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Chiral Amplification in Prebiotic Chemistry: A Mechanistic Study on Amino Acids and Peptides

Explore the intriguing chiral amplification process from amino acids to sugars in prebiotic chemistry. Learn about the role of L-proline as a catalyst, enantioenrichment mechanisms, and the potential impact on the selection of amino acids in the RNA world. Discover how small peptides can catalyze aldol reactions, shedding light on the catalytic abilities of amino acids and peptides.

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Chiral Amplification in Prebiotic Chemistry: A Mechanistic Study on Amino Acids and Peptides

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  1. Cordova et al. Chem. Commun., 2005, 2047-2049 • An intriguing example of how chirally enriched amino acids in the prebiotic world can generate sugars with D-configuration & with enantioenrichment: The Model: L-proline: a 2° amine; popular as an organocatalyst because it forms enamines readily

  2. Mechanism: enamine formation CO2H participates as acid

  3. Enantioenrichment % ee of sugar vs % ee of AA • Initially used 80% ee proline to catalyze reaction → >99% ee of allose • Gradually decreased enatio-purity of proline • Found that optical purity of sugar did not decrease until about 30% ee of proline! • Non-linear relationship!

  4.  chiral amplification • % ee out >> % ee in! • Suggests that initial chiral pool was composed of amino acids • Chirality was then transferred with amplification to sugars → “kinetic resolution” • Could this mechanism have led to different sugars diastereomers? • Sugars →→ RNA world →→ selects for L-amino acids? • Small peptides?

  5. Catalysis by Small Peptides • Small peptides can also catalyze aldol reactions with enantioenrichment (See Cordova et al. Chem. Commun. 2005, 4946) • Found to catalyze formation of sugars • It is clear that amino acids & small peptides are capable of catalysis i.e., do not need a sophisticated protein!

  6. From Amino Acids  Peptides • Peptides are short oligomers of AAs (polypeptide ~ 20-50 AAs); proteins are longer (50-3000 AAs) • Reverse reaction is amide hydrolysis, catalyzed by proteases

  7. At first sight, this is a simple carbonyl substitution reaction, however, both starting materials & products are stable: • RCO2- -ve charge is stabilized by resonance • Amides are also delocalized &  carbon & nitrogen are sp2 (unlike an sp3 N in an amine):

  8. Primary structure: AA sequence with peptide bonds • Secondary structure: local folding (i.e. -sheet & -helix) -sheet  helix

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