ABSTRACT
Deoxyribose is a key component of nucleotides in adenosine triphosphate (ATP) that transfer energy from glucose to chemical bonds through the glycolysis pathway in all present-day organisms. Therefore, saccharides have clearly been selected as the primary essential molecules for energy transfer reactions during the course of evolution. During chemical evolution in the primitive sea, the rst monosaccharides were presumably generated by processes similar to the “formose reaction“ which is autopolymerization of formaldehydes to make glyceraldehyde and dihydroxyacetone (nucleophilic) in the presence of basic catalysts (e.g., clay) under the prevailing weak reducing conditions. These two carbonyl compounds were able to bind to ketose sugars such as fructose (Frc) through the aldol condensation reaction. Further, catalytic reactions generated hexoses such as glucose (Glc). Following the appearance of Frc, both enolization (e.g., from Frc [ketose] to Glc [aldose]) and
epimerization of an hydroxyl group at the contiguous position (e.g., from Glc to mannose [Man; the C2 epimer of Glc]) led to the widespread production of Frc, Glc, and Man by nonenzymatic processes, through the so-called Lobry de Bruyn transformation (Hirabayashi 1996). In monosaccharides and other cyclohexane compounds, chair forms are more stable than boat forms, according to the concept of 1,3-diaxial interaction. The chair form of β-Glc does not involve 1,3-diaxial interaction because all hydroxyl groups at the C1-C4 positions take an equatorial conguration, which is the most stable thermodynamically. In contrast, Man and galactose (Gal) have a single axial hydroxyl group at the 2-and 4-positions, respectively (Figure 33.1; Glc vs. Man, Gal). Thus, each monosaccharide derived from a Glc epimer has one 1,3-diaxial interaction between C2-OH (axial) and C4-H Man, whereas Gal has an interaction between C2-H and C4-OH (axial).
