Lignin linkages

As seen, lignin structure and composition differs from plant to plant, means that lignin obtained from one source may need to be handled differently than lignin from another.

Plant lignins can be broadly divided into three classes: softwood (gymnosperm),  hardwood (angiosperm) and grass or annual plant (graminaceous) lignin.

Lignin abundance generally decreasing in the order of softwood, hardwoods and grasses

Lignins have in commons lots of different linkages. The linkages of lignin, individually depicted in Figure 1, include b-O-4, 5-5, b-5, 4-O-5, b-1, dibenzodioxocin, spiridienone, a-O-4 and b-b linkages, of which the b-O-4 linkages is dominant, consisting of more than half of the linkage structure of lignin. The relative abundance of the various linkages in softwoods (i.e. spruce) and hardwoods (i.e. birch), are given in Table 1

Fig. 1 Bonds in Lignin

Table 1. Linkages found in softwood and hardwood lignin 

References:

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

J. Zakzeski, P.C.A. Bruijnincx et al., Chem. Rev. 110 (2010) 355

2P. T. Patil et al., Energy Fuels, 25 (2011) 4713

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