Investigation on the hydrogen abstraction from methyl glucoside by active oxygen species under oxygen delignification conditions IV: appearance of kinetic isotope effect in the reaction between methyl glucoside and deuterated methyl glucoside

Abstract

In our previous studies, a clear kinetic isotope effect was observed when a pair of carbohydrate model compounds, methyl β-d-glucopyranoside (MGPβ) and a deuterated MGPβ labeled at the anomeric (methyl β-d-(1-2H)glucopyranoside) or C-2 position (methyl β-d-(2-2H)glucopyranoside), was reacted with active oxygen species (AOS) generated by reactions between O2 and a phenolic compound, 2,4,6-trimethylphenol (TMPh), under oxygen delignification conditions. These results indicate that the AOS abstract the anomeric and C-2 hydrogens of MGPβ. Contrarily, no clear kinetic isotope effects were observed when AOS were generated by reactions between O2 and another phenolic compound, 4-hydroxy-3-methoxybenzyl alcohol (vanillyl alcohol, Valc), and hence, the abstraction of the anomeric and C-2 hydrogens by the AOS was not confirmed. In this study, a pair of MGPβ and the deuterated MGPβ labeled at all the positions, (2H3)methyl β-d-(1,2,3,4,5,6,6-2H7)glucopyranoside, was reacted with AOS generated from TMPh or Valc under oxygen delignification conditions to further examine the appearance of a kinetic isotope effect. A clear kinetic isotope effect was observed when either of TMPh or Valc was the origin of AOS, indicating that some AOS abstract at least a hydrogen of MGPβ in either case. The results are further discussed focusing on the type of AOS generated from TMPh and Valc.