title: Impact of steam explosion on the wheat straw lignin structure studied by solution-state nuclear magnetic resonance and density functional methods.
abstract: 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 (31)P 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).
authors.affiliation: VTT Technical Research Centre of Finland , Biologinkuja 7, Espoo, FI-02044 VTT, Finland.
authors.forename: Harri
authors.initials: H
authors.lastname: Heikkinen
authors.forename: Thomas
authors.initials: T
authors.lastname: Elder
authors.forename: Hannu
authors.initials: H
authors.lastname: Maaheimo
authors.forename: Stella
authors.initials: S
authors.lastname: Rovio
authors.forename: Jenni
authors.initials: J
authors.lastname: Rahikainen
authors.forename: Kristiina
authors.initials: K
authors.lastname: Kruus
authors.forename: Tarja
authors.initials: T
authors.lastname: Tamminen
mesh_terms: D006358:Hot Temperature; D008031:Lignin / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D018547:Plant Stems / Q000737:chemistry*; D013227:Steam; D014908:Triticum / Q000737:chemistry*
chemical_list: D013227:Steam; D008031:Lignin
keywords: 2D NMR; 31P NMR; lignin; steam explosion; tricin; wheat straw