Investigation on hydrogen abstraction from methyl glucoside by active oxygen species under oxygen delignification conditions III: effects of the origin of active oxygen species

Carbohydrate model compounds methyl beta-d-glucopyranoside (MGP beta), methyl alpha-d-glucopyranoside (MGP alpha), and methyl beta-d-mannopyranoside (MMP beta) and the deuterium compounds of MGP beta labeled at the anomeric or C-2 positions (MGP beta-1D, MGP beta-2D) were reacted with active oxygen species (AOS) generated in situ by reactions between O-2 and a co-treated phenolic lignin model compound, 4-hydroxy-3-methoxybenzyl alcohol (VAlc), under conditions simulating oxygen delignification (0.5 mol/l NaOH, 0.36 mmol/l Fe3+, 1.1 MPa O-2, 95A degrees C). MGP beta was degraded more than MGP alpha but less than MMP beta when the pairs MGP beta/MGP alpha and MGP beta/MMP beta, respectively, were treated, which indicates that the configurational differences at the anomeric and C-2 positions influence the reactivity of AOS toward these compounds. When the pairs MGP beta/MGP beta-1D and MGP beta/MGP beta-2D were treated, no clear kinetic isotope effects were observed in either case. These results contrasted with those obtained when another phenolic compound, 2,4,6-trimethylphenol (TMPh), was used as the AOS generator instead of VAlc under exactly the same conditions. Clear kinetic isotope effects were observed when using TMPh. Because it is not easily accepted that the anomeric and C-2 hydrogen abstractions are minor reaction modes only for AOS generated in the VAlc system, it is suspected that the AOS do not show any clear kinetic isotope effect even though the AOS abstract an objective hydrogen.