Protoenzymes: The Case of Hyperbranched Polymer-Scaffolded ZnS Nanocrystals

Abstract

Enzymes are biological catalysts that are comprised of small-molecule, metal, or cluster catalysts augmented by biopolymeric scaffolds. It is conceivable that early in chemical evolution, ancestral enzymes opted for simpler, easier to assemble scaffolds. Herein, we describe such possible protoenzymes: hyperbranched polymer-scaffolded metal-sulfide nanocrystals. Hyperbranched polyethyleneimine (HyPEI) and glycerol citrate polymer-supported ZnS nanocrystals (NCs) are formed in a simple process. Transmission electron microscopy (TEM) analyses of HyPEI-supported NCs reveal spherical particles with an average size of 10 nm that undergo only a modest aggregation over a 14-day incubation. The polymer-supported ZnS NCs are shown to possess a high photocatalytic activity in an eosin B photodegradation assay, making them an attractive model for the study of the origin of life under the “Zn world” theory dominated by a photocatalytic proto-metabolic redox reaction network. The catalyst, however, could be easily adapted to apply broadly to different protoenzymatic systems.

Recent progress in primitive polyester synthesis and membraneless microdroplet assembly

“…For example, replacement of citric acid in the ternary starting mixture with methylsuccinic acid or adipic acid also resulted in similar hyperbranched polyester products, which even had the ability to act as catalysts to the Kemp elimination reaction by increasing the reaction rate by two- to three-fold, thus suggesting that polyesters with sufficient chemical, structural, and functional complexity could have been early catalysts (i.e., “protoenzymes”) on early Earth before the emergence of peptide or protein-based catalysts [ 51 ]. Further development of this system revealed that hyperbranched polyesters could even scaffold photocatalytic zinc sulfide nanoparticles [ 53 ]. However, other than the literature described here, there are not many studies which focus on prebiotic hyperbranched polyesters, and we suggest that this should be a priority for future study.…”

A Physicochemical Consideration of Prebiotic Microenvironments for Self-Assembly and Prebiotic Chemistry

“…Hygroscopic salts also sequester water [ 39 ]. These environments enable unique aqueous-phase chemistries such as the synthesis of polymer-supported zinc sulfide nanocrystals [ 40 ] or photochemical phosphorylation [ 38 ], and can also reduce evaporation rates and prevent total desiccation.…”

A material‐based panspermia hypothesis: The potential of polymer gels and membraneless droplets

“…For example, hyperbranched polymers have been shown to be able to scaffold the assembly of zinc sulfide nanocrystals. [ 172 ] For example, perhaps polymeric gel compartments could have actively scaffolded lipid assembly on its surface [ 127 ] to promote the formation of an enclosing membrane. This ability for a membraneless droplet to actively acquire and assemble a lipid membrane could have been a key stage of evolution connecting a membraneless protocellular world (polymeric gel material droplets) to a membrane‐bound world (more cell‐like state).…”

“…However, some gels have been found to be more heat‐resistant than others, namely hyperbranched polymer gel‐based materials, [ 197 ] some of which appear to be stable >850 °C [ 198 ] ; as mentioned before, hyperbranched gel‐based polymers have been investigated in the context of OoL and prebiotic chemistry. [ 167,172 ] Although this would still be far less than the maximum temperatures reached by particles upon atmospheric entry, not all particles reach the maximum temperature, and a recent study suggested that the average entry temperature of micrometeorites to Earth (radius of <~1 mm) was closer to 1300 °C, rather than 2000 °C. [ 199 ] In fact, it was proposed that 3.5% of all such micrometeorites would not reach 900 °C during entry into the Earth atmosphere [ 199 ] ; if entry of M‐BPS onto a receiving planet with an Earth‐like atmosphere resulted in similar entry temperatures of the gel materials, then at least some percentage of the materials could have arrived onto the planet surface in‐tact.…”