Prebiotic condensation through wet-dry cycling regulated by deliquescence

Abstract

Wet-dry cycling is widely regarded as a means of driving condensation reactions under prebiotic conditions to generate mixtures of prospective biopolymers. A criticism of this model is its reliance on unpredictable rehydration events, like rainstorms. Here, we report the ability of deliquescent minerals to mediate the oligomerization of glycine during iterative wet-dry cycles. The reaction mixtures evaporate to dryness at high temperatures and spontaneously reacquire water vapor to form aqueous solutions at low temperatures. Deliquescent mixtures can foster yields of oligomerization over ten-fold higher than non-deliquescent controls. The deliquescent mixtures tightly regulate their moisture content, which is crucial, as too little water precludes dissolution of the reactants while too much water favors hydrolysis over condensation. The model also suggests a potential reason why life evolved to favor the enrichment of potassium: so living systems could acquire and retain sufficient water to serve as a solvent for biochemical reactions.

Fig. 1

Demonstration of deliquescent wet–dry cycles. a An illustration of our deliquescent model with its cycling conditions. b A demonstration of the magnitude, reversibility, and consistency of moisture sorption/desorption during wet–dry cycling of a 50 mg mixture of K⁺, Na⁺, Cl⁻, OH^–, and glycine. Containers were weighed over the course of ten complete 24-h cycles to monitor moisture gain and loss at 70 %RH (red triangles, above RH₀ for the mixture) and 30 %RH (blue circles, below RH₀ for the mixture). Each cycle consisted of two phases: a cool phase for 20 h at 40 °C (blue background) followed by a hot phase for 4 h at 120 °C (orange background). The error bars represent 95% confidence intervals (n = 3 identical experiments). Source data are provided as a Source Data file

Fig. 2

The condensation of glycine in a deliquescent system. a The condensation of glycine into oligoglycines. We report yields based on the percentage of initial glycine converted into oligomers, excluding cyclic DKP dimer. b The total yields of glycine oligomers in the presence of (K⁺/Na⁺)(⁻Cl/⁻OH) after 1, 2, 3, 5, and 10 cycles. Each cycle was 24 h. For the red samples, one cycle included 20 h at 40 °C and 70 %RH (circles), 50 %RH (diamonds), or 30 %RH (triangles) followed by 4 h at 120 °C. For the blue samples, one cycle included 18 h at 40 °C and 70, 50 or 30 %RH (circles, diamonds, triangles) followed by 6 h at 100 °C. The error bars represent 95% confidence intervals (n = 3 identical experiments). The downturn in total yield observed at higher temperature is addressed in the Supplementary Discussion. Source data are provided as a Source Data file

Fig. 3

The advantage of limited rehydration in wet–dry cycling. Yields of glycine oligomers (excluding DKP) in the presence of (K⁺, Na⁺)(Cl⁻, OH⁻) after 1, 2, 3, 5, and 10 cycles. Each cycle was 24 h. For the samples marked with triangles, one cycle included 18 h at 40 °C and 70 %RH, followed by 6 h at 100 °C. The samples marked with circles were exposed to the same environmental cycles, but 20 mL of water was added to the mixture before each drying period. This addition simulated heavy rain and overhydration of the sample to verify a shortcoming of the standard model for wet–dry cycling that is obviated by self-regulated, limited rehydration through deliquescence. The error bars represent 95% confidence intervals (n = 3 identical experiments). Source data are provided as a Source Data file

Fig. 4

Glycine oligomerization in K₂HPO₄ vs. Na₂HPO₄. a The total yields of glycine oligomers (excluding DKP) in the presence of K₂HPO₄ (red diamonds) or Na₂HPO₄ (blue circles) after 1, 2, 3, 5, and 10 cycles. Each cycle was 24 total hours: 20 h at 40 °C and 50 %RH followed by 4 h at 120 °C. The error bars represent 95% confidence intervals (n = 3 identical experiments). b Distribution of yields by oligomer length after cycles 1, 2, 3, 5, and 10 in the sample prepared with K₂HPO₄. Source data are provided as a Source Data file