β-structures of polypeptides with L- and D-residues

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Abstract
The possible formation ofβ-structures from polypeptide chains with L-and D-residues randomly distributed was statistically analyzed within the frame of two hypotheses. Firstly, only those segments containing residues of identical chirality can associate to form antiparallelβ-structures, and secondly these segments must have a minimum length. The influence of different factors was examined: initial ratio of the L-and D-monomer, minimum length required for the segments to be incorporated intoβ-sheets, average length of the peptide molecules, and stereoselectivity in the course of the polymerization process. The results show that in all cases nuclei ofβ-sheets surrounded by random coil segments are formed, the optical activity of which very rapidly increases to purity when the initial ratio of monomers deviates from the racemic mixture. This suggests experiments to enrich the system in one enantiomer. Comparison is made with the corresponding behavior and properties of the a-helical structure.