Lignin structure

The word lignin is derived from the Latin word lignum meaning wood. It is a main component of vascular plants. Indeed, lignin is second only to polysaccharides in natural abundance, contributing 24–33% and 19–28%, respectively, to dry wood weights of normal softwoods and temperate-zone hardwoods [8]. Although the lignin exact structure found in plants is uncertain, we know that lignin is a three dimensional amorphous polymer consisting of methoxylated phenylpropane structures. The three monolignol monomers that form lignin structure are: p-coumaryl, coniferyl alcohol, sinapyl alcohol.

Fig.  Lignin structure and  three building blocks of lignin

These lignols are incorporated into lignin in the form of the phenylpropanoids p-hydroxyphenyl (H), guaicacyl (G) and syringyl (S) respectively. Gymnosperms have a lignin that consists almost entirely of G with small quantities of H. That of dicotyledonous angiosperms is more often than not mixture of G and S (with very little H), and monocotyledonous lignin is a mixture of all three. Many grasses have mostly G, while some palms have mainly S. All lignins contain small amounts of incomplete or modified monolignols, and other monomers are prominent in non-woody plants.

References:

J. Zakzeski, P.C.A. Bruijnincx et al., Chem. Rev. 110 (2010) 3552

W. Boerjan et al., Annu. Rev. Plant Biol.54 (2003) 519