RNA world and Beta-D-ribonucleotides?

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RNA world and Beta-D-ribonucleotides?

Beta-D-ribonucleotides are compounds which are composed of a purine (adenine or guanine) or a pyrimidine (uracil or cytosine) linked to the 1-position of ribose in the beta-configuration. There is also a phosphate group attached to the 1-position of ribose in the beta-configuration. For the four different ribonucleotides in this prebiotic scenario, there would be hundreds of other possible isomers.

Each of the four ribonucleotides is built up of three components: a purine or pyrimidine, a sugar (ribose) and phosphate. It is exceptionally improbable that any of these subunits could have accumulated in any more than trace amounts on the early earth. The suggested prebiotic pathway leading to ribose, for instance, is very problematic. The reaction does not proceed if the reaction mixture includes the various nitrogenous substances thought to have been present in the primordial oceans. These nitrogenous substances react with formaldehyde, as well as the intermediates involved in the pathways to the various sugars, and indeed the sugars themselves to form non-biological materials. Moreover, ribose has an extremely short half-life for decomposition at neutral pH, making it highly unlikely that the sugars were available as prebiotic reagents.

Reaction pathways proposed for the synthesis of adenine start with HCN in alkaline solutions of NH4OH. The reactions yield small amounts of adenine and other nitrogenous bases (provided the HCN concentration is greater than 0.01 M. The reaction mixtures, however, contain a large variety of nitrogenous substances that would interfere with the formose reaction. Thus, the conditions suggested for the formation of purines and pyrimidines are not compatible with those proposed for the formation of the sugar ribose. Adenine is also highly susceptible to deamination and ring-opening reactions making its prebiotic accumulation extremely unlikely. It seems unlikely, therefore, that substantial quantities of nucleotides and nucleosides could have been formed and have accumulated on the early earth. If the key components of nucleotides were not present, the possibility of obtaining the desired pool of the four beta-D-ribonucleotides with correct linkages would be very remote indeed.

If there was not in fact a prebiotic pool of beta-D-ribonucleotides (as is required by the RNA world model), then the entire hypothesis of an RNA world formed by natural processes is invalid.



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