PAH world hypothesis
Encyclopedia
The PAH world hypothesis is a speculative hypothesis that proposes that polycyclic aromatic hydrocarbon
s (PAH), 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. As yet it is untested.
The Miller–Urey experiment in 1952, and others since, demonstrated the synthesis of organic compounds, such as nucleobase
s, amino acids, formaldehyde
and sugars, from the original inorganic precursors presumed to have been present in the primordial soup. The RNA world hypothesis
shows how RNA can become its own catalyst (a ribozyme
), and so become the basis for evolution of life. 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. A more thoroughly elaborated idea has been published by Ehrenfreund et al..
, and are considered a likely constituent of the primordial sea. PAHs, along with fullerenes (or "buckyballs"), have been recently detected in nebulae. (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.”)
PAH's 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 in water.
These modified PAHs are amphiphilic, which means that they have parts that are both hydrophilic and hydrophobic. When in solution, they assemble in discotic
mesogen
ic stacks which, like lipids, tend to organize with their hydrophobic parts protected.
s 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.
(methanal) oligomer
s, 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.
(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).
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.
Polycyclic aromatic hydrocarbon
Polycyclic aromatic hydrocarbons , also known as poly-aromatic hydrocarbons or polynuclear aromatic hydrocarbons, are potent atmospheric pollutants that consist of fused aromatic rings and do not contain heteroatoms or carry substituents. Naphthalene is the simplest example of a PAH...
s (PAH), assumed to be abundant in the primordial soup of the early Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, played a major role in the origin of life by mediating the synthesis of RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
molecules, leading into the RNA world. As yet it is untested.
Background
Nucleobase
Nucleobases are a group of nitrogen-based molecules that are required to form nucleotides, the basic building blocks of DNA and RNA. Nucleobases provide the molecular structure necessary for the hydrogen bonding of complementary DNA and RNA strands, and are key components in the formation of stable...
s, amino acids, formaldehyde
Formaldehyde
Formaldehyde is an organic compound with the formula CH2O. It is the simplest aldehyde, hence its systematic name methanal.Formaldehyde is a colorless gas with a characteristic pungent odor. It is an important precursor to many other chemical compounds, especially for polymers...
and sugars, from the original inorganic precursors presumed to have been present in the primordial soup. The RNA world hypothesis
RNA world hypothesis
The RNA world hypothesis proposes that life based on ribonucleic acid pre-dates the current world of life based on deoxyribonucleic acid , RNA and proteins. RNA is able both to store genetic information, like DNA, and to catalyze chemical reactions, like an enzyme protein...
shows how RNA can become its own catalyst (a ribozyme
Ribozyme
A ribozyme is an RNA molecule with a well defined tertiary structure that enables it to catalyze a chemical reaction. Ribozyme means ribonucleic acid enzyme. It may also be called an RNA enzyme or catalytic RNA. Many natural ribozymes catalyze either the hydrolysis of one of their own...
), and so become the basis for evolution of life. In between there are some missing steps such as how the first RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
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. A more thoroughly elaborated idea has been published by Ehrenfreund et al..
Polycyclic aromatic hydrocarbons
Polycyclic aromatic hydrocarbons are the most common and abundant of the known polyatomic molecules in the visible UniverseUniverse
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...
, and are considered a likely constituent of the primordial sea. PAHs, along with fullerenes (or "buckyballs"), have been recently detected in nebulae. (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.”)
PAH's are not normally very soluble in sea water, but when subject to ionizing radiation such as solar UV light, the outer hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
atoms can be stripped off and replaced with a hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
group, rendering the PAHs far more soluble in water.
These modified PAHs are amphiphilic, which means that they have parts that are both hydrophilic and hydrophobic. When in solution, they assemble in discotic
Columnar phase
The columnar phase is a class of mesophases in which molecules assemble into cylindrical structures to act as mesogens. Originally, these kinds of liquid crystals were called discotic liquid crystals because the columnar structures are composed of flat-shaped discotic molecules stacked...
mesogen
Mesogen
Mesogen is the fundamental unit of a liquid crystal that induces structural order in the crystals.Typically, a liquid-crystalline molecule consists of a rigid moiety and one or more flexible parts. The rigid part aligns molecules in one direction, whereas the flexible parts induce fluidity in the...
ic stacks which, like lipids, tend to organize with their hydrophobic parts protected.
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 nucleotideNucleotide
Nucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides participate in cellular signaling , and are incorporated into important cofactors of enzymatic reactions...
s of RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
and DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
. 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
Pyrimidine
Pyrimidine is a heterocyclic aromatic organic compound similar to benzene and pyridine, containing two nitrogen atoms at positions 1 and 3 of the six-member ring...
and purine
Purine
A purine is a heterocyclic aromatic organic compound, consisting of a pyrimidine ring fused to an imidazole ring. Purines, including substituted purines and their tautomers, are the most widely distributed kind of nitrogen-containing heterocycle in nature....
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 formaldehydeFormaldehyde
Formaldehyde is an organic compound with the formula CH2O. It is the simplest aldehyde, hence its systematic name methanal.Formaldehyde is a colorless gas with a characteristic pungent odor. It is an important precursor to many other chemical compounds, especially for polymers...
(methanal) oligomer
Oligomer
In chemistry, an oligomer is a molecule that consists of a few monomer units , in contrast to a polymer that, at least in principle, consists of an unlimited number of monomers. Dimers, trimers, and tetramers are oligomers. Many oils are oligomeric, such as liquid paraffin...
s, 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.
Detachment of the RNA-like strands
A subsequent transient drop in the ambient pHPH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
(increase in acidity), for example as a result of a volcanic
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
discharge of acidic gases such as sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
or carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, 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).
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 acidity levels drop again, they would tend to fold back on themselves, with complementary sequencesBase pair
In molecular biology and genetics, the linking between two nitrogenous bases on opposite complementary DNA or certain types of RNA strands that are connected via hydrogen bonds is called a base pair...
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
RNA world hypothesis
The RNA world hypothesis proposes that life based on ribonucleic acid pre-dates the current world of life based on deoxyribonucleic acid , RNA and proteins. RNA is able both to store genetic information, like DNA, and to catalyze chemical reactions, like an enzyme protein...
, which speculates about further evolutionary developments from that point.
See also
- History of the Earth
- Iron-sulfur world theoryIron-sulfur world theoryThe iron-sulfur world theory is a set of proposals for the origin of life and the early evolution of life advanced by Günter Wächtershäuser, a Munich patent lawyer with a degree in chemistry who had been encouraged and supported by philosopher Karl R. Popper to publish his ideas. The theory...
- ThermosynthesisThermosynthesisThermosynthesis is a theoretical mechanism proposed by Anthonie Muller for biological use of the free energy in a temperature gradient to drive energetically uphill anabolic reactions. It makes use of this thermal gradient, or the dissipative structure of convection in this gradient, to drive a...
- Other possible RNARNARibonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
precursors:- Threose nucleic acid (TNA)
- Glycol nucleic acid (GNA)
- Peptide nucleic acid (PNA)
External links
- The 'PAH World'
- Astrobiology magazine Aromatic World An interview with Pascale Ehrenfreund on PAH origin of life. - Accessed June 2006
- Life's ingredients found in early universe New Scientist Magazine 14:49 29 July 2005
- RNA-directed amino acid homochirality