Syringyl units in lignin

S-Lign = syringyl unit in lignin, G-Lign = guaiacyl unit in lignin. 

Syringyl Lignin Pathways Two key enzymes (beyond the coniferyl alcohol pathway) are required for the production of sinapyl alcohol, which is the essential monolignol for production of syringyl units in lignins. Ferulate 5-hydroxylase, FH5, now perhaps more appropriately called CAld-5H to reflect coniferaldehyde as the preferred substrate [347,348], effects the 5-hydroxylation. Caffeate 0-methyltransferase,... 

Noguchi et al. (8) synthesized precursors of lignin biosynthesis (i.e., coniferyl and sinapyl alcohols) in which the side chain and methoxy groups were doubly labeled by and H, prepared the DHP (synthetic lignin) from them, and treated the DHP with the laccase. These results showed that about 25% of Hs of the a- and -positions were removed in a ratio of one to one, respectively, and that the methoxyl group of a syringyl unit was de-methylated easier than that of a guaiacyl. 

Figure 9. Molar yields, obtained after 20 minutes at pH 3.0 with 0.13 units mL lignin peroxidase in the presence of H2O2 and veratryl alcohol, of C -Cg, B-O-4 and aromatic ring cleavage products from 4.ethoxy-3-methoxyphenylgfycerol-B-syringyl ether moieties in dehydrogenative copolymer of conifery] alcohol and 4.ethoxy 3-methoryphet lgfycerol-B-syringaresinol (54).

The only very slight photochromic behaviour obtained upon irradiation of bleached aspen CTMP may be due to the presence of syringyl instead of guaiacyl units in hardwood lignin. These structures are less able to form orr/to-quinones during irradiation than softwood pulps (8). 

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