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Project Progress

4. Prebiotic Molecular Selection and Organization

1. Molecular Self-Organization

Studies in molecular self-organization focused on two types of amphiphilic molecules, which are molecules that possess both hydrophobic and hydrophilic regions. These molecules tend to self-organize spontaneously in an aqueous environment.

Hazen and Deamer investigated the self-organization of a suite of molecules produced when an aqueous solution of pyruvate is subjected to hydrothermal conditions (250°C at 0.2 GPa). They then observed a significant production of amphiphilic, lipid-like molecules that form vesicles in water.

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The behavior of this molecular suite is similar to that found previously in studies of natural amphiphiles from the Murchison meteorite.

In a second study of prebiotic molecular self-organization, Postdoctoral Fellow Nicholas Platts developed the “PAH World” model of life’s origins, based on the discotic self-organization of functionalized PAH molecules in an aqueous environment. In this model, a stack of PAHs becomes decorated at the edges with the bases of nucleic acids, which are linked into a genetic-type sequence. Platts proposes this model as a precursor to the RNA World.

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2. Molecular Selection and Organization on Mineral Surfaces

Several of the group’s studies in molecular selection focused on the knotty problem of life’s homochirality — the characteristic of life to incorporate overwhelmingly left-handed amino acids and right-handed sugars. By what processes might such molecular selectivity have occurred on the prebiotic Earth. The team’s studies proceeded on two fronts — experimental and theoretical.

Two questions frame these studies. First, what molecules are adsorbed selectively onto what mineral surfaces? Given the vast numbers of different mineral surfaces and different organic molecules of interest, a combinatoric approach is needed. Therefore, Maule, Steele, and Hazen focused on the application of microarray technologies to apply small amounts of many different molecules to several different mineral surfaces in a single experiment. Preliminary results indicate that a variety of mineral surfaces are suitable for this approach, and that small amounts of adsorbed amino acids can be detected using time-of-flight secondary ion mass spectrometry (ToF-SIMS) in the macro-raster mode. These advances will allow the group to investigate a wide range of mineral-molecule interactions.

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In addition to the phenomenology of selective molecular adsorption, Hazen and colleagues want to know the details of mineral-molecule interactions at an atomic scale. To this end, Asthagiri and Hazen have initiated a theoretical study of molecular adsorption using density functional theory — a first principles, ab initio computational approach. They find that strong chiral adsorption requires three points of interaction (bonding) between the molecule and mineral surface. Such interactions occur, for example, when the amino acid, aspartic acid, bonds to the calcite (214) surface. Thus D-Asp is strongly selected in preference to L-Asp, just as observed in experiments.

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However, the amino acid alanine has only two strong bonded interactions with the calcite (214) surface, so there is no chiral selection, in agreement with experiments.

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In addition to these studies of chiral selection, Ertem conducted studies of the formation of RNA oligomers on Na-montmorillonite (a clay mineral). She has demonstrated the formation of phosphodiester bonds in the presence of this clay. Furthermore, she finds that the rates of formation of different oligomers differ significantly. Thus, only a limited number of oligomers could have formed on the early Earth, rather than equal amounts of all possible isomers.

Ultimately, the objective of this project is to understand molecular selection and organization as it might have occurred in a natural prebiotic setting. To this end, Deamer and coworkers have studied the fate of a suite of organic molecules (including four different amino acids, four different nucleobases, glycerol, myristic acid, and phosphate) when poured into a natural, acidic (pH = 3.1) hot spring with a volume of approximately 10 liters. Virtually all of the solutes except myristic acid rapidly disappeared from solution with half-times ranging from 30 minutes to 2 hours. Subsequent analyses of clay minerals lining the edge of the pool revealed that the organic compounds and phosphate were adsorbed by the clay particles. Furthermore, these molecules could be released at alkaline pH ranges. The myristic acid, which was not adsorbed to the clay, nonetheless was precipitated as an iron or aluminum soap that formed an insoluble curd. They conclude that organic compounds and phosphate in an acidic hydrothermal spring exposed to mineral surfaces would not be available as soluble reactants, nor would a fatty acid amphiphile be able to produce stable membranes. These observations provide constraints on conditions that would be conducive to the origin of cellular life.