Prebiotic ribose synthesis: a critical analysis.

The discovery of catalytic ability in RNA has given fresh impetus to speculations that RNA played a critical role in the origin of life. This question must rest on the plausibility of prebiotic oligonucleotide synthesis, rather than on the properties of the final product. Many claims have been published to support the idea that the components of RNA were readily available on the prebiotic earth. In this article, the literature cited in support of the prebiotic availability of one subunit, D-ribose, is reviewed to determine whether it justifies the claim. Polymerization of formaldehyde (the formose reaction) has been the single reaction cited for prebiotic ribose synthesis. It has been conducted with different catalysts: numerous basic substances, neutral clays and heat, and various types of radiation. Ribose has been identified (yields are uncertain, but unlikely to be greater than 1%) in reactions run with concentrated (0.15 M or greater) formaldehyde. It has been claimed in reactions run at lower concentration, but characterization has been inadequate, and experimental details have not been provided. The complex sugar mixture produced in the formose reaction is rapidly destroyed under the reaction conditions. Nitrogenous substances (needed for prebiotic base synthesis) would interfere with the formose reaction by reacting with formaldehyde, the intermediates, and sugar products in undesirable ways. The evidence that is currently available does not support the availability of ribose on the prebiotic earth, except perhaps for brief periods of time, in low concentration as part of a complex mixture, and under conditions unsuitable for nucleoside synthesis.