Lignin: Functional Biomaterial with Potential in Surface Chemistry and Nanoscience - The Nanoscience and Technology of Renewable Biomaterials - page 175

HO HO

HO

MeO

OMe OMe

OH OH OH

Figure7.1 Thecommonligninmonomersp-coumarylalcohol,coniferylalcoholandsinapyl
alcohol.

Table7.1 Proportionsofmonolignolsinplants.

p-Coumaryl alcohol Coniferyl alcohol Sinapyl alcohol

Softwood < 5 > 95 Trace amounts

Hardwood 0-8 25-50 46-75

Grasses 5-33 33-80 20-54

lignin, found in coniferous trees, contains predominantly repeat units of coniferyl alco-
hol with very little trace of sinapyl alcohol whereas hardwood lignin, present in the
angiosperms or broad leaf trees, contains both of these monolignols in significant pro-
portions (however often the ratio of sinapyl alcohol to coniferyl alcohol may be as high
as 3:1). Neither hardwood nor softwood lignin contains high levels of p -coumaryl alco-
hol which is found in grass lignin along with both the coniferyl and sinapyl alcohol
lignin monomers.
Now that the basic units of lignin have been defined, the way in which these monomers
are inter-connected should be discussed. Analysis of isolated lignin samples over the
past decades has led to many advances in knowledge of the prevalence of the various
covalent linkages present in the three-dimensionally branched lignin macromolecule.
However, it is very important to note that it is still beyond the realms of possibility to
directly study naturally occurring lignin in its unaltered form although some extraction
methods are more benign than others. All investigations of lignin isolated from native
sources result in some form of chemical modification of the three dimensional network
through cleavage of bonds from other lignin monomers or from the covalent attachments
to polysaccharides. In this chapter, we will limit the discussion to what is known about
types of bonding interactions and only mention in passing the biosynthetic pathways for
the native production of the lignin macromolecules as there is continuing conjecture as
to whether there is full biochemical control over the monolignol polymerisation or if the
coupling reactions proceed in a random fashion (Boerjan et al . 2003, Vanholme et al .
2008, Lewis 1999). Such a discussion is beyond the scope of this chapter focussing on the
surface properties and possibilities for use in nanotechnology from sustainable sources
so the interested reader is referred to other reviews which discuss the relative merits of
the 'random' and 'directed' synthetic pathways (Vanholme et al . 2008, Lewis 1999).
It is commonly accepted that lignin is synthesised through the enzymatic dehydro-
genation of the monolignols, transported to the cell wall, to radical species followed