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.
过氧化物酶/
H2O2 介导的
4-羟基肉桂醛自由基偶联会产生 8-O-4-、8-5-和 8-8 偶联脱氢二聚体(如前所述)以及 5-5 偶联脱氢二聚体。不过,8-5-脱氢二聚体是在中性 pH 值下以环状苯基
香豆素形式产生的。由羟基
肉桂醛和正常单
木质素混合物产生的合成聚合物提供了更高的复杂性。通过二聚体、低聚物和合成聚合物的光谱数据,我们可以对从 CAD 缺失的松树突变体和反义-CAD 下调的转
基因烟草中分离出来的
木质素中的醛组分进行更实质性的分配。CAD 缺失的松树
木质素显示出典型的
苯甲醛和
肉桂醛末端基团含量增加,同时还显示出两种 8-O-4 偶联的
松柏醛单元。CAD 下调的烟草也有较高
水平的羟基
肉桂醛和羟基
苯甲醛(主要是
丁香醛)掺入,但类似的两种 8-O-4 偶联产物是主要特征。8-8 偶联单元也很明显。有明显证据表明羟基
肉桂醛相互偶联,然后掺入
木质素中,以及羟基
肉桂醛单体掺入生长中的
木质素聚合物中。
松柏醛和
山奈醛(以及
香兰素和
丁香醛)与传统的单
木质素共同聚合成
木质素,当 CAD 缺乏影响到正常的单
木质素生产时,它们的聚合
水平会提高。这意味着,特别是在被子植物中,
醛类的行为与传统的单
木质素相似,在正常和 CAD 缺乏的植物中,
醛类可能应被视为真正的
木质素单体。