Reactivity of syringyl quinone methide intermediates in dehydrogenative polymerization. Part 2: pH effect in horseradish peroxidase-catalyzed polymerization of sinapyl alcohol

Abstract

The solvent pH effect in the horseradish peroxidase (HRP)-catalyzed polymerization of sinapyl alcohol (S-alc) and analogously, sinapyl alcohol g-O-glucoside (isosyringin, iso-S) was investigated particularly focusing on the behavior of syringyl-type quinone methide intermediates (S-type QMs) under acidic conditions. At first, the HRP-catalyzed polymerization of iso-S at pH 6.5-2.5, which produces watersoluble dehydrogenation polymer (DHP) intermediates in a homogeneous phase, was monitored by UV spectroscopy and gel permeation chromatography with photodiode array detection (GPC-PDA). Under acidic conditions at pH below 4.5, unstablilized S-type QMs from iso-S are rapidly quenched resulting in efficient productions of DHPs, although substantial loss of HRP activity and the resulting insufficient polymerization were inevitable at pH below 3.5. In addition, it was found that a small addition of guaiacyl-type comonomer (isoconiferin, iso-G) effectively promotes the polymerization of iso-S under acidic conditions, in which the comonomer serves as a radical mediator to facilitate the HRP-catalyzed oxidations of iso-S. Next, the HRP-catalyzed polymerization of S-alc at various pH values was conducted and the resulting DHPs were characterized by GPC and NMR measurements. The yields of isolated DHPs significantly increased as solvent pH decreased below 4.5. The structural analyses of the DHPs demonstrated that reaction selectivity of S-type QMs during the polymerization drastically changed at pH below 4.5: they react efficiently with water molecules as solvent leading to the formation of benzyl alcohol type b-O-4 substructures preferentially to the formation of a-O-aryl type substructures. Consequently, the data in this study demonstrated that acidic conditions at pH below 4.5 are favored in the dehydrogenative polymerization of S-alc from the viewpoint of the reactivity of S-type QMs.

Effect of pH on the Dehydrogenative Polymerization of Monolignols by Laccases fromTrametes versicolorandRhus vernicifera

“…It is reported that in the quinone methide reactions with vanillyl alcohol in aqueous solution, only the addition of water was observed at low pH, and the phenol addition was prominent under neutral conditions . A similar pH effect has been reported for the dehydrogenative polymerization of SA using HRP . These results suggest that in the biosynthesis of lignin, the type of laccase does not significantly affect the structure of lignin.…”

“… 27 A similar pH effect has been reported for the dehydrogenative polymerization of SA using HRP. 28 These results suggest that in the biosynthesis of lignin, the type of laccase does not significantly affect the structure of lignin. It is likely that the surrounding environment during lignin biosynthesis, such as pH, has a greater influence on the structure of lignin.…”

“…Because the high proportion of the β- O -4/α- O -4 structure is not observed for native lignins, 20 − 23 lower pH is most likely a lignin biosynthesis condition as suggested by some studies on the reactivity of quinone methide 29 , 30 and on the dehydrogenative polymerization of monolignol using HRP. 28 …”

“…It is likely that the surrounding environment during lignin biosynthesis, such as pH, has a greater influence on the structure of lignin. Because the high proportion of the β- O -4/α- O -4 structure is not observed for native lignins, − lower pH is most likely a lignin biosynthesis condition as suggested by some studies on the reactivity of quinone methide , and on the dehydrogenative polymerization of monolignol using HRP …”

Epigallocatechin gallate incorporation into lignin enhances the alkaline delignification and enzymatic saccharification of cell walls

“…Therefore in the present study, we examined the copolymerization of EGCG and CA into dehydrogenation polymers (synthetic lignins, DHPs), utilizing an in vitro horseradish peroxidase (HRP)-catalyzed polymerization system that models lignin polymerization in vivo [ 35 - 38 ]. Two-dimensional nuclear magnetic resonance (NMR) experiments with the DHPs revealed one major cross-coupling mode between CA and both epigallocatechin and gallate moieties of EGCG.…”