Figure 1: Schematic representation of citric acid and glycerol polyesterification. Credit: Irena Mamajanov, ELSI

The research described here employs the process of gelation in hyperbranched polymer as a model for tar formation in prebiotic reaction. By subjecting hyperbranched polyesters forming reactions to wet-dry cycles, the onset of gelation was delayed. The finding suggests that wet-dry cycling systems, associated with tidal or geyser activity, could impose rudimentary control over prebiotic chemistry, guiding the products toward selection and function rather than deterioration to tar.

Chemical reactions that lack enzymatic controlled or skilled chemist supervision often result in a dark intractable polymeric material, or so-called "tar" or "asphalt." S. A. Benner refers to the tar formation as a paradox of origin of life and considers the tar "better suited for paving roads rather than supporting Darwinism." Indeed, if prebiotic chemical reactions are fated to produce tar and not life's building blocks, how did life arise? What was the mechanism of control that guided the prebiotic chemical systems toward the emergence of life rather than tar formation? Here, ELSI PI Irena Mamajanov investigates the gelation of hyperbranched polymers, one of the processes contributing to the tar formation, under wet-dry cycles.

Gelation occurs when a branched polymer forms an extensive network between strands through crosslinking. The gel transition is associated with a drastic change in polymer properties, such as viscosity and solubility. Gelation is sharply dependent upon the degree of polymerization. The experimental setup compares the branched polyesters prepared under mild continuous heating and the wet-dry cycling associated with environmental conditions, such as dew formation or tidal activities. The results show that periodic wetting, which promotes partial hydrolysis of the polyester, serves to constrain the chain growth and delays the gel transition. Furthermore, spectral analyses indicate that continuously dried samples contain higher quantities of crosslinked and macrocyclic products, whereas cycled systems are enriched in branched structures. Purportedly, environmental conditions can exert a rudimentary pressure to selectively enrich the polyesterification products in polymers of different structures and properties.