Recent studies demonstrate that several polyphenolic compounds produced from beyond the canonical monolignol biosynthetic pathways can behave as lignin monomers, participating in radical coupling reactions and being incorporated into lignin polymers. Here, we show various classes of flavonoids, the chalconoid naringenin chalcone, the flavanones naringenin and dihydrotricin, and the flavone tricin, incorporated into the lignin polymer of papyrus (Cyperus papyrus L.) rind. These flavonoids were released from the rind lignin by Derivatization Followed by Reductive Cleavage (DFRC), a chemical degradative method that cleaves the β-ether linkages, indicating that at least a fraction of each was integrated into the lignin as β-ether-linked structures. Due to the particular structure of tricin and dihydrotricin, whose C-3ʹ and C-5ʹ positions at their B-rings are occupied by methoxy groups, these compounds can only be incorporated into the lignin through 4ʹ–O–β bonds. However, naringenin chalcone and naringenin have no substituents at these positions and can therefore form additional carbon–carbon linkages, including 3ʹ– or 5ʹ–β linkages that form phenylcoumaran structures not susceptible to cleavage by DFRC. Furthermore, Nuclear Magnetic Resonance analysis indicated that naringenin chalcone can also form additional linkages through its conjugated double bond. The discovery expands the range of flavonoids incorporated into natural lignins, further broadens the traditional definition of lignin, and enhances the premise that any phenolic compound present at the cell wall during lignification could be oxidized and potentially integrated into the lignin structure, depending only on its chemical compatibility. This study indicates that papyrus lignin has a unique structure, as it is the only lignin known to date that integrates such a diversity of phenolic compounds from different classes of flavonoids. This discovery will open up new ways to engineer and design lignins with specific properties and for enhanced value.

中文翻译：

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黄酮类化合物柚皮素查尔酮、柚皮素、二氢甘油三酯和甘油三酯是纸莎草中的木质素单体

最近的研究表明，从经典的单木质素生物合成途径之外产生的几种多酚化合物可以充当木质素单体，参与自由基偶联反应并掺入木质素聚合物中。在这里，我们展示了各种黄酮类化合物，即 chalcone 柚皮素查耳酮、黄烷酮柚皮素和二氢蓖麻毒素以及黄酮 tricin，它们被掺入到纸莎草 (Cyperus papyrus L.) 外皮的木质素聚合物中。这些黄酮类化合物通过衍生化后还原裂解 (DFRC) 从外皮木质素中释放出来，DFRC 是一种裂解 β-醚键的化学降解方法，表明至少有一部分以 β-醚键结构整合到木质素中. 由于tricin和dihydrotricin的特殊结构，其B环的C-3′和C-5′位被甲氧基占据，这些化合物只能通过4′-O-β键结合到木质素中。然而，柚皮素查尔酮和柚皮素在这些位置没有取代基，因此可以形成额外的碳-碳键，包括形成不易被 DFRC 裂解的苯基香豆素结构的 3'- 或 5'-β 键。此外，核磁共振分析表明柚皮素查尔酮还可以通过其共轭双键形成额外的键。这一发现扩大了天然木质素中黄酮类化合物的范围，进一步拓宽了木质素的传统定义，并增强了木质化过程中存在于细胞壁上的任何酚类化合物都可能被氧化并可能整合到木质素结构中的前提，这仅取决于其化学相容性。这项研究表明，纸莎草木质素具有独特的结构，因为它是迄今为止已知的唯一一种整合了来自不同类黄酮类化合物的多种酚类化合物的木质素。这一发现将为设计和设计具有特定特性并提高价值的木质素开辟新的途径。