Azide ion as a quinone methide scavenger in the horseradish peroxidasecatalyzed polymerization of sinapyl alcohol

Abstract

The enzymatic dehydrogenative polymerizations of monolignols, mainly using horseradish peroxidase (HRP)/hydrogen peroxide as a catalytic system, has been widely used as a model to study lignifi cation, yet there still remain several points to be clarifi ed. One of them is the peculiar behavior of sinapyl alcohol (SA) in the in vitro polymerizations. Many researchers have reported that enzymatic dehydrogenative polymerizations of SA afforded synthetic lignin (dehydrogenation polymer: DHP), but in extremely low yields, whereas the polymerizations of coniferyl alcohol (CA) or p-coumaryl alcohol (PA) gave their DHPs in high yields. Enzymatic dehydrogenative polymerizations of monolignols proceed mainly by three steps: (1) radical formation by enzymatic dehydrogenation of phenolic hydroxyl groups; (2) radical coupling; (3) nucleophilic addition of nucleophiles to the quinone methide intermediates (QMs), re sulting in the reproduction of phenolic compounds for subsequent oxidative couplings. The polymerization of SA is shown in Figure 1. Until now, the problems regarding the polymerizations of SA have been mainly discussed in terms of the reaction steps (1) and (2): the low reactivity of HRP to SA and preferential b-b coupling reactions to b-O-4. However, there is no report focused on reaction step (3) concerning the low polymerizability of SA. Brunow et al. reported that syringyl-type (S-type) quinone methide model compound was signifi cantly less