title: Effect of lignin content on changes occurring in poplar cellulose ultrastructure during dilute acid pretreatment.
abstract: BACKGROUND
  Obtaining a better understanding of the complex mechanisms occurring during lignocellulosic deconstruction is critical to the continued growth of renewable biofuel production. A key step in bioethanol production is thermochemical pretreatment to reduce plant cell wall recalcitrance for downstream processes. Previous studies of dilute acid pretreatment (DAP) have shown significant changes in cellulose ultrastructure that occur during pretreatment, but there is still a substantial knowledge gap with respect to the influence of lignin on these cellulose ultrastructural changes. This study was designed to assess how the presence of lignin influences DAP-induced changes in cellulose ultrastructure, which might ultimately have large implications with respect to enzymatic deconstruction efforts.
  
  
  RESULTS
  Native, untreated hybrid poplar (Populus trichocarpa x Populus deltoids) samples and a partially delignified poplar sample (facilitated by acidic sodium chlorite pulping) were separately pretreated with dilute sulfuric acid (0.10 M) at 160°C for 15 minutes and 35 minutes, respectively . Following extensive characterization, the partially delignified biomass displayed more significant changes in cellulose ultrastructure following DAP than the native untreated biomass. With respect to the native untreated poplar, delignified poplar after DAP (in which approximately 40% lignin removal occurred) experienced: increased cellulose accessibility indicated by increased Simons' stain (orange dye) adsorption from 21.8 to 72.5 mg/g, decreased cellulose weight-average degree of polymerization (DPw) from 3087 to 294 units, and increased cellulose crystallite size from 2.9 to 4.2 nm. These changes following DAP ultimately increased enzymatic sugar yield from 10 to 80%.
  
  
  CONCLUSIONS
  Overall, the results indicate a strong influence of lignin content on cellulose ultrastructural changes occurring during DAP. With the reduction of lignin content during DAP, the enlargement of cellulose microfibril dimensions and crystallite size becomes more apparent. Further, this enlargement of cellulose microfibril dimensions is attributed to specific processes, including the co-crystallization of crystalline cellulose driven by irreversible inter-chain hydrogen bonding (similar to hornification) and/or cellulose annealing that converts amorphous cellulose to paracrystalline and crystalline cellulose. Essentially, lignin acts as a barrier to prevent cellulose crystallinity increase and cellulose fibril coalescence during DAP.
authors.affiliation: School of Chemistry and Biochemistry, Renewable Bioproducts Institute, Georgia Institute of Technology, 500 10th Street, N.W. Atlanta, GA 30332-0620 USA.
authors.forename: Qining
authors.initials: Q
authors.lastname: Sun
authors.affiliation: Department of Energy, Environmental and Chemical Engineering, Washington University, 1 Brookings Drive, Saint Louis, MO 63130 USA.
authors.forename: Marcus
authors.initials: M
authors.lastname: Foston
authors.affiliation: School of Chemistry and Biochemistry, Renewable Bioproducts Institute, Georgia Institute of Technology, 500 10th Street, N.W. Atlanta, GA 30332-0620 USA.
authors.forename: Xianzhi
authors.initials: X
authors.lastname: Meng
authors.affiliation: Center for Structural Molecular Biology and the Biology and Soft Matter Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.
authors.forename: Daisuke
authors.initials: D
authors.lastname: Sawada
authors.affiliation: Center for Structural Molecular Biology and the Biology and Soft Matter Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.
authors.forename: Sai Venkatesh
authors.initials: SV
authors.lastname: Pingali
authors.affiliation: Center for Structural Molecular Biology and the Biology and Soft Matter Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.
authors.forename: Hugh M
authors.initials: HM
authors.lastname: O'Neill
authors.affiliation: Center for Environmental Research and Technology (CE-CERT), Bourns College of Engineering, University of California, 1084 Columbia Avenue, Riverside, CA 92507 USA.
authors.forename: Hongjia
authors.initials: H
authors.lastname: Li
authors.affiliation: Center for Environmental Research and Technology (CE-CERT), Bourns College of Engineering, University of California, 1084 Columbia Avenue, Riverside, CA 92507 USA ; Department of Chemical and Environmental Engineering, Bourns College of Engineering, University of California, 900 University Avenue, Riverside, CA 92521 USA ; BioEnergy Science Center (BESC), Oak Ridge National Laboratory (ORNL), Oak Ridge, TN 37831 USA.
authors.forename: Charles E
authors.initials: CE
authors.lastname: Wyman
authors.affiliation: Center for Structural Molecular Biology and the Biology and Soft Matter Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.
authors.forename: Paul
authors.initials: P
authors.lastname: Langan
authors.affiliation: School of Chemistry and Biochemistry, Renewable Bioproducts Institute, Georgia Institute of Technology, 500 10th Street, N.W. Atlanta, GA 30332-0620 USA ; BioEnergy Science Center (BESC), Oak Ridge National Laboratory (ORNL), Oak Ridge, TN 37831 USA ; Department of Chemical and Biomolecular Engineering, Department of Forestry, Wildlife, and Fisheries, Center for Renewable Carbon, University of Tennessee, Knoxville, TN 37996-2200 USA.
authors.forename: Art J
authors.initials: AJ
authors.lastname: Ragauskas
authors.affiliation: Center for Environmental Research and Technology (CE-CERT), Bourns College of Engineering, University of California, 1084 Columbia Avenue, Riverside, CA 92507 USA ; BioEnergy Science Center (BESC), Oak Ridge National Laboratory (ORNL), Oak Ridge, TN 37831 USA.
authors.forename: Rajeev
authors.initials: R
authors.lastname: Kumar
keywords: Biomass recalcitrance; Cellulose ultrastructure; Delignification; Dilute acid pretreatment; Enzymatic sugar release; Lignin content