Li Zhenglun, Bansal Namita, Azarpira Ali, Bhalla Aditya, Chen Charles H, Ralph John, Hegg Eric L, Hodge David B
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.
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.
Biotechnol Biofuels. 2015 Aug 20;8:123. doi: 10.1186/s13068-015-0300-5. eCollection 2015.
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.
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.
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.
先前已确定由Cu(II) 2,2'-联吡啶配合物催化的碱性过氧化氢预处理可通过适度脱木质素显著改善包括杂交杨树在内的木本植物的酶水解。在本研究中，对这种预处理方法的细胞壁形态和木质素结构变化进行了表征，以深入了解预处理结果，特别是确定木质素修饰的程度和性质。
通过透射电子显微镜成像显示，这种催化氧化过程破坏了杂交杨树的细胞壁层。主要以Cu(I)氧化态存在的含铜纳米颗粒与破坏区域共定位，提供了催化活性的间接证据，即可溶性Cu(II)配合物在预处理过程中被还原并沉淀。对于铜催化的氧化预处理，碱溶性聚合物和低聚木质素的浓度显著更高。在催化氧化过程中，这种碱溶性木质素含量随时间增加，尽管分子量分布未改变。发现木质素上芳香族单体（包括酚酸和醛）的产率低于0.2%（重量/重量）。在溶解木质素的核磁共振光谱中，木质素侧链中的苄醇氧化明显，而预处理不溶性木质素的变化最小。
这些结果为细胞壁内的催化活性提供了间接证据。木质素衍生的芳香族单体的低产率，以及预处理可溶性和预处理不溶性木质素的详细表征表明，这两种木质素库中的大多数仍然相对未被修饰。因此，由此过程产生的木质素保留了与天然木质素非常相似的特征，因此可能适合后续的增值利用。
