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PAH world hypothesis

The PAH world hypothesis is a biological hypothesis that 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, though in 2007 Cassini spacecraft found the presence of heavy negative ions of tholin in the upper regions of Titan’s atmosphere.^[1]

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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. Thus 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 is 0.34 nm. This is the same separation found in 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.