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. 2016 Jul 26;6(3):28.
doi: 10.3390/life6030028.

Dry/Wet Cycling and the Thermodynamics and Kinetics of Prebiotic Polymer Synthesis

David S Ross  1 , David Deamer  2
Affiliations

Dry/Wet Cycling and the Thermodynamics and Kinetics of Prebiotic Polymer Synthesis

David S Ross et al. Life (Basel). .

Abstract

The endoergic nature of protein and nucleic acid assembly in aqueous media presents two questions that are fundamental to the understanding of life's origins: (i) how did the polymers arise in an aqueous prebiotic world; and (ii) once formed in some manner, how were they sufficiently persistent to engage in further chemistry. We propose here a quantitative resolution of these issues that evolved from recent accounts in which RNA-like polymers were produced in evaporation/rehydration cycles. The equilibrium Nm + Nn ↔ Nm+n + H₂O is endoergic by about 3.3 kcal/mol for polynucleotide formation, and the system thus lies far to the left in the starting solutions. Kinetic simulations of the evaporation showed that simple Le Châtelier's principle shifts were insufficient, but the introduction of oligomer-stabilizing factors of 5-10 kcal/mol both moved the process to the right and respectively boosted and retarded the elongation and hydrolysis rates. Molecular crowding and excluded volume effects in present-day cells yield stabilizing factors of that order, and we argue here that the crowded conditions in the evaporites generate similar effects. Oligomer formation is thus energetically preferred in those settings, but the process is thwarted in each evaporation step as diffusion becomes rate limiting. Rehydration dissipates disordered oligomer clusters in the evaporites, however, and subsequent dry/wet cycling accordingly "ratchets up" the system to an ultimate population of kinetically trappedthermodynamically preferred biopolymers.

Keywords: RNA; evaporites; hydrothermal ponds; kinetics; molecular crowding; polynucleotides; thermodynamics.

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Conflict of interest statement

One of the authors (DWD) serves as editor-in-chief of Life but did not participate in reviewing the manuscript.

Figures

Figure 1
Figure 1
The activity of water and the corresponding Gibbs energy benefit driving the equilibrium in Equation (1) to the right at 85 °C as a function of the characteristic cavity size.
Figure 2
Figure 2
Numerical simulations of the evaporation and rehydration of 6.6 mM solutions of mononucleotide. (A) Starting solution concentrated by a factor of 1400 and ΔG′ = ΔG°′; (B) products from A at 30 min and ΔG′ = −3.5 kcal/mol; (C) products from A at 30 min and ΔG′ = −10 kcal/mol; (D) products at 1 s in C diluted by a factor of 1400 and ΔG′ = ΔG°′; (E) products at 20 h in (D) concentrated by a factor of 1400 and ΔG′ = −10 kcal/mol. The 30-min mark in (A) is nominally the point at which the evaporations were complete.
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