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The asphalt problem

16 February 2019 - Origins

The asphalt problem is closely related to the water problem and coined by Benner and Carrigan in 2012 (DOI) discussing RNA in the RNA world. Any laboratory model for the abiogenesis of this biomolecule suffers from the complex mixtures (asphalt) typically obtained from simple precursor molecules such as formamide and from the intrinsic instability of RNA in water (RNA and DNA have different backbones). In 2012 the experimental system for the synthesis of RNA building blocks consisted of olivine, borate-stabilized carbohydrates, formamide, and ammonium formate. The stabilization of the RNA backbone by borate minerals was already investigated by the same research group in 2004. (DOI)

There have been two recent developments in this research field both focussing on the role played by the geothermal element (rocks and heat) in the synthetic equation. Last September Saladino, Di Mauro and Garcia-Ruiz presented a Universal Geochemical Scenario. In it, throwing a lot of different minerals at formamide did not exactly solve the asphalt problem. To give a sampling of the results chalcopyrite gives a mixture of purines, pyrimidines and condensing agents. With borax the main product is a mixture of carboxylic acids. Only with iron silicate (an olivine as with the 2012 study by Benner et al.) the single reaction products are pyrimidines.

Two rather unusual geothermal constructs are discussed as well. One is the chemical garden. The classical chemical garden based on sodium silicate and iron chloride does not a lot of synthesis but related silica vesicles (drop-shaped membranes rather than tubes) produce quantities of amino acids. The other geothermal concept discussed in the article relates to the formation of methane in early Earth. It is a rather exotic reaction called serpentinization where olivine reacts with water to form serpentine, magnetite and (surprise!) hydrogen gas. From there it is small step to methane when hydrogen reacts with carbon dioxide in the Sabatier reaction. This reaction is relevant because it generates the alkaline silica environment required for the silica vesicles to build. Early planet Earth as a giant chemical factory.

In a very recent contribution Colón-Santos, Cooper and Cronin do not mention asphalt but rather speak of an combinatorial explosion (Link). Their target is different by focusing on the formose reaction with as reactants formaldehyde, glycolaldehyde and calcium hydroxide with formamide and water as solvent. Only one of the minerals selected in this study, chalcopyrite matches with one of the Saladino study. As an additional hack the procedure went through several cycles with a portion of the reaction mixture isolated with new raw feedstock added. The reaction products were also not isolated but it was found that on each cycle the number of different reaction products was reduced as measured by ultra-performance liquid chromatography - mass spectroscopy. In the chalcopyrite example the number of compounds (features) is reduced by over a hundred on going from cycle one to cycle three.