| Hemicellulose is a high quality and renewable biomass with high value for research and utilization.Owing to the existence of lignin-carbohydrate complex(LCC)linkages,the extracted hemicellulose contains lignin,which is difficult to remove.Such as alkali extraction of hemicellulose contains more lignin,it limits its utilization.Therefore,it is very necessary to analyze the structure of LCC for its high-value utilization.However,LCC structure identification is challenging,especially for the carbohydrate-rich ones.In the previous study of hemicellulose purification by chlorine dioxide oxidation,it was difficult to observe the structure of the residual lignin after purification due to the high content of sugar fraction in the sample,which created an obstacle to the study of the structural changes of LCC during purification.Therefore,it is necessary to enrich for the LCC which in hemicellulose samples,thus improving the sensitivity of structure identification.In this paper,a xylanase-macroporous resin method was used to enrich the LCC in hemicellulose samples extracted from bamboo by alkali method.Afterwards,the effects of xylanase dosage and enzyme treatment time on LCC enrichment were discussed.Finally,the structural changes of LCC in hemicellulose samples after chlorine dioxide oxidation were analyzed by xylanase-macroporous resin method.The main conclusions are as follows:After xylanase-macroporous resin treatment,the sensitivity of signal recognition of LCC structure in hemicellulose samples was significantly enhanced.The main chain of xylose in LCC was destroyed,the proportion of arabinose was increased,and lignin mainly existed in the form of acid-soluble lignin after enrichment.2D NMR-HSQC NMR and Py-GC/MS analyses revealed that the LC linkage were dominated by phenyl glycosidic bonds,while the FA-Ara-Xly structure was retained.The lignin content showed a pattern of S > H > G.The main lignin linkage was β-O-4′,and a less of β-β′ and β-5′structures were also tested.The dose of xylan endonuclease had a more significant effect on the composition and structure of LCC after enrichment than the treatment time.The enzyme dosage of 112 U/g had the highest lignin content of 17.34%.With the increase of enzyme dosage,the sensitivity of the NMR signal of LCC structure was enhanced,and the signals such as β-O-4′ and phenyl glycosidic bond were more clearly identified.The enzyme treatment time of 120 min can make low molecular weight lignin isolated from LCC and affect the integrity of LCC structure identification in hemicellulose.The enzyme dosage of 112 U/g ～ 140U/g and the enzyme treatment time of 60 ～ 90 min could meet the enrichment requirements of both lignin content and structural integrity when using the xylanase-macroporous resin method.After the hemicellulose samples oxidized by chlorine dioxide were enriched with xylanase-macroporous resin,the unreacted LCC structure information was identified.The residual lignin in the purified hemicellulose was predominantly H-type.The phenyl glycosidic bond was the primary LCC linkage.the β-O-4′ signal was the strongest in the lignin linkage.Most of the residual lignin in bamboo-alkali hemicellulose is presumed to be non-phenolic lignin.The oxidation of lignin in LCC by chlorine dioxide reaction mainly within 10 min.The reaction efficiency increased with increasing chlorine dioxide concentration.At the concentration of chlorine dioxide of 90.93 mg/L,the reaction requires about 0.7g chlorine dioxide per oxidation 1g LCC of lignin. |
