School of Chemistry and Biochemistry, Renewable Bioproducts Institute, Georgia Institute of Technology, 500 10th Street, N.W. Atlanta, GA 30332-0620 USA.
Department of Energy, Environmental and Chemical Engineering, Washington University, 1 Brookings Drive, Saint Louis, MO 63130 USA.
Biotechnol Biofuels. 2014 Oct 14;7(1):150. doi: 10.1186/s13068-014-0150-6. eCollection 2014.
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.
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%.
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.
深入了解木质纤维素解构过程中发生的复杂机制对于可再生生物燃料生产的持续增长至关重要。生物乙醇生产的关键步骤是热化学预处理，以降低植物细胞壁的抗降解性，便于下游加工。先前的稀酸预处理（DAP）研究表明，预处理过程中纤维素超微结构发生了显著变化，但对于木质素对这些纤维素超微结构变化的影响，仍存在很大的知识空白。本研究旨在评估木质素的存在如何影响 DAP 诱导的纤维素超微结构变化，这可能最终对酶促解构工作产生重大影响。
分别对天然、未经处理的杂交杨（Populus trichocarpa x Populus deltoids）样品和部分脱木质素的杨木样品（通过酸性亚氯酸钠蒸煮实现）用 0.10 M 稀硫酸在 160°C 下分别预处理 15 分钟和 35 分钟。经过广泛的表征，部分脱木质素的生物质在 DAP 后显示出比天然未经处理的生物质更显著的纤维素超微结构变化。与天然未经处理的杨木相比，DAP 后脱木质素的杨木（其中约 40%的木质素去除）经历了：纤维素可及性增加，表明 Simons 染色（橙色染料）吸附从 21.8 增加到 72.5 mg/g；纤维素重均聚合度（DPw）从 3087 降低到 294 个单位；纤维素微晶尺寸从 2.9 增加到 4.2 nm。这些 DAP 后的变化最终使酶解糖产率从 10%增加到 80%。
总体而言，结果表明木质素含量对 DAP 过程中发生的纤维素超微结构变化有很强的影响。随着 DAP 过程中木质素含量的降低，纤维素微纤维尺寸和微晶尺寸的增大变得更加明显。此外，纤维素微纤维尺寸的增大归因于特定的过程，包括由不可逆链间氢键驱动的结晶纤维素的共结晶（类似于角质化）和/或将无定形纤维素转化为准晶和结晶纤维素的纤维素退火。本质上，木质素起到了阻止纤维素结晶度增加和 DAP 过程中纤维素纤维聚结的屏障作用。
