by neufer » Sun Sep 02, 2012 1:37 am
http://en.wikipedia.org/wiki/PAH_world_hypothesis wrote:
<<The Miller–Urey experiment in 1952, and others since, demonstrated the synthesis of organic compounds, such as nucleobases, 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.
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. 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.)
http://apod.nasa.gov/apod/ap091230.html
http://apod.nasa.gov/apod/ap080605.html
http://apod.nasa.gov/apod/ap080428.html
http://apod.nasa.gov/apod/ap080111.html
http://apod.nasa.gov/apod/ap060414.html
http://apod.nasa.gov/apod/ap050409.html
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 mesogenic stacks which, like lipids, tend to organize with their hydrophobic parts protected.
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.
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.
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).
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.>>
[quote=" http://en.wikipedia.org/wiki/PAH_world_hypothesis"]
<<The Miller–Urey experiment in 1952, and others since, demonstrated the synthesis of organic compounds, such as nucleobases, 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.
[float=right][img3="[b][color=#0000FF]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.[/color][/b]"]http://upload.wikimedia.org/wikipedia/commons/d/d1/PAHWorld.png[/img3][/float]
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. 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.)
http://apod.nasa.gov/apod/ap091230.html
http://apod.nasa.gov/apod/ap080605.html
http://apod.nasa.gov/apod/ap080428.html
http://apod.nasa.gov/apod/ap080111.html
http://apod.nasa.gov/apod/ap060414.html
http://apod.nasa.gov/apod/ap050409.html
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 mesogenic stacks which, like lipids, tend to organize with their hydrophobic parts protected.
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.
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.
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).
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.>>[/quote]