Enzymatic bioconversion of beechwood xylan into the antioxidant 2′-O-α-(4-O-methyl-D-glucuronosyl)-xylobiose

Abstract

Acidic xylooligosaccharides (XOS), also called aldouronics, are hetero-oligomers of xylose randomly branched with 4-O-methyl-D-glucuronic acid residues linked by α(1 → 2) bonds, which display bioactive properties. We have developed a simple and integrated method for the production of acidic XOS by enzymatic hydrolysis of a glucurono-xylan from beechwood. Among the enzymes screened, Depol 670L (a cellulolytic preparation from Trichoderma reesei) displayed the highest activity (70.3 U/mL, expressed in reducing xylose equivalents). High-performance anion-exchange chromatography coupled with pulsed amperometric detection (HPAEC-PAD) analysis revealed the formation of a neutral fraction (corresponding to linear XOS, mainly xylose and xylobiose) and a group of more retained products (acidic XOS), which were separated using strong anion-exchange cartridges. The acidic fraction contained a major product, characterized by matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry and mono- and two-dimensional nuclear magnetic resonance spectroscopy (NMR) as 2′-O-α-(4-O-methyl-α-D-glucuronosyl)-xylobiose (X2_MeGlcA). Starting from 2 g of beechwood xylan, 1.5 g of total XOS were obtained, from which 225 mg (11% yield) corresponded to the aldouronic X2_MeGlcA. The acidic XOS exhibited higher antioxidant activity (measured by the ABTS·+ discoloration assay) than xylan, whilst neutral XOS displayed no antioxidant activity. This work demonstrates that it is possible to obtain a safe and natural antioxidant by enzymatic biotransformation of hardwood hemicellulose.

Recovery of xylan from Acacia mearnsii using ultrasound‐assisted alkaline extraction

“…We suppose the antioxidant activity exhibited by acacia xylan may be due to the enrichment in acidic groups (MeGlc p A and 2‐ O ‐acetyl‐Xylp or 3‐ O ‐acetyl‐Xylp) compared to beech xylan with lower content of these acidic groups. Previous studies have confirmed uronic acid 6,51,53 and ferulic acid 53 substituents to be contributors to the antioxidant profile of xylan and its oligosaccharides. Considering that no further purification or purity evaluation was conducted on the xylan extracts, we cannot completely rule out the possibility that an impurity, besides protein and phenolics, which were not detected in the xylan, may have contributed to the antioxidant activity of the xylan.…”

“…The acacia xylan exhibited an EC50 value (the concentration of a compound that gives half‐maximal response) of ≈1.5 mg/mL, whereas the beech xylan did not display any antioxidant capacity. Interestingly, a recent study showed that acidic XOS derived from beech xylan exhibited higher antioxidant activity (measured by the ABTS· + discolouration assay) than the unmodified xylan, whereas neutral XOS displayed no antioxidant activity 51 . We suppose the antioxidant activity exhibited by acacia xylan may be due to the enrichment in acidic groups (MeGlc p A and 2‐ O ‐acetyl‐Xylp or 3‐ O ‐acetyl‐Xylp) compared to beech xylan with lower content of these acidic groups.…”

“…Interestingly, a recent study showed that acidic XOS derived from beech xylan exhibited higher antioxidant activity (measured by the ABTS• + discolouration assay) than the unmodified xylan, whereas neutral XOS displayed no antioxidant activity. 51 We suppose the antioxidant activity exhibited by acacia xylan may be due to the enrichment in acidic groups (MeGlcpA and 2-O-acetyl-Xylp or 3-O-acetyl-Xylp) compared to beech xylan with lower content of these acidic groups. Previous studies have confirmed uronic acid 6,51,53 and ferulic acid 53 substituents to be contributors to the antioxidant profile of xylan and its oligosaccharides.…”

“…50 Using the information mentioned above, the M v were calculated and found to be 25.7 kDa, 21.4 kDa and 21 kDa for beech, alkaline, and ultrasound-assisted alkaline extracted acacia xylan, respectively. Several studies have reported that beech xylan has a molecular weight of around 20 kDa; 21.5 kDa, 51 20.84 kDa, 52 and 11.1 kDa. 34 On the other hand, Acacia mangium-derived xylan was reported to be 28 kDa, 10 which is similar to the weight reported in the current study.…”