Wet-dry cycles promote the formation of depsipeptides

It is demonstrated that subjecting a mixture α-hydroxy acids and α-amino acids to wet-dry cycles results in the formation of depsipeptides. The products are characterized by electrospray mass spectroscopy and IR spectroscopy.

Authors

Forsythe JG,Yu SS,Mamajanov I,Grover MA,Krishnamurthy R,Fernández FM,Hud NV

Abstract

Although it is generally accepted that amino acids were present on the prebiotic Earth, the mechanism by which α-amino acids were condensed into polypeptides before the emergence of enzymes remains unsolved. Here, we demonstrate a prebiotically plausible mechanism for peptide (amide) bond formation that is enabled by α-hydroxy acids, which were likely present along with amino acids on the early Earth. Together, α-hydroxy acids and α-amino acids form depsipeptides-oligomers with a combination of ester and amide linkages-in model prebiotic reactions that are driven by wet-cool/dry-hot cycles. Through a combination of ester-amide bond exchange and ester bond hydrolysis, depsipeptides are enriched with amino acids over time. These results support a long-standing hypothesis that peptides might have arisen from ester-based precursors.

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Wet-dry cycles in hot (85 °C), salty environments promote RNA polymerization

Seasonal and daily wet-dry cycles are typical in hot or warm ponds on land. This group used 1–16 wet-dry cycles to polymerize RNA to chain lengths up to ~300 monomers.

Authors

{Da Silva}, L. and {Maurel}, M.-C. and {Deamer}, D.

Abstract

A fundamental problem in origins of life research is how the first polymers with the properties of nucleic acids were synthesized and incorporated into living systems on the prebiotic Earth. Here, we show that RNA-like polymers can be synthesized non-enzymatically from 5'-phosphate mononucleosides in salty environments. The polymers were identified and analyzed by gel electrophoresis, nanopore analysis, UV spectra, and action of RNases. The synthesis of phosphodiester bonds is driven by the chemical potential made available in the fluctuating hydrated and anhydrous conditions of hydrothermal fields associated with volcanic land masses.

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Meteorites delivered nucleobases to thousands of ponds during the Hadean

Using numerical modeling, this group showed that carbonaceous meteorites landed in thousands of ponds on the early Earth, and the nucleobases contained within these meteorites provided ppm–ppb-level pond concentrations for up to a few years.

Authors

{Pearce}, B.~K.~D. and {Pudritz}, R.~E. and {Semenov}, D.~A. and {Henning}, Th.

Abstract

Before the origin of simple cellular life, the building blocks of RNA (nucleotides) had to form and polymerize in favorable environments on early Earth. At this time, meteorites and interplanetary dust particles delivered organics such as nucleobases (the characteristic molecules of nucleotides) to warm little ponds whose wet–dry cycles promoted rapid polymerization. We build a comprehensive numerical model for the evolution of nucleobases in warm little ponds leading to the emergence of the first nucleotides and RNA. We couple Earth’s early evolution with complex prebiotic chemistry in these environments. We find that RNA polymers must have emerged very quickly after the deposition of meteorites (less than a few years). Their constituent nucleobases were primarily meteoritic in origin and not from interplanetary dust particles. Ponds appeared as continents rose out of the early global ocean, but this increasing availability of “targets” for meteorites was offset by declining meteorite bombardment rates. Moreover, the rapid losses of nucleobases to pond seepage during wet periods, and to UV photodissociation during dry periods, mean that the synthesis of nucleotides and their polymerization into RNA occurred in just one to a few wet–dry cycles. Under these conditions, RNA polymers likely appeared before 4.17 billion years ago.

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Fatty acids readily form membranous vesicles when dispersed in freshwater, but do cannot do so in the high ionic concentrations of seawater

This group shows experimentally that decanoic acid (C10) and dodecanoic acid (C12) readily form membranous vesicles at range of pHs, when the ionic concentration is similar to those in hydrothermal fields like Yellowstone. At the ionic concentrations of seawater, however, these fatty acids crystallize instead of forming vesicle compartments.

Authors

Milshteyn, Daniel and Damer, Bruce and Havig, Jeff and Deamer, David

Abstract

There is a general assumption that amphiphilic compounds, such as fatty acids, readily form membranous vesicles when dispersed in aqueous phases. However, from earlier studies, it is known that vesicle stability depends strongly on pH, temperature, chain length, ionic concentration and the presence or absence of divalent cations. To test how robust simple amphiphilic compounds are in terms of their ability to assemble into stable vesicles, we chose to study 10- and 12-carbon monocarboxylic acids and a mixture of the latter with its monoglyceride. These were dispersed in hydrothermal water samples drawn directly from hot springs in Yellowstone National Park at two pH ranges, and the results were compared with sea water under the same conditions. We found that the pure acids could form membranous vesicles in hydrothermal pool water, but that a mixture of dodecanoic acid and glycerol monododecanoate was less temperature-sensitive and assembled into relatively stable membranes at both acidic and alkaline pH ranges. Furthermore, the vesicles were able to encapsulate nucleic acids and pyranine, a fluorescent anionic dye. None of the amphiphiles that were tested formed stable vesicles in sea water because the high ionic concentrations disrupted membrane stability.

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A model is proposed to explain how wet-dry cycling can lead to self-replicating polymers

Damer and Deamer propose a model for selection of encapsulated catalytic polymers, driven by wet-dry cycles in hydrothermal pools on land. In the wet phase, polymers that, for example, stabilize the vesicle membrane, will survive until the dry phase and make it into the next generation. During the dry phase, polymerization will occur and allow for new polymers to cycle into the subsequent wet phase for potential selection. The hypothesis is that, through extended wet-dry cycling, you get increasing complexity of catalytic polymers, eventually leading to one that can self-replicate (i.e. life).

Authors

{Damer}, B. and {Deamer}, D.~W.

Abstract

Hydrothermal fields on the prebiotic Earth are candidate environments for biogenesis. We propose a model in which molecular systems driven by cycles of hydration and dehydration in such sites undergo chemical evolution in dehydrated films on mineral surfaces followed by encapsulation and combinatorial selection in a hydrated bulk phase. The dehydrated phase can consist of concentrated eutectic mixtures or multilamellar liquid crystalline matrices. Both conditions organize and concentrate potential monomers and thereby promote polymerization reactions that are driven by reduced water activity in the dehydrated phase. In the case of multilamellar lipid matrices, polymers that have been synthesized are captured in lipid vesicles upon rehydration to produce a variety of molecular systems. Each vesicle represents a protocell, an “experiment” in a natural version of combinatorial chemistry. Two kinds of selective processes can then occur. The first is a physical process in which relatively stable molecular systems will be preferentially selected. The second is a chemical process in which rare combinations of encapsulated polymers form systems capable of capturing energy and nutrients to undergo growth by catalyzed polymerization. Given continued cycling over extended time spans, such combinatorial processes will give rise to molecular systems having the fundamental properties of life.

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There is no evidence for steep proton gradients across inorganic membranes in modern hydrothermal vents

There are 5 models for the way in which a sharp pH gradient could exist in inorganic membranes in hydrothermal vents. However, there is no evidence for any of these membrane systems (or any inorganic membranes) in modern hydrothemal vents such as Lost City.

Authors

Jackson, J. B.

Abstract

The hypothesis that a natural pH gradient across inorganic membranes lying between the ocean and fluid issuing from hydrothermal alkali vents provided energy to drive chemical reactions during the origin of life has an attractive parallel with chemiosmotic ATP synthesis in present-day organisms. However, arguments raised in this review suggest that such natural pH gradients are unlikely to have played a part in life’s origin. There is as yet no evidence for thin inorganic membranes holding sharp pH gradients in modern hydrothermal alkali vents at Lost City near the Mid-Atlantic Ridge. Proposed models of non-protein forms of the H+-pyrophosphate synthase that could have functioned as a molecular machine utilizing the energy of a natural pH gradient are unsatisfactory. Some hypothetical designs of non-protein motors utilizing a natural pH gradient to drive redox reactions are plausible but complex, and such motors are deemed unlikely to have assembled by chance in prebiotic times. Small molecular motors comprising a few hundred atoms would have been unable to function in the relatively thick (>1 μm) inorganic membranes that have hitherto been used as descriptive models for the natural pH gradient hypothesis. Alternative hypotheses for the evolution of chemiosmotic systems following the emergence of error-prone gene replication and translation are more likely to be correct.

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