Abstract
The origin of biopolymers is a central question in origins of life research. In extant life, proteins are coded linear polymers made of a fixed set of twenty alpha-L-amino acids. It is likely that the prebiotic forerunners of proteins, or protopeptides, were more heterogenous polymers with a greater diversity of building blocks and linkage stereochemistry. To investigate a possible chemical selection for alpha versus beta amino acids in abiotic polymerization reactions, we subjected mixtures of alpha and beta hydroxy and amino acids to single-step dry-down or wet-dry cycling conditions. The resulting model protopeptide mixtures were analyzed by a variety of analytical techniques, including mass spectrometry and NMR spectroscopy. We observed that amino acids typically exhibited a higher extent of polymerization in reactions that also contained alpha hydroxy acids over beta hydroxy acids, whereas the extent of polymerization by beta amino acids was higher compared to their alpha amino acid analogs. Our results suggest that a variety of heterogenous protopeptide backbones existed during the prebiotic epoch, and that selection towards alpha backbones occurred later as a result of polymer evolution.
Highlights
All living organisms utilize the same limited sets of small molecules as building blocks of polynucleotides, polypeptides, and polysaccharides [1]
We found that the extent of polymerization of amino acids into depsipeptides is higher in the presence of alpha hydroxy acids than with beta hydroxy acids
Four hydroxy acids were studied: glycolic acid, lactic acid, 3-hydroxypropionic acid, and 3-hydroxybutanoic acid. glc and lac are alpha hydroxy acids, whereas hpa and hba are beta hydroxy acids. We examined their corresponding amino acid analogs: glycine (Gly), alanine (Ala), β-alanine (β-Ala), and β-aminobutyric acid (β-Aba)
Summary
All living organisms utilize the same limited sets of small molecules as building blocks of polynucleotides, polypeptides, and polysaccharides [1]. Reasonable pathways for the origins of some building blocks, especially amino acids, have been proposed [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18]. Around four billion years ago, a creative phase of pre-Darwinian chemical evolution pared down the number of potential building blocks to form oligomers and polymers with levels of functionally required for Darwinian processes [15,19,20,21,22,23,24]. The chemistry of water and the condensation-dehydration of building blocks into chimeric metabolites, oligomers and polymers are important features of these models
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