Much uncertainty exists with regard to the possibility of abiotic formation of nucleotide constituents in geological environments and the combination of them into larger entities. The components that would probably be most easily formed are the purine nitrogen bases, i.e. primarily adenine. The ribose and deoxyribose could, in principle, be formed by the formose (Butlerow) reaction from formaldehyde, but so far it has been very difficult to produce pure pentoses via that pathway. The addition of the phosphodiester backbone is also difficult to achieve. Phosphorylation has, however, been carried out experimentally using pyrophosphate. The potential of pyrophosphate formation upon heating of hydrogenated orthophosphates like whitlockite ((Ca18Mg2H2(PO4)14) to a few hundred ºC has probably been underestimated. This reaction requires low water to rock ratio, conditions that are met at subduction of an oceanic plate. Once pyrophosphate is available, phosphorylation of pentoses, ribose in particular, may occur. Experiments involving heating of sodium dihydrogen phosphate have even shown high yields of trimetaphosphate. This compound is an even better phosphorylating agent than pyrophosphate and has been identified in volcanic fumaroles, but not yet in other geological settings. Ribose may be formed from formaldehyde and glycolaldehyde, because the ribose molecule is stabilized by borate of interstitial fluids. Borate binds to the 2’ and 3’ positions of the ribose molecule. Mechanistically, aldehydes can be formed directly from elemental carbon in the presence of water. The initial reaction of elemental carbon with water gives hydroxymethylene, which can rearrange to formaldehyde. A new hydroxymethylene molecule can then add onto the formaldehyde (and larger aldehyde molecules) and form glycolaldehyde. In this way, the known lag in the formation of glycolaldehyde from formaldehyde is avoided. This lag has previously been a drawback and a reason that the formose reaction was for a while outdated as a possible mechanism for abiotic synthesis of carbohydrates. The reason why pentoses are stabilized by borate is that it forms trigonal and tetrahedral complexes with oxygen groups and, therefore, has a strong affinity for organic material. Boric acid and borate readily form complexes with a wide variety of sugars, particularly the furanose form of pentoses, and other compounds containing cis-hydroxyl groups like humic substances. Borate is continuously scavenged from seawater by secondary layer minerals of oceanic lithosphere and is released again at moderate heating of the subducting plate at convergent margins. The Mariana back-arc is a good example of this process. The fact that ribose is stabilized by borate may change our opinion of the formose reaction as a seemingly random and nonselective reaction into a very precise geochemical abiotic pre-RNA process.
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