A comparative study of alanine adsorption and condensation to peptides in two clay minerals

Abstract

The polycondensation of amino acids to oligopeptides is an important step in the origins of life, and known to be effective on several mineral surfaces. The data available on clay mineral surfaces are heterogeneous and sometimes contradictory, however. The objective of the present work is to investigate the adsorption of a selected amino acid, alanine, in expanding and non-expanding clays and then study the possible peptide condensation by thermal activation. A multi-technique approach was used, including macroscopic measurements of the adsorption process (adsorption isotherms and pH-metry), and in situ molecular-level characterization of the solids obtained (X-ray diffraction, thermogravimetric analysis and infrared spectroscopy). The results indicate that only weak interaction is established between alanine molecule and kaolinite surfaces, most of the deposited molecules being only physically retained. Thermal activation of alanine/kaolinite only led to desorption. In contrast, higher-energy adsorption mechanisms were at play in hectorite, including cation exchange and coordinative adsorption to the interlayer ions, and alanine species adsorbed in this way were observed to form cyclic dimers upon activation between 160 and 270 °C.