Syringyltype

Syringyltype refers to a form of lignin in which syringyl units are the predominant monomeric components. These syringyl units originate from sinapyl alcohol during lignin biosynthesis. In contrast, guaiacyltype lignin is rich in guaiacyl units derived from coniferyl alcohol, and p-hydroxyphenyl (H) lignin contains p-coumaryl units. Syringyl-rich lignins are especially common in many hardwoods and certain non-woody angiosperms, while softwoods tend to be more guaiacyl-rich. Some grasses exhibit mixed compositions with notable S and G contents and varying H units.

The relative amount of syringyl and guaiacyl units is summarized by the S/G ratio. A higher S/G ratio indicates a syringyltype lignin, whereas a lower ratio points toward a guaiacyltype. This ratio is determined using analytical techniques such as thioacidolysis, pyrolysis-GC/MS, or two-dimensional NMR (HSQC), which quantify the abundance of S and G units in a lignin sample.

Biosynthetically, the production of syringyl units is influenced by the enzyme ferulate 5-hydroxylase (F5H), which converts coniferyl derivatives toward sinapyl derivatives. Regulation of F5H and related phenylpropanoid pathway steps affects the S content and the overall lignin architecture.

Significance: syringyl-type lignin tends to form fewer condensed carbon–carbon linkages and more β-O-4 linkages, which can facilitate chemical pulping and enzymatic bleaching and influence biomass recalcitrance. Understanding S/G composition is important in forestry, pulping, and biofuel research.