Enantiomer enrichment in early peptides.

The two main ordered conformations of polypeptides, the alpha-helix and the beta-sheet, were considered as possible asymmetric supports for stereoselective processes leading to the enrichment in one enantiomer. alpha-Helices can accommodate residues of both chirality distributed in the same chain, whereas the formation of beta-sheets arises from the association of chain segments containing at least seven residues of the same chirality. When the residue composition departs from the racemic mixture, one handedness of the helix is favored, whereas the beta-sheet nuclei become predominantly of a unique chirality. Taking into account these observations, processes of enrichment specific for alpha- amd beta-structures were envisaged theoretically theoretically as well as experimentally.