Selective emergence and survival of early polypeptides in water

Abstract

Oligopeptides essential to primitive cells could not be obtained just by raising the background noise of organic compounds produced by a prebiotic chemistry working at random. Selection pathways were required. Experimental evidence is given for selective condensation of amino acids in water as well as for selective resistance to degradation. It is shown that N-carboxyanhydrides are good candidates for chemical selection in water. They are formed when active esters of amino acids are left in the presence of bicarbonate ions or when N,N'-carbonyldiimidazole is used as condensing agent. Polymerization of a mixture of proteinaceous and non-proteinaceous amino acids leads to an enrichment in the proteinaceous ones plus alpha-aminobutyric acid. Selective resistance toward degradation of beta-pleated sheet conformation is used to exemplify a possible accumulation of homochiral sequences made of hydrophilic and strong hydrophobic residues. Amino acids with branched aliphatic side-chains are selected but those having short linear aliphatic side-chains such as alpha-aminobutyric acid or norvaline are not.

Identification of diamino acids in the Murchison meteorite

“…This ␤-sheet structure has the hydrophilic side chains arranged on one side of the sheet and the hydrophobic side chains on the other (38). It was concluded earlier that the first biological systems might have contained only a limited number of different amino acids (39). The results presented here for the Murchison chondrite indicate that the diamino acids are potential candidates as hydrophilic constituents within the ␤-sheet structure.…”

Energy flows, metabolism and translation

“…Living organisms depending only on RNA for genetic continuity and lacking encoded proteins (in agreement with the RNA world hypothesis) would only have to improve this process to take advantage of the formation of non-coded random peptides, without the need of specifically built machinery. It must be emphasized that, like aa-AMP, the large majority of carboxy-activated amino acids, including thioesters, would have been converted into NCA (scheme 4) [62] because of the reaction of the amino group with CO 2 as a result of the abundance of CO 2 and bicarbonate on the early Earth [63]. Moreover, additional experiments [57] have shown that aa-AMP like other amino acid phosphatemixed anhydrides do not give rise to efficient peptide chain formation except in the presence of carbon dioxide demonstrating that peptides are mainly formed through the intermediacy of NCA, which removes any evolutionary advantage of adenylates as peptide precursors.…”

Primitive Membrane Formation, Characteristics and Roles in the Emergent Properties of a Protocell

“…Polycondensation of amino acids is possible in solution when monomers are activated using N-carboxyanhydride [65][66][67][68], however sequence specificity is impossible in such a situation. The introduction of a membrane changes the dynamics of the condensation reaction such that hydrophobic amino acids can be joined more efficient (high rates, yields and length of products) on membranes than in solution [69,70].…”