PAH world hypothesis
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The PAH world hypothesis proposes that the use of polycyclic aromatic hydrocarbons (PAH) was a means for a pre-RNA World basis for the origin of life. As yet it is untested.
Experiments such as the Miller experiment and others allow the simple construction of primitive organic molecules including amino acids. 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 2005.
It is known that polycyclic aromatic hydrocarbons are a likely constituent of the primordial sea. 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 PAH's far more soluble in water.
PAH's are amphiphilic, which means that they have parts that are both hydrophilic and hydrophobic. So when in solution, like lipids, they tend to self organise themselves in stacks, with the hydrophobic parts protected.
In this self ordering stack, the separation between rings has a 0.34nm separation. This is the same separation 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 bases. These bases are similarly amphiphilic and so also tend to line up in similar stacks. This ends up making an effective scaffold for a nucleic acid backbone to form along the bases.
A small change in acidity would then allow the bases to break off from the original stack of PAH's and so form molecules like RNA.
The details are expected to be published in Astrobiology (Liebert Publ.) in summer 2006.
[edit] 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 - Accessed June 2006
