Racemisation of organic compounds by montmorillonite - Implications for age determinations and stereocatalysis

The catalytic effect of montmorillonite surfaces on the racemisation of amino acids and terpenes was studied using isoleucine and limonene as models. The correlation between surface acidity of the clay measured with Hammett indicators and the amount of material racemised is quantitative for isoleucine and qualitative for limonene. In sediments of high surface acidity this effect may invalidate age or palaeotemperature determinations based on racemisation of amino acids, as well as conclusions drawn from the distribution patterns of optical isomers. Minor amounts of impurities can inactivate some of the acid sites. If such impurities are associated with only one optical enantiomer, reactions occurring on the clay may appear to be stereoselective.