In Situ Adsorption of Red Onion (Allium cepa) Natural Dye on Cellulose Model Films and Fabrics Exploiting Chitosan as a Natural Mordant.

Synthetic dyes and chemicals create an enormous impact on environmental pollution both in textile manufacturing and after the product's lifetime. Biobased plant-derived colorants and mordants have great potential for the development of more sustainable textile dyeing processes. Colorants isolated from biomass residues are renewable, biodegradable, and usually less harmful than their synthetic counterparts. Interestingly, they may also bring additional functions to the materials. However, the extraction and purification of the biocolorants from biomass as well as their dyeing efficiency and color fastness properties require a more thorough examination. Here, we extracted red onion (Allium cepa) skins to obtain polyphenolic flavonoids and anthocyanins as biocolorants, characterized the chemical composition of the mixture, and used a quartz crystal microbalance and thin films of cellulose nanofibrils to study the adsorption kinetics of dyes onto cellulose substrates in situ. The effect of different mordants on the adsorption behavior was also investigated. Comparison of these results with conventional dyeing experiments of textiles enabled us to determine the interaction mechanism of the dyes with substrates and mordants. Chitosan showed high potential as a biobased mordant based both on its ability to facilitate fast adsorption of polyphenols to cellulose and its ability to retain the purple color of the red onion dye (ROD) in comparison to the metal mordants FeSO4 and alum. The ROD also showed excellent UV-shielding efficiency at low concentrations, suggesting that biocolorants, due to their more complex composition compared to synthetic ones, can have multiple actions in addition to providing aesthetics.

Depolymerization of biorefinery lignin by improved laccases of the white-rot fungus Obba rivulosa.

Fungal laccases are attracting enzymes for sustainable valorization of biorefinery lignins. To improve the lignin oxidation capacity of two previously characterized laccase isoenzymes from the white-rot fungus Obba rivulosa, we mutated their substrate-binding site at T1. As a result, the pH optimum of the recombinantly produced laccase variant rOrLcc2-D206N shifted by three units towards neutral pH. O. rivulosa laccase variants with redox mediators oxidized both the dimeric lignin model compound and biorefinery poplar lignin. Significant structural changes, such as selective benzylic α-oxidation, were detected by nuclear magnetic resonance analysis, although no polymerization of lignin was observed by gel permeation chromatography. This suggests that especially rOrLcc2-D206N is a promising candidate for lignin-related applications.

Selective Cleavage of Lignin ß-O-4 Aryl Ether Bond by ß-Etherase of the White-Rot Fungus Dichomitus squalens.

Production of value-added compounds from a renewable aromatic polymer, lignin, has proven to be challenging. Chemical procedures, involving harsh reaction conditions, are costly and often result in nonselective degradation of lignin linkages. Therefore, enzymatic catalysis with selective cleavage of lignin bonds provides a sustainable option for lignin valorization. In this study, we describe the first functionally characterized fungal intracellular ß-etherase from the wood-degrading white-rot basidiomycete Dichomitus squalens. This enzyme, Ds-GST1, from the glutathione-S-transferase superfamily selectively cleaved the ß-O-4 aryl ether bond of a dimeric lignin model compound in a glutathione-dependent reaction. Ds-GST1 also demonstrated activity on polymeric synthetic lignin fractions, shown by a decrease in molecular weight distribution of the laccase-oxidized guaiacyl dehydrogenation polymer. In addition to a possible role of Ds-GST1 in intracellular catabolism of lignin-derived aromatic compounds, the cleavage of the most abundant linkages in lignin under mild reaction conditions makes this biocatalyst an attractive green alternative in biotechnological applications.

Synthesis and antioxidant activity of hydroxycinnamic acid xylan esters.

Naturally occurring hydroxycinnamic acids, such as ferulic and sinapic acids, are known to possess antioxidant activity. In this study, ferulic acid and sinapic acid were covalently attached to oat spelt arabinoxylan and birch wood glucuronoxylan by esterification in a two-step feasible synthesis to generate modified xylans with various degrees of substitution. The obtained derivatives were fully analyzed by FT-IR, NMR, and HPSEC experiments to confirm the esterification of xylans and the degree of substitution. The antioxidative potential of the conjugates was evaluated using the emulsion lipid oxidation test. The results demonstrate that the derivatized xylans inhibited lipid oxidation notably better than the native oat spelt and birch wood xylans. It was found that ferulic acid esters of glucuronoxylan were more efficient antioxidants than those of arabinoxylan and that sinapic acid xylan esters were more efficient than their ferulic acid counterparts.

Antioxidant potential of hydroxycinnamic acid glycoside esters.

Hydroxycinnamic acids are natural antioxidants found in fruits, vegetables, and cereals. In this study, the antioxidant activity of various types of hydroxycinnamoyl glycoside esters that mimic the structure of polymeric carbohydrates was studied in different model systems prone to oxidation, namely, liposomes and emulsions. In addition, radical scavenging activity against the stable 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical was tested. It was found that the esterification in the primary hydroxyl group of the glycoside resulted in the improved radical scavenging activity of both sinapoyl and feruloyl glycosides compared to conjugation to the secondary hydroxyl group. Increased activity was also observed, particularly in the case of feruloyl glucosides in inhibiting the oxidation of liposomes emulsions. The results showed that sinapic and ferulic acid glycoside esters were as effective or more efficient antioxidants than their free forms. In conclusion, the strength of their antioxidant effect depends on the nature of conjugation.