Evolution of Homochirality by Epimerization of Random Peptide Chains. A Stochastic Model

Abstract

A cyclic process is described which is constituted of polymerization, epimerization, and hydrolysis steps. During the first cycle peptides with random sequences are formed from racemic amino acids. A small portion of these peptides have substructures with a terminal residue linked to a homochiral sequence of optical antipodes. In such a substructure the terminal residue is assumed to invert into its mirror image so that a thermodynamically favourable epimeric stucture with continuous homochirality is formed. In the hydrolysis step the peptides are split back to monomeric units with retention of configuration. Due to stochastic differences between L- and D-substructures a net excess of one of the enantiomers results. This excess enhances the probability of the formation of substructures having the dominant configuration in the next cycle. It is shown by probabilistic considerations and computer simulations that this mechanism generates an autocatalytic growth of one of the enantiomers which finally results in homochiral populations of amino acids. The number of cycles necessary to attain homochirality depends on the number of residues of the substructure, on the chain length distribution of the polymers and on the total number of amino acid units.

Emergence of homochirality in far‐from‐equilibrium systems: Mechanisms and role in prebiotic chemistry

“…This process turns out to actually be necessary in systems with constant numbers of chiral units. 22,23 The asymmetry brought by non-equilibrium steady-states can stand, against the ''calamity of racemization.'' 24 Initial Rupture of Symmetry: The Central Role of Statistical Fluctuations…”

“…A representative example is given by systems composed of activated cycles of polymerization/depolymerization of amino acids. 22,23 Energy can flow inside the system through the activation of amino acids into activated derivatives like Ncarboxyanhydrides of amino acids (NCAs). For example, this can be performed through the consumption of HNCO and the release of NH 3 and CO 2 as gaseous wastes, 45 while the total number of amino acid residues inside the system remains constant.…”

“…This model shows how the addition of such a recycling mechanism allows a closed system to reach a real fixed point. The last model in closed environment is based on polymerization/depolymerization systems 17,22,23 ( Fig. 7c).…”

“…53 (c) APED model. 22,23 When starting the reaction by the addition of a non-racemic pyrimidylalkanol, an amplification of the initial ee is observed. Successive amplifications from an initial very low ee, involving consecutive steps of synthesis/purification of final products, gives rise to very high enantiomeric excesses.…”

“…17 In order to obtain real non-equilibrium systems with stable non-racemic states, the presence of polymerization/depolymerization is required. 22,23 If these systems are still mostly theoretical, first steps towards their experimental realization were reported. 15,40 In these experiments, successive formations of cyclic peptides together with depolymerization back to monomeric amino acids can be performed, resulting in enantiomeric amplification.…”