Homochiral oligopeptides by chiral amplification within two-dimensional crystalline self-assemblies at the air-water interface; relevance to biomolecular handedness

A possible role that might have been played by ordered clusters at interfaces for the generation of homochiral oligopeptides under prebiotic conditions has been probed by a catalyzed polymerization of amphiphilic activated alpha-amino acids, in racemic and chiral non-racemic forms, which had self-assembled into two-dimensional (2D) ordered crystallites at the air-aqueous solution interface. As model systems we studied N(epsilon)-stearoyl-lysine thioethyl ester (C(18)-TE-Lys), gamma-stearyl-glutamic thioethyl ester (C(18)-TE-Glu), N(alpha)-carboxyanhydride of gamma-stearyl-glutamic acid (C(18)-Glu NCA) and gamma-stearyl-glutamic thioacid (C(18)-thio-Glu). According to in-situ grazing incidence X-ray diffraction measurements on the water surface, (R,S)-C(18)-TE-Lys, (R,S)-C(18)-TE-Glu, and (R,S)-C(18)-Glu-NCA amphiphiles self-assembled into ordered racemic 2D crystallites. Oligopeptides 2-12 units long were obtained at the air-aqueous solution interface after injection of appropriate catalysts into the water subphase. The experimental relative abundance of oligopeptides with homochiral sequence generated from (R,S)-C(18)-TE-Lys and (R,S)-C(18)-TE-Glu, as determined by mass spectrometry on enantioselectively deuterium-labeled samples, was found to be significantly larger than that obtained from (R,S) C(18)-thio-Glu which polymerizes randomly. An efficient chiral amplification was obtained in the polymerization of non-racemic mixtures of C(18)-Glu-NCA since the monomer molecules in the racemic 2D crystallites are oriented such that the reaction occurs between heterochiral molecules related by glide symmetry to yield heterochiral oligopeptides whereas the enantiomer in excess, in the enantiomorphous crystallites, yield oligopeptides of a single handedness.