Enolization reducing sugars

Figure 25.8 Fructose, a ketose, is a reducing sugar because it undergoes two base-catalyzed keto-enol tautomerizations that result in conversion to an aldose.

Fig. 6.25. Simplified mechanism of two degradation reactions between peptides and reducing sugars occurring in solids, a) Maillard reaction between a side-chain amino (or amido) group showing the formation of an imine (Reaction a), followed by tautomerization to an enol (Reaction b) and ultimately to a ketone (Reaction c). Reaction c is known as the Amadori rearrangement (modified from [8]). b) Postulated mechanism of the reaction between a reducing sugar and a C-terminal serine. The postulated nucleophilic addition yields an hemiacetal (Reaction a) and is followed by cyclization (intramolecular condensation Reaction b). Two subsequent hydrolytic steps (Reactions c and d) yield a serine-sugar conjugate and the des-Ser-peptide...

Rates of Enolization Reactions. For a better understanding of the transformation and oxidation reactions of reducing sugars, methods have been developed to measure the primary rates of enolization (18). One of these methods depends on the rate at which tritium ions are released from aldoses-2- to the solvent. This is measured by separation of the water-, sublimation, and radiochemical assay of the water as the reaction proceeds. The rate constant is calculated from the first-order equation ... 

H. S. Isbell, Enolization and oxidation reactions of reducing sugars, in R. F. Gould (Ed.), Carbohydrates in solution, Advances in Chemistry, ACS, Washington, 117 (1973) 70-87. 

The reaction is agreed to start by the formation of the phenylhydrazone of the reducing sugar, which then enolises. Two mechanisms are then proposed (Figure 6.17). In mechanism A, the lone pairs on the enol oxygen (perhaps with deprotonation) provide enough electronic push to cleave the N-N bond of the hydrazine. In mechanism B, the enol tautomerises to the ketone, which then forms a hydrazone. After re-tautomerisation to an enediamine, the N-N bond is cleaved by a push from a nitrogen lone pair. 

The formation of metasaccharinic acids from reducing sugars involves 1,2-enolization (43), elimination of the functional group at C-3 to give (44), which tautomerizes to the glycosulose (45), and a benzilic acid rearrangement of (49) to (50). ... 

A 2-ketose may enolize in two ways, namely, by the loss of a proton from either C-1 or C-3. A jS-elimination can then take place from C-3 in the first case, and, in the second, from either C-1 or C-4. Under alkaline conditions, the enolizations are rapid and reversible, so that the direction of the over-all reaction depends on the relative ease of elimination of the functional groups at C-1, C-3, and C-4. The saccharinic, metasaccharinic, and isosaccharinic acids are derived by the elimination of a group from C-1, C-3, or C-4, respectively, from a reducing sugar in alkaline solution. - ... 

Any ketose that is in equilibrium with its aldehyde is a reducing sugar.This is because under the basic conditions of theTollens test, keto-enol tautomer-ism occurs and converts the ketose to an aldose (it is the aldose that is the actual reducing sugar) ... 

Under mild acid conditions (viz., pH 5-6 at 0-60 C) reducing sugars ionize and mu-tarotate, at lower pH (v/z., down to pH 3 or 4) and at higher temperatures viz., up to ca. 100°C) enolization and isomerization occurs. In acid solution enolization is initiated by direct protonation of the carbonyl group (see Figure 2). In fact, acids are far less effective enolization catalysts than alkalies and as a consequence D-glucose and D-fructose in aqueous solution show maximum stability between pH 3 and 4 e.g., McDonald, 1950). 

Figure 2. Enolization of reducing sugars in acidic, aqueous solution.

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