Self-replication, homochiral peptides

Chiral surfaces Enantiomorphous crystals Homochiral peptides Mirror symmetry breaking Non-linear kinetics Self-replication of peptides... 

In accordance with the autocatalytic process, matrices are again formed. It is surprising that the autocatalysis decreases when only 1 of the 15 building blocks of the peptide has the opposite handedness, e.g., when the N-peptide fragment contains one D-amino acid as well as the 14 L-amino acids. These experimental results show that such a system is able to form homochiral products via self-replication. It can be assumed that similar mechanisms influenced the origin of homochirality on Earth (Saghatelian et al., 2001 Siegel, 2001). 

Once again, the Ghadiri group used the peptide with the leucine zip. Starting from a racemic mixture of the peptide fragments E and N (with 15 and 17 amino acids respectively), homochiral products were preferentially formed in a homochiral selection process in the catalytic self-replication cycle. The initial mixture contained the two peptide fragments, which each consisted of d- and L-amino acids, i.e., a total of four competing molecular species (NL, N°, EL and E°). Thus, four different products could be formed in the condensation reaction TLL, TDD, TLD and TDL. As stated above, TLL and TDD were formed preferentially. The experimental results can be summarized as shown in Fig. 5.8. 

Enantioselective enriching processes lead to homochirality [20] in amino acids, which is not a consequence of, but a prerequisite for the origin of life. Mutation and selection formed the first self-replicating, but stUl prebiotic, molecular systems of catalytically highly active peptides and oligoribonucleotides. Ultimately, macroscopic asymmetry derived a posteriori from non-linear amplification mechanisms. [21,22]... 

Self-replication has been shown to work with peptides as well. Ghadiri and coworkers created a homochiral 32-residue polypeptide that could stereoselectively replicate itself from a racemic pool of fragments (Figure 6) (53). 

Figure 6. Schematic representation of chiroselective self-replication of the homochiral peptide LL. Adapted with permission from reference 53. Copyright 2001 Nature Publishing Group.

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