Prebiotic chemists often study how modern biopolymers, e.g., peptides and nucleic acids, could have originated in the primitive environment, though most contemporary biomonomers don't spontaneously oligomerize under mild conditions without activation or catalysis. However, life may not have originated using the same monomeric components that it does presently. There may be numerous non-biological (or "xenobiological") monomer types that were prebiotically abundant and capable of facile oligomerization and self-assembly. Many modern biopolymers degrade abiotically preferentially via processes which produce thermodynamically stable ring structures, e.g. diketopiperazines in the case of proteins and 2', 3'-cyclic nucleotide monophosphates in the case of RNA. This weakness is overcome in modern biological systems by kinetic control, but this need not have been the case for primitive systems. We explored here the oligomerization of a structurally diverse set of prebiotically plausible xenobiological monomers, which can hydrolytically interconvert between cyclic and acyclic forms, alone or in the presence of glycine under moderate temperature drying conditions. These monomers included various lactones, lactams and a thiolactone, which varied markedly in their stability, propensity to oligomerize and apparent modes of initiation, and the oligomeric products of some of these formed self-organized microscopic structures which may be relevant to protocell formation.
Nucleic acid segregation and compartmentalization were likely essential functions that primitive compartment systems resolved during evolution. Recently, polyester microdroplets generated from dehydration synthesis of various α-hydroxy acids (αHA) were suggested as potential primitive compartments. Some of these droplets can differentially segregate and compartmentalize organic dyes, proteins, and nucleic acids. However, the previously studied polyester microdroplets included limited αHA chemical diversity, which may not reflect the chemical diversity available in the primitive Earth environment. Here, we increased the chemical diversity of polyester microdroplet systems by combinatorially adding an αHA monomer with a basic side chain, 4-amino-2-hydroxybutyric acid (4a2h), which was incorporated with different ratios of other αHAs containing uncharged side chains to form combinatorial heteropolyesters via dehydration synthesis. Incorporation of 4a2h in the polymers resulted in the assembly of some polyester microdroplets able to segregate fluorescent RNA or potentially acquire intrinsic fluorescent character, suggesting that minor modifications of polyester composition can significantly impact the functional properties of primitive compartments. This study suggests one process by which primitive chemical systems can increase diversity of compartment "phenotype" through simple modifications in their chemical composition.
Compartmentalization was likely essential for primitive chemical systems during the emergence of life, both for preventing leakage of important components, i.e., genetic materials, and for enhancing chemical reactions. Although life as we know it uses lipid bilayer-based compartments, the diversity of prebiotic chemistry may have enabled primitive living systems to start from other types of boundary systems. Here, we demonstrate membraneless compartmentalization based on prebiotically available organic compounds, α-hydroxy acids (αHAs), which are generally coproduced along with α-amino acids in prebiotic settings. Facile polymerization of αHAs provides a model pathway for the assembly of combinatorially diverse primitive compartments on early Earth. We characterized membraneless microdroplets generated from homo- and heteropolyesters synthesized from drying solutions of αHAs endowed with various side chains. These compartments can preferentially and differentially segregate and compartmentalize fluorescent dyes and fluorescently tagged RNA, providing readily available compartments that could have facilitated chemical evolution by protecting, exchanging, and encapsulating primitive components. Protein function within and RNA function in the presence of certain droplets is also preserved, suggesting the potential relevance of such droplets to various origins of life models. As a lipid amphiphile can also assemble around certain droplets, this further shows the droplets' potential compatibility with and scaffolding ability for nascent biomolecular systems that could have coexisted in complex chemical systems. These model compartments could have been more accessible in a "messy" prebiotic environment, enabling the localization of a variety of protometabolic and replication processes that could be subjected to further chemical evolution before the advent of the Last Universal Common Ancestor.