Structural characterization and antioxidant activity of water-soluble lignin-carbohydrate complexes (LCCs) isolated from wheat straw

Abstract

Lignin-carbohydrate complex (LCC) was proven to possess antioxidant activities. To understand structures and antioxidant activities of LCC, water-soluble LCC-stalk, LCC-sheath and LCC-leaf were isolated from ball-milled wheat straw stalk, sheath and leaf by successive dissolution in LiCl/DMSO solvent, water extraction and purification. LCCs were later structurally characterized by wet chemistry, chromatography and spectroscopy respectively. Their antioxidant activities were evaluated by ferric reducing antioxidant power (FRAP) and DPPH radical scavenging assays. The results showed that three LCCs were carbohydrate-rich (≈70%) and presented relatively narrow molecular weight distribution (PI < 2.0). The lignin moieties of LCCs were mainly connected with β-O-4′ structures, and phenyl glycoside and γ-ester linkages were main LCC linkages in LCCs. However, guaiacyl units were the predominant lignin units in LCC-stalk and LCC-sheath, while syringyl units were predominant in LCC-leaf. Intermolecular cross-linkages were mainly pCA-bridges in LCC-stalk and FA-bridges in LCC-sheath and LCC-leaf. Besides, LCC-sheath featured higher polysaccharide content exhibited higher molecular weight, fewer LCC linkages and better antioxidant activities (DPPH radical scavenging rate up to 74.91%) than both LCC-stalk (74.55%) and LCC-leaf (64.52%). This work helped to know LCCs in wheat straw well and inspire the application of LCC as potential antioxidants.