A Theoretical Study of the Difference in the Behavior of L- and D-Alanine Toward the Two Inverse Forms of Kaolinite

The adsorption of L- and D-alanine on kaolinite is studied by means of the self-consistent field method, the field created by the crystal being simulated by that of 230 point charges, corresponding to two layers. The absolute structure we have adopted for kaolinite is that for whichthe ((a) over right arrow, (b) over right arrow, (c) over right arrow) trihedron is direct. Indeed, it has been shown that, owing to the nuclear weak interactions, this structure is more stable than the inverse structure. On the adopted structure, the L forms of the positive ion and of the zwitterion are more easily adsorbed than the o forms. The difference between the adsorption energies of the L and D forms are 0.14 and 0.04 kJ mol(-1) for the positive ion and the zwitterion, respectively. These differences are considerably larger than those arising from the weak interactions (ca. 10(-17) kJ mol(-1)) so that the asymmetrical adsorption on kaolinite seems to be one of the causes of the L homochirality observed for the amino acids of proteins.