Pump 'gas' from biomass: the journey from dream to reality (4)

Chemical Components: Lignin

Lignin is one of the major chemical components in plant cell wall. It is an amorphous, cross-linked, 3-dimensional branched polymer (Fig.1) with C9- phenylpropane unit (Fig.2.). The biosynthesis of lignin stems from the polymerization of three types of phenylpropane units as monolignols: coniferyl, sinapyl,and p-coumaryl alcohols (Fig.3). Softwood lignin is composed mainly of coniferyl alcohol units, while hardwood lignin is composed mainly of coniferyl and sinapyl alcohol units.

Fig.1. Lignin macromoleculle

Fig. 2. C9 phenylpropane unit

Fig. 3. Three building blocks of lignin


The three building blocks are connected through C-O-C and C-C linkages to form the 3-D structure, which has the strength to support plants. The exact structure of protolignin is unknown. But the improvements in methods for identifying lignin-degradation products and advancements in spectroscopic methods have enabled scientists to elucidate the predominant structural features of lignin. Fig. 4 depicts some of the common link ages found in soft wood lignin.

Fig. 4. Common linkages in softwood

Th e typical abundance of these types of linkages in softwoods are shown in Fig.5

Fig. 5. Proportions of different types of linkages connecting the phenylpropane units in softwood lignin.

In addition, lignin blocks are also linked with some sugar unit to form Lignin-Carbohydrate Complex (LCC) (Fig. 6). So, lignin also functions like glue to stick different cell structures together.

Fig. 6. Typical LCCs

Because of these structural and chemical characteristics, lignin is the most recalcitrant component of the plant cell wall. In general, the higher the proportion of lignin, the lower the bioavailability of the substrate. Th effect of virgin lignin, redeposited lignin after pretreatment, and LCC on the bioavailability of other cell-wall components is thought to play a large role in the physical restriction mechanism. This require some pretreatments to partially delignify to remove the barrier and open cell wall channels so that hydrolysis chemicals or enzymes can penetrate inside the cell wall more easilly and fast, accordingly speed up the transport and overall reaction rate.