MnO₂ can potentially suppress the degradation of carbohydrates in oxygen delignification, because MnO₂ catalyzes the decomposition of H₂O₂ to H₂O and O₂ and possibly that of organic peroxides to alcohols and O₂ without the formation of any active oxygen species, which degrade carbohydrates. The addition of MnO₂ actually suppressed the degradation of a carbohydrate model compound, methyl β-D-glucopyranoside, when reacted with active oxygen species generated from reactions between a phenolic lignin model compound, vanillyl alcohol, and O₂ under oxygen delignification conditions. However, the addition of MnO₂ did not have any meaningful effect when hardwood unbleached kraft pulp was oxygen-delignified. The addition surprisingly had a deleterious effect on pulp viscosity when MnO2 was generated in situ in pulp fibers. This deleterious effect would result from a phenomenon whereby the oxidation of Mn²⁺ to MnO₂ was not complete in the in situ generation and Mn³⁺-related species, along others, were generated. In contrast, substitution of the latter half of oxygen delignification with a MnO₂ oxidation stage at a pH of 2 substantially suppressed the degradation of carbohydrates, compared to the common oxygen delignification without substitution.

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The antioxidant properties of technical lignins have been extensively investigated but there are still gaps of knowledge that should be filled to facilitate the practical applications of lignins as antioxidants (AOs). In the present investigation, we compared the short-term (60 min) and long-term (48h) AO performance of lignins with different contents of functional groups and lignin model compounds (LMCs) with different aromatic ring substituents in the 2,2-diphenyl-1-picrylhydrazyl radical (DPPH*) antioxidant assay. We found some LMCs to quickly expend their AO capacity while others started off slowly but after 48h had consumed more DPPH* per phenolic hydroxyl group. Reaction time was also a factor in the relative AO performance of lignins. For softwood lignins, a higher phenolic hydroxyl content was associated with increased DPPH* reactivity. CatLignin, a thermally treated lignin rich in phenolic units and especially catechol groups, consumed more than twice as much DPPH* than any other lignin during 48h (over two mol/lignin unit of 180 g/mol). CatLignin also had the lowest 60 min half-maximal effective concentration (EC50). In polypropylene, lignins provided better UV protection than commercial primary antioxidants applied at similar loadings. Similarly, the better performance of lignin over commercial AOs was observed against thermal oxidation.

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Dehydrogenation polymers (DHPs) were prepared by laccase from Rhus vernicifera and horseradish peroxidase (HRP). The enzymatic ability of oxidation and polymerization was compared between these enzymes. Laccase showed higher enzyme activity against syringaldazine than ABTS, while HRP exhibited lower enzyme activity against syringaldazine than 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid). These enzymes had various enzyme activity against these substrates. DHPs from sinapyl alcohol (SA) were hardly produced by each enzyme, whereas DHPs from coniferyl alcohol (CA) were produced by both enzymes. The laccase oxidized sinapyl alcohol faster than coniferyl alcohol. The nuclear magnetic resonance (NMR) analysis demonstrated that acetylated-DHP (Ac-DHP) from CA by laccase contained β-5 and β-β linkages, but not β-O-4 linkage. On the other hand, Ac-DHP from CA by HRP carried all these three linkages.

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Lignocellulose biomass is indispensable for establishing sustainable societies. Trees and large-sized grasses are the major sources of lignocellulose biomass. Moreover, large-sized grasses greatly surpass trees in terms of lignocellulose biomass productivity. With an overall aim to improve lignocellulose usability, it is vital to improve lignin content and simplify lignin structures in biomass plants via lignin metabolic engineering. In this mini review, recent studies of lignin metabolic engineering of grass biomass plants mainly from the authors’ research group are summarized, which includes characterization of lignocellulose properties of large-sized grass biomass plants and the augmentation of lignin content and simplification of lignin structures in grasses.

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The mulberry cultivar with unusual red-colored wood, “Sekizaisou,” was discovered in the bushland of Okushiri Island, Hokkaido, Japan, around a century ago. The leaves of this cultivar were used as feed for sericulture on the island for a short duration from 1916. Although propagules of Sekizaisou have been preserved by sequential vegetative propagation in several public research institutes in Japan, Sekizaisou is believed to have already become extinct on the island. Recently, a point mutation in the first exon of the CINNAMYL ALCOHOL DEHYDROGENASE 1 locus in Sekizaisou was identified to be responsible for the change in the color of the wood and the structural alteration of lignin. In this study, we performed genotyping of the allele in nearly 600 mulberry individuals grown naturally on Okushiri Island for the rediscovery of Sekizaisou and its wild relatives. A simple protocol for the detection of the mutant allele using polymerase chain reaction followed by direct Sanger sequencing was applied. Although no individuals with the mutant allele were identified in the present study, our results will provide an insight into the flow of the mutant gene in the natural mulberry population.

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