Pathways to Carbohydrates on Prebiotic Earth: Hydrogen Cyanide as a Substrate and Catalyst in a Reducing Environment

We present alternate chemical routes for the production of simple sugars on prebiotic Earth that do not involve the formose reaction, whose mechanism of initiation involving the self-condensation of two formaldehyde electrophiles remains unclear. We show that HCN is the only carbon-containing prebiotic molecule required for the production of short- and medium-chain aldoses and ketoses, wherein HCN serves as either a reactant or catalyst. In situ generation of H₂ from the decomposition of formic acid, produced from HCN hydrolysis, supports the two-electron reduction of cyanohydrins to imines, catalyzed by heavy or transition metals provided by asteroid or meteorite collisions with prebiotic Earth and subsequent imine hydrolysis to give aldehydic functionalities. Experimental evidence is provided to support some of the proposed pathways, including the control of glycolaldehyde self-condensation in the presence of cyanide ions to preferentially give C₅ aldononitriles (cyanohydrins) and cyclic imido-1,4-lactones, both precursors to aldopentoses. Molybdate-catalyzed aldose epimerization is discussed as a chemical progenitor to the transketolase reaction of the pentose phosphate pathway, whose mechanism of action may be more complex than presently understood.