Impact of Steam Explosion on the Wheat Straw Lignin Structure Studied by Solution-State Nuclear Magnetic Resonance and Density Functional Methods

• Published in: Journal of agricultural and food chemistry Vol. 62; no. 43; pp. 10437 - 10444
• Main Authors: Heikkinen, Harri, Elder, Thomas, Maaheimo, Hannu, Rovio, Stella, Rahikainen, Jenni, Kruus, Kristiina, Tamminen, Tarja
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 29.10.2014; American Chemical Society, Books and Journals Division
• Subjects: acidolysis; dissociation; Hot Temperature; lignin; Lignin - chemistry; Magnetic Resonance Spectroscopy; Molecular Structure; nuclear magnetic resonance spectroscopy; Plant Stems - chemistry; raw materials; Steam; thermodynamics; Triticum - chemistry; wheat straw; wheat straw; lignin; tricin; 2D NMR; steam explosion; 31P NMR
• ISSN: 0021-8561; 1520-5118
• DOI: 10.1021/jf504622j

Chemical changes of lignin induced by the steam explosion (SE) process were elucidated. Wheat straw was studied as the raw material, and lignins were isolated by the enzymatic mild acidolysis lignin (EMAL) procedure before and after the SE treatment for analyses mainly by two-dimensional (2D) [heteronuclear single-quantum coherence (HSQC) and heteronuclear multiple-bond correlation (HMBC)] and 31P nuclear magnetic resonance (NMR). The β-O-4 structures were found to be homolytically cleaved, followed by recoupling to β-5 linkages. The homolytic cleavage/recoupling reactions were also studied by computational methods, which verified their thermodynamic feasibility. The presence of the tricin bound to wheat straw lignin was confirmed, and it was shown to participate in lignin reactions during the SE treatment. The preferred homolytic β-O-4 cleavage reaction was calculated to follow bond dissociation energies: G–O–G (guaiacyl) (69.7 kcal/mol) > G–O–S (syringyl) (68.4 kcal/mol) > G–O–T (tricin) (67.0 kcal/mol).