Ester Formation and Hydrolysis during Wet–Dry Cycles: Generation of Far-from-Equilibrium Polymers in a Model Prebiotic Reaction

Abstract

Biopolymers exist within living cells as far-fromequilibrium metastable polymers. Living systems must constantly invest energy for biopolymer synthesis. In the earliest stages of life on Earth, the complex molecular machinery that contemporary life employs for the synthesis and maintenance of polymers did not exist. Thus, a major question regarding the origin of life is how the first far-from-equilibrium polymers emerged from a prebiotic "pool" of monomers. Here, we describe a proof-of-principle system, in which L-malic acid monomers form far-from-equilibrium, metastable oligoesters via repeated, cyclic changes in hydration and temperature. Such cycles would have been associated with day−night and/or seasonal cycles on the early Earth. In our model system, sample heating, which promotes water evaporation and ester bond formation, drives polymerization. Even though periodic sample rehydration and heating in the hydrated state promotes ester bond hydrolysis, successive iterations of wet−dry cycles result in polymer yields and molecular weight distributions in excess of that observed after a single drying cycle. We term this phenomenon a "polymerization ratchet". We have quantitatively characterized the "ratchet" of our particular system. Ester bond formation rates and oligoester hydrolysis rates were determined for temperatures ranging from 60 to 95°C. Based on these rates, a mathematical model was developed using polycondensation kinetics, from which conditions were predicted for oligoester growth. This model was verified experimentally by the demonstration that L-malic acid monomers subjected to multiple wet−dry cycles form oligoesters, which reach a steady-state concentration and mean length after several cycles. The concentration of oligoesters that persist between subsequent steady-state cycles depends on the temperature and durations of the dry and wet phases of the cycle. These results provide insights regarding the potential for very simple systems to exhibit features that would have been necessary for initiation of polymer evolution, before the emergence of genomes or enzymes.

A Chemical Engineering Perspective on the Origins of Life

“…Prior to the addition of water, the system was cooled to room temperature and capped, and then was heated back to 60 o C, a temperature at which hydrolysis is low. Monomer conversion of 60% could then be achieved [35], as shown in Figure 7. The purpose of this case study is to investigate polymerization under more realistic scenarios, in which the temperature varies gradually and the water mass transfer is coupled to the temperature.…”

“…Orgel suggested that the ester bond may have preceded the amide bond in peptides and proteins [15]. Although the ester bond is less stable than the amide bond, this could actually have been beneficial in the early stages of life, preventing the irreversible formation of the cyclic dimer, a thermodynamic "dead end" [35].…”

“…Although the ester bond is less stable than the amide bond, its formation is thermodynamically favorable (unlike the amide bond), and it does not form the cyclic dimer trap associated with amino acids. Polyester oligomers were formed from unactivated monomers in a drying reaction [105] and more recently in a cyclic hydration/dehydration environment [35].…”

“…Here we consider three different environmental scenarios associated with a cycling environment. The chemical system studied is the polymerization of malic acid, the α-hydroxy acid previously studied by one of the authors [35]. This polyester was shown to form under alternating hot/dry and cold/wet conditions, yielding a potential proto-protein.…”

“…The units on T here are in o C, with P * w in [mmHg]. Using Antoine's equation at 85 o C and K P = 0.0022 mol/h [35], a value of K y,w = 0.0038 mol/h is obtained.…”