title: Chemical and structural changes associated with Cu-catalyzed alkaline-oxidative delignification of hybrid poplar.
abstract: BACKGROUND
  Alkaline hydrogen peroxide pretreatment catalyzed by Cu(II) 2,2'-bipyridine complexes has previously been determined to substantially improve the enzymatic hydrolysis of woody plants including hybrid poplar as a consequence of moderate delignification. In the present work, cell wall morphological and lignin structural changes were characterized for this pretreatment approach to gain insights into pretreatment outcomes and, specifically, to identify the extent and nature of lignin modification.
  
  
  RESULTS
  Through TEM imaging, this catalytic oxidation process was shown to disrupt cell wall layers in hybrid poplar. Cu-containing nanoparticles, primarily in the Cu(I) oxidation state, co-localized with the disrupted regions, providing indirect evidence of catalytic activity whereby soluble Cu(II) complexes are reduced and precipitated during pretreatment. The concentration of alkali-soluble polymeric and oligomeric lignin was substantially higher for the Cu-catalyzed oxidative pretreatment. This alkali-soluble lignin content increased with time during the catalytic oxidation process, although the molecular weight distributions were unaltered. Yields of aromatic monomers (including phenolic acids and aldehydes) were found to be less than 0.2 % (wt/wt) on lignin. Oxidation of the benzylic alcohol in the lignin side-chain was evident in NMR spectra of the solubilized lignin, whereas minimal changes were observed for the pretreatment-insoluble lignin.
  
  
  CONCLUSIONS
  These results provide indirect evidence for catalytic activity within the cell wall. The low yields of lignin-derived aromatic monomers, together with the detailed characterization of the pretreatment-soluble and pretreatment-insoluble lignins, indicate that the majority of both lignin pools remained relatively unmodified. As such, the lignins resulting from this process retain features closely resembling native lignins and may, therefore, be amenable to subsequent valorization.
authors: Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI USA ; DOE-Great Lakes Bioenergy Research Center, Michigan State University, East Lansing, MI USA ; College of Agricultural Sciences, Oregon State University, Corvallis, OR USA.
authors: Zhenglun
authors: Z
authors: Li
authors: DOE-Great Lakes Bioenergy Research Center, Michigan State University, East Lansing, MI USA ; Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI USA.
authors: Namita
authors: N
authors: Bansal
authors: DOE-Great Lakes Bioenergy Research Center, University of Wisconsin, Madison, WI USA.
authors: Ali
authors: A
authors: Azarpira
authors: DOE-Great Lakes Bioenergy Research Center, Michigan State University, East Lansing, MI USA ; Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI USA.
authors: Aditya
authors: A
authors: Bhalla
authors: Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI USA ; Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, MD USA.
authors: Charles H
authors: CH
authors: Chen
authors: DOE-Great Lakes Bioenergy Research Center, University of Wisconsin, Madison, WI USA ; Department of Biochemistry, University of Wisconsin, Madison, WI USA.
authors: John
authors: J
authors: Ralph
authors: DOE-Great Lakes Bioenergy Research Center, Michigan State University, East Lansing, MI USA ; Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI USA.
authors: Eric L
authors: EL
authors: Hegg
authors: Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI USA ; DOE-Great Lakes Bioenergy Research Center, Michigan State University, East Lansing, MI USA ; Department of Biosystems and Agricultural Engineering, Michigan State University, East Lansing, WI USA ; Division of Sustainable Process Engineering, Luleå University of Technology, Luleå, Sweden.
authors: David B
authors: DB
authors: Hodge
keywords: Alkaline hydrogen peroxide (AHP) pretreatment; Catalytic oxidation; Electron microscopy; Lignin; NMR spectroscopy; Plant cell walls; Pretreatment