Interaction of clay and amino acid mixtures on rocky (exo)planets

In prebiotic warm pools on rocky exoplanets, water, minerals and organic materials interact in a dynamic fashion. In these settings in particular, exogenous organics can have an important contribution to the organic reservoir, being delivered to planetary surfaces by meteorites and/or cosmic dust. Chemical complexity can arise due to interactions of organics with clay surfaces, which have a high affinity to adsorb organics and catalyse chemical reactions. For example, polymerisation of amino acids to form short peptides have been shown to be catalysed by first being adsorbed on clay surfaces. Yet, studies are often limited to only a single organic species and is biased towards the incorporation of protein-forming (proteinogenic) amino acids.

In this study we examine the adsorption interaction between calcium-montmorillonite clay (Mt) and mixtures of proteinogenic and non-proteinogenic amino acids. The proteinogenic amino acids, L-lysine and L-arginine, are known to strongly adsorb to Mt surfaces. The non-proteinogenic species, gamma-aminobutyric acid (GABA), is a common component in carbonaceous meteorites and possesses a structural similarity yet different charge distribution, compared to L-lysine and L-arginine. The competitive and collaborative adsorption behaviour of single-component vs. amino acid mixture is characterised using Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy and powder X-ray diffraction (pXRD). GABA has been found to affect not only the adsorption behaviour of the proteinogenic species, but to also irreversibly alter the structure of the clay by inducing exfoliation, despite the interlayer incorporation of these strongly adsorbing organics. This work provides new insights on processes relevant to the formation of biomolecules on lifeless rocky exoplanets and has implications for the role of extraterrestrial organics in these processes.