REACTIVITY OF ACTIVE OXYGEN SPECIES TOWARD C-2 POSITION OF CARBOHYDRATE AND ITS DEPENDENCE ON THE ORIGIN OF ACTIVE OXYGEN SPECIES UNDER OXYGEN BLEACHING CONDITIONS

摘要

To examine whether the C-2 position of carbohydrate is attacked by active oxygen species (AOS) under oxygen bleaching conditions, a pair of carbohydrate model compounds, methyl β-D-glucopyranoside (MGP), methyl β-D-mannopyranoside (MMP), methyl β-D-(l-2H)glucopyranoside (MGP-1D) or methyl β>(2-2H)glucopyranoside (MGP-2D), was treated together with a phenolic compound, 2,4,6-trimethylphenol (TMPh) or 4-hydroxy-3-methoxybenzyl alcohol (vanillyl alcohol, VA), under conditions similar to practical oxygen bleaching (0.5 mol/L NaOH, 0.36 mmol/L FeCl3, 1.1 MPa O2, 95°C). AOS were generated in situ by reacting TMPh or VA with O2 and attacked the carbohydrate models. MMP was degraded greater than MGP when the pair of them were treated together with either phenolic compound, which suggests that the configurational difference in their C-2 positions has certain effects on their reactions with AOS. A clear kinetic isotope effect was observed, when a pair of MGP and MGP-2D was treated together with TMPh. This indicates that the C-2 hydrogen of MGP is certainly abstracted by some AOS. Contrarily, the difference observed in the degradations between MGP and MGP-2D or between MGP and MGP-1D was not great enough to conclude that the C-2 or anomeric hydrogen of MGP, respectively, is certainly abstracted by AOS, when VA was co-treated with each pair of them. The pair of MGP and MGP-2D was also treated with an alkaline hydrogen peroxide in which oxyl anion radical, the conjugate base of hydroxyl radical, is the only possible AOS. The difference observed in their degradations was too small to conclude that oxyl anion certainly attacks the C-2 position of MGP. The C-2 hydrogen of MGP is abstracted at least by AOS that are not generated from VA but from TMPh. It is the urgent problem to examine whether AOS generated from VA and oxyl anion radical do not show clear kinetic isotope effects even when they abstract the target hydrogens.