Experiments of the amplification of optical activity

By way of investigating possible mechanisms for the abiotic amplification of small enantiomeric excesses (e.e.'s) in almost racemic mixtures of amino acid enantiomers, we have undertaken a quantitative study of the base-initiated partial polymerization of leucine and valine N-carboxy-anhydride (NCA) enantiomer mixtures containing known excesses of both the R- and S-forms. Polymerization to the extent of ca. 50% of leucine NCA having an 8-70% e.e. of either the R- or S-enantiomer led to an e.e. enhancement in the polymer, which contained a 12-84% e.e. of that enantiomer which predominated in the original monomer NCA. A corresponding decrease in the e.e. of the initially predominant enantiomer was noted in the unpolymerized residue from each reaction. Polymerization to the extent of 25-50% of mixtures of valine NCA enantiomers containing a 12-13% e.e. of either R- or S-isomer led to polymers showing a 7-8%decrease in the e.e. of the initially predominant enantiomer, and to an increase of its e.e. in the unpolymerized residue. These divergent results, the latter of which is quite novel, are compared with earlier qualitative results in the literature and are discussed briefly from the viewpoint of both mechanism and the amplification of optical activity.