Kim Hoon, Ralph John, Lu Fachuang, Ralph Sally A, Boudet Alain M, MacKay John J, Sederoff Ronald R, Ito Takashi, Kawai Shingo, Ohashi Hideo, Higuchi Takayoshi
US Dairy Forage Research Center, USDA Agricultural Research Service, Department of Forestry, University of Wisconsin, Madison, WI 53706-1598, USA.
Org Biomol Chem. 2003 Jan 21;1(2):268-81. doi: 10.1039/b209686b.
Peroxidase/H2O2-mediated radical coupling of 4-hydroxycinnamaldehydes produces 8-O-4-, 8-5-, and 8-8-coupled dehydrodimers as has been documented earlier, as well as the 5-5-coupled dehydrodimer. The 8-5-dehydrodimer is however produced kinetically in its cyclic phenylcoumaran form at neutral pH. Synthetic polymers produced from mixtures of hydroxycinnamaldehydes and normal monolignols provide the next level of complexity. Spectral data from dimers, oligomers, and synthetic polymers have allowed a more substantive assignment of aldehyde components in lignins isolated from a CAD-deficient pine mutant and an antisense-CAD-downregulated transgenic tobacco. CAD-deficient pine lignin shows enhanced levels of the typical benzaldehyde and cinnamaldehyde end-groups, along with evidence for two types of 8-O-4-coupled coniferaldehyde units. The CAD-downregulated tobacco also has higher levels of hydroxycinnamaldehyde and hydroxybenzaldehyde (mainly syringaldehyde) incorporation, but the analogous two types of 8-O-4-coupled products are the dominant features. 8-8-Coupled units are also clearly evident. There is clear evidence for coupling of hydroxycinnamaldehydes to each other and then incorporation into the lignin, as well as for the incorporation of hydroxycinnamaldehyde monomers into the growing lignin polymer. Coniferaldehyde and sinapaldehyde (as well as vanillin and syringaldehyde) co-polymerize with the traditional monolignols into lignins and do so at enhanced levels when CAD-deficiency has an impact on the normal monolignol production. The implication is that, particularly in angiosperms, the aldehydes behave like the traditional monolignols and should probably be regarded as authentic lignin monomers in normal and CAD-deficient plants.
如先前文献所记载，过氧化物酶/H₂O₂介导的4-羟基肉桂醛自由基偶联反应会生成8-O-4、8-5和8-8偶联的脱氢二聚体，以及5-5偶联的脱氢二聚体。然而，8-5脱氢二聚体是在中性pH条件下以其环状苯基香豆素形式动力学生成的。由羟基肉桂醛和普通单木质醇混合物制成的合成聚合物带来了更高层次的复杂性。二聚体、寡聚物和合成聚合物的光谱数据使得从CAD缺陷型松树突变体和反义CAD下调转基因烟草中分离出的木质素中醛类成分的分配更加充实。CAD缺陷型松树木质素显示出典型苯甲醛和肉桂醛端基水平的提高，同时有证据表明存在两种类型的8-O-4偶联松柏醛单元。CAD下调的烟草中羟基肉桂醛和羟基苯甲醛（主要是丁香醛）的掺入水平也更高，但类似的两种8-O-4偶联产物是主要特征。8-8偶联单元也清晰可见。有明确证据表明羟基肉桂醛相互偶联然后掺入木质素，以及羟基肉桂醛单体掺入不断增长的木质素聚合物中。松柏醛和芥子醛（以及香草醛和丁香醛）与传统单木质醇共聚形成木质素，当CAD缺陷影响正常单木质醇生成时，它们的共聚水平会提高。这意味着，特别是在被子植物中，醛类的行为类似于传统单木质醇，在正常和CAD缺陷型植物中可能应被视为真正的木质素单体。
