Chemical evolution postulates the prebiotic formation of organic compounds and their accumulation as a necessary preamble to the appearance of life on the earth. Of the surface constituents on the primitive earth, clay minerals may have contributed to the process of chemical evolution because of their wide distribution in geological time and space and their strong affinity for organic compounds.
The possible role of clay minerals in the different stages of chemical evolution may include 1) catalysis in the reaction of monomer synthesis from gaseous constituents of the primordal atmosphere, 2) adsorption of the monomers on their surface, providing a highly concentrated system with specific monomers, 3) dehydration-condensation in polymerization of the monomers to biologically important polymers, and 4) formation of organo-clay complexes through which some replicating systems may evolve. Of these roles, the monomer formation, adsorption, and polycondensation processes are discussed in this paper with particular emphasis on amino acids, peptides, and proteins.
A simulation experiment of prebiotic amino acid formation produced both protein and non-protein amino acids and their racemic mixtures in the presence of montmorillonite. The role of the clay is to promote the formation of amino acids of greater carbon numbers. Adsorption experiments show that there is no selection of protein amino acids over non-protein ones by the clay. However, α-amino acids are less adsorbed than non-α-amino acids under neutral or acidic soluton pH. Neither of D-or L-enantiomers are adsorbed preferentially by the clay, although some reports claim that clay minerals (kaolinite and montmorillonite) can adsorb L-enantiomers selectively. Under fluctuating wet/dry cycles clay minerals can polymerize amino acids upto oligomers, acting probably as a dehydration-condensation agent. The interlayer surfaces and edges of clay minerals are unique areas for chemical evolution and should be investigated more in the study of prebiotic chemistry.
