Spontaneous onset of homochirality in oligopeptide chains generated in the polymerization of N-carboxyanhydride amino acids in water.

This article is concerned with the spontaneous onset of homochiral oligopeptide sequences.We will show that the polymerization of hydrophobic NCA (N-carboxyanhydride = cyclicanhydride)-amino acid racemates (i.e. tryptophane, leucine and isoleucine) in aqueous solution yieldsoligopeptides that are characterized by a high degree of homochiral sequences. Furthermore we willshow that quartz enhances efficiently the mole fraction of oligopeptides with homochiral sequence byselectively adsorbing the more stereoregular oligopeptides from an aqueous solution of oligo-D,Lleucine.We find in particular that the mole fraction of the adsorbed homochiral 7mers is 17 timeslarger than the mole fraction calculated for a theoretical, random process. Experimentally the stereoisomerdistribution for each oligomer length can be determined by the use of enantio-labeling andLC-MS (Liquid Chromatography-Mass Spectrometry). Furthermore, if we start the polymerizationwith an enantiomeric excess (e.e.) of 20% of L-leucine (L-amino acid : D-amino acid = 6 : 4, molarratio) we observe a chiral amplification in the enantiomeric homochiral oligopeptides. We think thatsuch processes are relevant to the chemical evolution of single handedness.