Nimlos Mark R, Qian Xianghong, Davis Mark, Himmel Michael E, Johnson David K
National Bioenergy Center, National Renewable Energy Laboratory, Golden, Colorado 80401, USA.
J Phys Chem A. 2006 Oct 26;110(42):11824-38. doi: 10.1021/jp0626770.
The decomposition of xylose has been studied using quantum mechanical calculations supported by NMR data. Proposed mechanisms for the decomposition of xylose have been investigated by obtaining the structures and energies of transition states and products. The intent of this study was to understand the experimentally observed formation of furfural and formic acid that occurs during the decomposition of xylose in mildly hot acidic solutions. A mechanism of furfural formation involving the opening of the pyranose ring and subsequent dehydration of the aldose was compared to a direct intramolecular rearrangement of the protonated pyranose. Energies were determined using CBS-QB3, and it was shown that the barriers for dehydration of the aldose were high compared to intramolecular rearrangement. This result suggests that the latter mechanism is a more likely mechanism for furfural formation. The intramolecular rearrangement step results from protonation of xylose at the O2 hydroxyl group. In addition, it has been shown that formic acid formation is a likely result of the protonation of xylose at the O3 hydroxyl group. Finally, solvation of xylose decomposition was studied by calculating energy barriers for xylose in selected water clusters. The mechanisms proposed here were supported in part by 13C-labeling studies using NMR.
利用核磁共振数据支持的量子力学计算研究了木糖的分解。通过获得过渡态和产物的结构与能量，对提出的木糖分解机制进行了研究。本研究的目的是了解在温和热酸性溶液中木糖分解过程中实验观察到的糠醛和甲酸的形成。将涉及吡喃糖环开环以及随后醛糖脱水的糠醛形成机制与质子化吡喃糖的直接分子内重排进行了比较。使用CBS-QB3确定了能量，结果表明与分子内重排相比，醛糖脱水的势垒较高。这一结果表明，后一种机制是糠醛形成更可能的机制。分子内重排步骤是木糖在O2羟基处质子化的结果。此外，研究表明，甲酸的形成可能是木糖在O3羟基处质子化的结果。最后，通过计算木糖在选定水簇中的能量势垒，研究了木糖分解的溶剂化作用。这里提出的机制部分得到了使用核磁共振的13C标记研究的支持。
