(494c) Condensation Polymerization Under Primordial Conditions
Biopolymers, such as polypeptides and nucleic acids, are far-from-equilibrium metastable structures; in living systems, energy is invested into biopolymer synthesis and protection from hydrolysis via enzymatic catalytic networks. In the study of chemical evolution, the question must be asked: How the first biopolymers evolve without the aid of enzymatic (protein biopolymer) catalysts? There are two significant impediments to polymerization in the primordial world. One is the extreme dilution of monomers in the prebiotic oceans, severely retarding the rate of condensation polymerization; the other is the inability of such polymerizations to achieve significant degree of polymerization due the fact that condensation polymerizations are generally reversible, with the hydrolysis reaction causing depolymerization. In a highly dilute aqueous environment, equilibrium between condensation and hydrolysis will be quickly established, preventing significant degree of polymerization.
One postulated solution to this dilemma is polymerization in tidal pools. In this scenario, alternating cycles of wet and cool (night) and hot and dry (day) have been postulated to cause a “ratchet effect” in which hydrolysis is inhibited by low temperatures (night) and low water content (day), while monomer concentration is effected by repeated cycles of evaporation. Herein we describe a primitive metastable condensation polymer system that is plausible under prebiotic condition, in the absence of enzymatic energy harvesting and catalytic machinery. The system is driven far from equilibrium by day and night cycles involving temperature and hydration differentials. As a model system, we have chosen the condensation polymerization of malic acid (thought to exist in the prebiotic environment). Malic acid (and other simple organic acids) is structurally very similar to the peptides forming simple proteins, the difference being only the substitution of an amine group for the alcohol functionality.
A “day-night machine” has been built, based on a PCR (polymerase chain reaction) machine, which allows 96 wells to be treated with individual conditions of diurnal temperature and moisture variations. Various wells are sampled at specific time intervals, and the resulting polymer is analyzed for degree of polymerization via gel permeation chromatography. Very good fits have been obtained to the Arrhenius function for both the condensation and hydrolysis rate constants.
A mathematical model has been developed based on standard polycondensation kinetics, taking into the account the effects of different environmental conditions, and using the rate constants derived from the day-night machine. Using this model, conditions for ratcheting kinetics have been predicted and verified experimentally. Using the combination of experiment and modeling, it is possible to show that ratcheting is, indeed, a viable explanation for the evolution of primitive biomacromolecules and under what environmental conditions this will occur.