Spontaneous Generation of Chirality via Chemistry in Two Dimensions

Abstract

An experimental model for the spontaneous separation of enantiomers of racemic α-amino acids, between crystals of glycine grown at the air-aqueous solution interface and the solution itself, is presented. This process involves several steps. A small enantiomeric excess of chiral α-amino acids is first achieved by oriented growth of few glycine crystals at the solution surface. The growing glycine crystals occlude only one of the α-amino acid enantiomers through the appropriate enantiotopic {010} face exposed to the solution, yielding an enrichment in the solution of the other enantiomer. This chiral bias created in the solution is preserved and amplified by virtue of two effects. First is the formation, at the air-solution interface, of monolayer clusters of partially resolved hydrophobic α-amino acids arranged in a way akin to the glycine layer structure. Such monolayer clusters serve as templates for an oriented crystallization of fresh glycine crystals. Second, the water-soluble hydrophobic and hydrophilic α-amino acids enantiomerically enriched in the aqueous solution inhibit nucleation of "wrongly" oriented glycine crystals. Recent grazing incidence diffraction studies have provided direct information on the structure and dynamics of the monolayer clusters at the solution surface. Of particular importance are the observations that certain crystallites of racemic amphiphilic α-amino acids undergo spontaneous resolution in two dimensions. This effect opens new opportunities for the generation of homochiral oligopeptides from hydrophobic α-amino acids by their polymerization at interfaces.