4.3.2.1 Polysaccharide-polysaccharide cross-linking.

4.3.2.1.1 DIMERIZATION OF ESTERIFIED HCA

In grasses, hydroxycinnamate esters on GAX (see Section 4.2.1.2.2) in primary and lignified secondary walls are covalently cross-linked by oxidative dimerization of HCA units on neighboring AX chains by radical coupling reactions (32, 79) (Figure 4.3a). The homo — and hetero-dehydrodimers formed involve mostly FA and sometimes SA (80); pCA does not appear in these dehydrodimers. Dehydrotrimers (32, 81) and a dehydrotetramer (82) of FA have also been described and could participate in cross-linking. A number of isomeric cross-linking homo — and hetero-dimers have been encountered involving both the aromatic ring and the propenoic acid side chain of the HCA (Figure 4.4) (83, 84).

4.3.2.1.2 ESTERIFIED HCA CYCLODIMERIZATION

In addition to dimerization of ester-linked HCA by oxidative coupling, cross-linking dimers are formed from FA and/or pCA monomers by photodimerization (83). These ester-linked cyclodimers are cyclobutane derivatives (truxillic acids and truxinic acids) (Figure 4.3b). Both the esterified HCA dimers and cyclodimers are readily released by treatment with dilute alkali (0.5 M NaOH, 20°C, 18 h) (85).

4.3.2.1.3 DIRECT ASSOCIATIONS BETWEEN POLYSACCHARIDES

Direct covalent linkages between pectic polysaccharides and xyloglucans in angiosperm walls have been reported (86-88), but the detailed chemistry of the linkage(s) is not known.

Figure 4.3 Three modes of covalent cross-linking involving feruloylated GAXs: (a) a dehydrodiferulate cross-link(5-5), (b)aferulatecyclodimer (truxillicacid) cross-link, and (c)tyrosyl-ferulatecrosslinkbetween a protein and a feruloylated GAX. Ph = 4-Hydroxy-3-methoxy-benzene.