Bharadwaj Vivek S, Crowley Michael F, Peña Maria J, Urbanowicz Breeanna, O'Neill Malcolm
Renewable Resources and Enabling Sciences Center, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States.
Complex Carbohydrate Research Center, University of Georgia, 315 Riverbend Road, Athens, Georgia 30602, United States.
J Phys Chem B. 2020 Nov 12;124(45):10117-10125. doi: 10.1021/acs.jpcb.0c06920. Epub 2020 Oct 28.
Rhamnogalacturonan II (RG-II)-the most complex polysaccharide known in nature-exists as a borate cross-linked dimer in the plant primary cell wall. Boric acid facilitates the formation of this cross-link on the apiosyl residues of RG-II's side chain A. Here, we detail the reaction mechanism for the cross-linking process with ab initio calculations coupled with transition state theory. We determine the formation of the first ester linkage to be the rate-limiting step of the mechanism. Our findings demonstrate that the regio- and stereospecific nature of subsequent steps in the reaction itinerary presents four distinct energetically plausible reaction pathways. This has significant implications for the overall structure of the cross-linked RG-II dimer assembly. Our transition state and reaction path analyses reveal key geometric insights that corroborate previous experimental hypotheses on borate ester formation reactions.
鼠李半乳糖醛酸聚糖II（RG-II）——自然界中已知的最复杂的多糖——在植物初生细胞壁中以硼酸交联二聚体的形式存在。硼酸促进了RG-II侧链A的芹糖基残基上这种交联的形成。在此，我们通过从头算计算结合过渡态理论详细阐述了交联过程的反应机制。我们确定第一个酯键的形成是该机制的限速步骤。我们的研究结果表明，反应历程中后续步骤的区域和立体特异性性质呈现出四种在能量上合理的不同反应途径。这对交联的RG-II二聚体组装体的整体结构具有重要意义。我们的过渡态和反应路径分析揭示了关键的几何见解，证实了先前关于硼酸酯形成反应的实验假设。
