Preferential polymerization and adsorption of L-optical isomers of amino acids relative to D-optical isomers on kaolinite templates

Experiments on the polymerization of the L- and D-optical isomers of aspartic acid and serine using kaolinite as a catalyst showed that the L-optical isomers were polymerized at a much higher rate than the D-optical isomers; racemic (DL-) mixtures were polymerized at an intermediate rate. The peptides formed from the L-monomers were preferentially adsorbed by the clay. In the absence of kaolinite, no significant or consistent difference in the behavior of the L- and D-optical isomers was observed. In experiments on the adsorption of L- and D-phenylalanine by kaolinite, the L-optical isomer was preferentially adsorbed. Adsorption of L-phenylalanine was sensitive to pH, whereas adsorption of D-phenylalanine was not. The experimental results indicate that catalytically active faces of the kaolinite crystals acted as specific templates which preferentially adsorbed and polymerized molecules with the L-configuration. The observed discrimination between the L- and D-optical isomers of the amino acids evidently occurred on the enantiomorphous edge (hk) faces of the kaolinite. These findings may have significance for the pre-biotic origin of proteins.