pass. In contrast, addition to clay interlayers already reacted with amino acid solutions resulted in little or no change in D/L ratio during the time of the experiment. Adsorption of small amounts of amphiphilic organic molecules in clay inter-layers is known to produce Layer-by-Layer or Langmuir-Blodgett films. Moreover atomistic simulations show that self-organization of organic species in clay interlayers is important. These noncentrosymmetric, chirally active nanofilms may cause clays to act subsequently as chiral amplifiers, concentrating organic material from dilute solution and having different adsorption energetics for D-and L-enantiomers. The additional role of clays in RNA oligomerization already postulated by Ferris and others, together with the need for the organization of amphiphilic molecules and lipids noted by Szostak and others, suggests that such chiral separation by clays in lagoonal environments at normal biological temperatures might also have played a significant role in the origin of biochirality.2

Formation of Replicating Saponite from a Gel in the Presence of Oxalate: Implications for the Formation of Clay Minerals in Carbonaceous Chondrites and the Origin of Life

“…The template-catalyzed polymerization of clay formation could be a possible amplification mechanism for the enrichment of chiral molecules in carbonaceous chondrite meteorites. In recent studies, Fraser et al (2011aFraser et al ( , 2011b provided experimental evidence for the chiral enrichment of amino acids on vermiculite crystals. It is therefore important from the point of view of prebiotic chemistry whether organic acids like oxalic acid might have catalyzed the formation of the phyllosilicate phases on these meteorites.…”

The Significance of Microbe-Mineral-Biomarker Interactions in the Detection of Life on Mars and Beyond

“…Adsorption of organic molecules on oxide surfaces is affected by complex surface speciation in relation to mineral structure, with inner and outer sphere species varying in relative importance over a range of pH, ionic strength, and surface coverage (Hazen and Sverjensky, 2010). The substitution of the inorganic interlayer cations that occur naturally in smectite clays by small organic cations alters the adsorption characteristics of clays for amino acids (Fraser et al, 2011). The differences in side chains between amino acids have an effect on their charge and polarity, although most amino acids are readily soluble in water.…”

Exfoliation and intercalation of montmorillonite by small peptides

“…Organoclays mediate the relationship of microbes to soils and sediments and may have played a role in the development of prebiotic life (Bouchoucha et al, 2011; Carneiro et al, 2011a,b; Fraser et al, 2011; Georgelin et al, 2013). Conversely, organic molecules may modify the structure of clays.…”