PAH world hypothesis

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The PAH world hypothesis is a speculative hypothesis that proposes that polycyclic aromatic hydrocarbons (PAHs), known to be abundant in the universe, [1] [2] [3] including in comets, [4] and assumed to be abundant in the primordial soup of the early Earth, played a major role in the origin of life by mediating the synthesis of RNA molecules, leading into the RNA world. However, as yet, the hypothesis is untested. [5]

Contents

A PAH stack assembling PAHWorld.svg
A PAH stack assembling

Background

The Miller-Urey experiment showed that organic compounds can be readily produced under the presumed conditions of the early Earth. Miller-Urey experiment-en.svg
The Miller–Urey experiment showed that organic compounds can be readily produced under the presumed conditions of the early Earth.

The 1952 Miller–Urey experiment demonstrated the synthesis of organic compounds, such as amino acids, formaldehyde and sugars, from the original inorganic precursors the researchers presumed to have been present in the primordial soup (but is no longer considered likely). This experiment inspired many others. In 1961, Joan Oró found that the nucleotide base adenine could be made from hydrogen cyanide (HCN) and ammonia in a water solution. [6] Experiments conducted later showed that the other RNA and DNA nucleobases could be obtained through simulated prebiotic chemistry with a reducing atmosphere. [7]

The RNA world hypothesis shows how RNA can become its own catalyst (a ribozyme). In between there are some missing steps such as how the first RNA molecules could be formed. The PAH world hypothesis was proposed by Simon Nicholas Platts in May 2004 to try to fill in this missing step. [8] A more thoroughly elaborated idea has been published by Ehrenfreund et al. [9]

Polycyclic aromatic hydrocarbons

The Cat's Paw Nebula lies inside the Milky Way Galaxy and is located in the constellation Scorpius. Green areas show regions where radiation from hot stars collided with large molecules and small dust grains called "polycyclic aromatic hydrocarbons" (PAHs), causing them to fluoresce. (Spitzer Space Telescope, 2018) PIA22568-CatsPawNebula-Spitzer-20181023.jpg
The Cat's Paw Nebula lies inside the Milky Way Galaxy and is located in the constellation Scorpius.
Green areas show regions where radiation from hot stars collided with large molecules and small dust grains called "polycyclic aromatic hydrocarbons" (PAHs), causing them to fluoresce.
(Spitzer Space Telescope, 2018)

Polycyclic aromatic hydrocarbons are the most common and abundant of the known polyatomic molecules in the visible universe, and are considered a likely constituent of the primordial sea. [1] [2] [3] PAHs, along with fullerenes (or "buckyballs"), have been recently detected in nebulae. [10] In April 2019, scientists, working with the Hubble Space Telescope, reported the confirmed detection of the large and complex ionized molecules of buckminsterfullerene (C60) in the interstellar medium spaces between the stars. [11] [12] (Fullerenes are also implicated in the origin of life; according to astronomer Letizia Stanghellini, "It's possible that buckyballs from outer space provided seeds for life on Earth." [13] ) In September 2012, NASA scientists reported that PAHs, subjected to interstellar medium (ISM) conditions, are transformed, through hydrogenation, oxygenation and hydroxylation, to more complex organics — "a step along the path toward amino acids and nucleotides, the raw materials of proteins and DNA, respectively". [14] [15] Further, as a result of these transformations, the PAHs lose their spectroscopic signature which could be one of the reasons "for the lack of PAH detection in interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers of protoplanetary disks." [14] [15]

In 2013, polycyclic aromatic hydrocarbons were detected in the upper atmosphere of Titan, the largest moon of the planet Saturn. [16]

In 2018, researchers reported low-temperature chemical pathways from simple organic compounds to complex PAHs. Such chemical pathways may help explain the presence of PAHs in the low-temperature atmosphere of Saturn 's moon Titan, and may be significant pathways, in terms of the PAH world hypothesis, in producing precursors to biochemicals related to life as we know it. [17] [18]

PAHs are not normally very soluble in sea water, but when subject to ionizing radiation such as solar UV light, the outer hydrogen atoms can be stripped off and replaced with a hydroxyl group, rendering the PAHs far more soluble.

These modified PAHs are amphiphilic, which means that they have parts that are both hydrophilic and hydrophobic. When in solution, they assemble in discotic mesogenic (liquid crystal) stacks which, like lipids, tend to organize with their hydrophobic parts protected.

In 2014, NASA announced a database [19] for tracking polycyclic aromatic hydrocarbons (PAHs) in the universe. More than 20% of the carbon in the universe may be associated with PAHs, [19] [20] possible starting materials for the formation of life. PAHs seem to have been formed as early as a couple of billion years after the Big Bang, are abundant in the universe, [1] [2] [3] and are associated with new stars and exoplanets. [19]

Reactions

Attachment of nucleobases to PAH scaffolding

In the self-ordering PAH stack, the separation between adjacent rings is 0.34 nm. This is the same separation found between adjacent nucleotides of RNA and DNA. Smaller molecules will naturally attach themselves to the PAH rings. However PAH rings, while forming, tend to swivel around on one another, which will tend to dislodge attached compounds that would collide with those attached to those above and below. Therefore, it encourages preferential attachment of flat molecules such as pyrimidine and purine nucleobases, the key constituents (and information carriers) of RNA and DNA. These bases are similarly amphiphilic and so also tend to line up in similar stacks.

Attachment of oligomeric backbone

According to the hypothesis, once the nucleobases are attached (via hydrogen bonds) to the PAH scaffolding, the inter-base distance would select for "linker" molecules of a specific size, such as small formaldehyde (methanal) oligomers, also taken from the prebiotic "soup", which will bind (via covalent bonds) to the nucleobases as well as each other to add a flexible structural backbone. [5] [8]

Detachment of the RNA-like strands

A subsequent transient drop in the ambient pH (increase in acidity), for example as a result of a volcanic discharge of acidic gases such as sulfur dioxide or carbon dioxide, would allow the bases to break off from their PAH scaffolding, forming RNA-like molecules (with the formaldehyde backbone instead of the ribose-phosphate backbone used by "modern" RNA, but the same 0.34 nm pitch). [5]

Formation of ribozyme-like structures

The hypothesis further speculates that once long RNA-like single strands are detached from the PAH stacks, and after ambient pH levels became less acidic, they would tend to fold back on themselves, with complementary sequences of nucleobases preferentially seeking out each other and forming hydrogen bonds, creating stable, at least partially double-stranded RNA-like structures, similar to ribozymes. The formaldehyde oligomers would eventually be replaced with more stable ribose-phosphate molecules for the backbone material, resulting in a starting milestone for the RNA world hypothesis, which speculates about further evolutionary developments from that point. [5] [8] [21]

See also

Related Research Articles

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Aromatic compounds, also known as "mono- and polycyclic aromatic hydrocarbons", are organic compounds containing one or more aromatic rings. The word "aromatic" originates from the past grouping of molecules based on smell, before their general chemical properties were understood. The current definition of aromatic compounds does not have any relation with their smell.

Pyrimidine is an aromatic, heterocyclic, organic compound similar to pyridine. One of the three diazines, it has nitrogen atoms at positions 1 and 3 in the ring. The other diazines are pyrazine and pyridazine.

<span class="mw-page-title-main">RNA world</span> Hypothetical stage in the early evolutionary history of life on Earth

The RNA world is a hypothetical stage in the evolutionary history of life on Earth, in which self-replicating RNA molecules proliferated before the evolution of DNA and proteins. The term also refers to the hypothesis that posits the existence of this stage.

<span class="mw-page-title-main">Uracil</span> Chemical compound of RNA

Uracil is one of the four nucleobases in the nucleic acid RNA. The others are adenine (A), cytosine (C), and guanine (G). In RNA, uracil binds to adenine via two hydrogen bonds. In DNA, the uracil nucleobase is replaced by thymine (T). Uracil is a demethylated form of thymine.

<span class="mw-page-title-main">Thymine</span> Chemical compound of DNA

Thymine is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The others are adenine, guanine, and cytosine. Thymine is also known as 5-methyluracil, a pyrimidine nucleobase. In RNA, thymine is replaced by the nucleobase uracil. Thymine was first isolated in 1893 by Albrecht Kossel and Albert Neumann from calf thymus glands, hence its name.

<span class="mw-page-title-main">Astrochemistry</span> Study of molecules in the Universe and their reactions

Astrochemistry is the study of the abundance and reactions of molecules in the universe, and their interaction with radiation. The discipline is an overlap of astronomy and chemistry. The word "astrochemistry" may be applied to both the Solar System and the interstellar medium. The study of the abundance of elements and isotope ratios in Solar System objects, such as meteorites, is also called cosmochemistry, while the study of interstellar atoms and molecules and their interaction with radiation is sometimes called molecular astrophysics. The formation, atomic and chemical composition, evolution and fate of molecular gas clouds is of special interest, because it is from these clouds that solar systems form.

<span class="mw-page-title-main">Nucleobase</span> Nitrogen-containing biological compounds that form nucleosides

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<span class="mw-page-title-main">Cosmochemistry</span> Study of the chemical composition of matter in the universe

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<span class="mw-page-title-main">Atomic and molecular astrophysics</span>

Atomic astrophysics is concerned with performing atomic physics calculations that will be useful to astronomers and using atomic data to interpret astronomical observations. Atomic physics plays a key role in astrophysics as astronomers' only information about a particular object comes through the light that it emits, and this light arises through atomic transitions.

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<span class="mw-page-title-main">Abiogenesis</span> Natural process by which life arises from non-living matter

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<span class="mw-page-title-main">Cyclopropenylidene</span> Chemical compound

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<span class="mw-page-title-main">2-Methylnaphthalene</span> Chemical compound

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Pseudo-panspermia is a well-supported hypothesis for a stage in the origin of life. The theory first asserts that many of the small organic molecules used for life originated in space. It continues that these organic molecules were distributed to planetary surfaces, where life then emerged on Earth and perhaps on other planets. Pseudo-panspermia differs from the fringe theory of panspermia, which asserts that life arrived on Earth from distant planets.

References

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  7. ↑ Oró J (1967). Fox SW (ed.). Origins of Prebiological Systems and of Their Molecular Matrices. New York Academic Press. p. 137.
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  13. ↑ Atkinson, Nancy (October 27, 2010). "Buckyballs Could Be Plentiful in the Universe". Universe Today . Retrieved October 28, 2010.
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