Epimerization and Decomposition of Kojibiose and Sophorose by Heat Treatment under Neutral pH Conditions

Generation of 3-deoxypentulose by the isomerization and β-elimination of 4-O-substituted glucose and fructose

Aldose-ketose isomerization is commonly used to prepare rare oligosaccharides such as maltulose (4-O-α-d-glucopyranosyl-d-fructose) and lactulose (4-O-β-d-galactopyranosyl-d-fructose). However, both sugars are degraded under alkaline conditions via β-elimination, while their subsequent benzylic acid rearrangement leads to the formation of isosaccharinic acids. Here, we investigated the behavior of maltose and maltulose upon heating in phosphate buffer solution at pH 7.5. Maltose was initially isomerized into maltulose. Maltulose was degraded via β-elimination, followed by keto-enol tautomerization, which led to the formation of a 1,3-dicarbonyl intermediate bearing an aldehyde group at the C-1 position and a ketone group at the C-3 position. Subsequent hydrolysis of this intermediate afforded formic acid and 3-deoxy-d-glycero-pent-2-ulose (1) as the major products based on HPLC and NMR data. In contrast, the formation of isosaccharinic acid via benzylic acid rearrangement, not the 3-deoxypentulose, was reported under the strongly alkaline conditions (Knill and Kennedy, 2003). The heat treatment of 1→4 linked oligo- and polysaccharides possessing glucose or fructose residue at the reducing end under neutral pH conditions could be applied for the practical preparation of a 3-deoxypentulose.

Alkoxycarbonyl elimination of 3-O-substituted glucose and fructose by heat treatment under neutral pH

3-O-Substituted reducing aldoses are commonly unstable under heat treatment at neutral and alkaline pH. In this study, to evaluate the decomposition products, nigerose (3-O-α-d-glucopyranosyl-d-glucose) and 3-O-methyl glucose were heated at 90 °C in 100 mM sodium phosphate buffer (pH 7.5). Decomposition via β-elimination was observed that formed a mixture of 3-deoxy-arabino-hexonic acid and 3-deoxy-ribo-hexonic acid; upon further acid treatment, it was converted to their γ-lactones. Similarly, turanose (3-O-α-d-glucopyranosyl-d-fructose), a ketose isomer of nigerose, decomposed more rapidly than nigerose under the same conditions, forming the same products. These findings indicate that 3-O-substituted reducing glucose and fructose decompose via the same 1,2-enediol intermediate. The alkoxycarbonyl elimination of 3-O-substituted reducing glucose and fructose occurs readily if an O-glycosidic bond is located on the carbon adjacent to the 1,2-enediol intermediate. Following these experiments, we proposed a kinetic model for the3- decomposition of nigerose and turanose by heat treatment under neutral pH conditions. The proposed model showed a good fit with the experimental data collected in this study. The rate constant of the decomposition for nigerose was (1.2 ± 0.1) × 10^-4 s^-1, whereas that for turanose [(2.6 ± 0.2) × 10^-4 s^-1] was about 2.2 times higher.

Analysis of the susceptibility of reducing disaccharides composed of d-glucose to glycation using the Maillard reaction and a novel sensitive method that measures the percentage of the open-ring form

The susceptibility to glycation of all d-glucose-containing reducing disaccharides (kojibiose, sophorose, nigerose, laminaribiose, maltose, cellobiose, isomaltose, and gentiobiose) was evaluated by Maillard browning and the percentages of their acyclic forms estimated using a novel method to evaluate reactivity toward oxime formation were compared for the first time. This new method is facile and applicable to non-labeled carbohydrates, and it is extremely sensitive, more so than any other previously reported methods. The disaccharides linked by 1-6 bonds displayed both high browning intensity and oxime formation reactivity, and they had the greatest amount of the acyclic form. On the other hand, the proportion of acyclic form was generally very low when glucoses were linked by 1-2, 1-3 and 1-4 bonds. The stability of the 1-3 linkage was drastically reduced when basicity was increased due to β-elimination and the production of a highly reactive dehydrated hexose. The 1-4-linked structures, involved in the formation of amylose and cellulose, respectively, were found to be advantageous due to their relatively low susceptibility to glycation.