Fundamental Science and Applications for Biomaterials 45
Given the various physical characteristics stated here for heteropolysaccharides, it is
no surprise that they tend to be more susceptible to degradation from both chemical
and enzymatic attack. In addition, they can sometimes be more amenable to chemical
derivatization and reaction than their cellulose analogue.
2.2.3 Lignin
This last polymer, albeit neglected for many years because of its molecular hetero-
geneity and seeming lack of tractability, is one of the most important polymers on
the planet. It forms the last element of the lignocellulosics that primarily comprise
the biomass on this planet. The term “lignin” is in fact taken from the Latin word
lignum, which is translated to “wood.” It is found virtually in all biomass especially
vascularized plants including herbs and grasses, but it is chiefly located (on a per capita
basis)inthecellwallofwoodytreespecies[8,9].Onamassbasis,uptoapproximately
30% of all carbon in the biosphere can be attributed to lignin. A representation of its
diverse native structure is provided in Figure 2.6 [10]. The structure seemingly lacks the
signature monomer associated with polymer structures, but in wood chemistry circles,
it is assumed that the monomer is based on a C 9 phenylpropanoid residue that will be
further elaborated within this section.
The structure elucidated here is speculative at best in nature and represents a best
guess attempt because not only is its X-ray structural characterization impossible,
but it is extremely divergent in its structure between species and even within the
same species. In terms of its role among the natural polymers in woody and plant
species, lignin maintains unique and powerful functionality within the bio-system.
It is essentially a natural “glue” that helps to keep the polysaccharide bundle intact
and whole. The current theory for its particular niche in lignocellulosics is that it
covalently complexes with the heteropolysaccharides in a so-called LCC. In addition to
its chemical attachment to polysaccharides, it is believed that lignin also acts as a plant’s
second line of defense (after the bark/extractives) against microbial attack/infestation.
Lignin as shown in Figure 2.6 is a complex aromatic network polymer (but it is
three-dimensional) that is polydisperse and contains a number of divergent function-
alities and branching points. Although PDI may not be objectively applicable in the
case of lignin, lignin nevertheless is a branched network polymer that has its origins
in well-characterized lignol subunits. Figure 2.7 shows the principal monomeric units
that constitute the biosynthesized lignin.
Each of the structures shown in Figure 2.7 is available in differing amounts in different
classes of woods or plants. For example, monocotyledonous grasses tend to have almost
exclusively thep-hydroxyphenyl monomer unit, whereas gymnosperms are almost
exclusively constructed out of the guaiacyl units. Angiosperms, on the other hand, tend
to have some combination of syringyl (principal component) and guaiacyl units.
The lignin extant in nature generally has a support role for the polysaccharide
matrix. In woody tissue, it tends to surround the wood cell helping to ensure not
only integrity but to maintain proper surface energetics (hydrophobicity) to allow
for the uninterrupted circulation of water and nutrients. Figure 2.8 demonstrates the
respective localization of lignin within a representative wood cell.
The cell wall is the region that contains the most lignin polymer, but it is highly
concentrated between cells, in essence, to ensure good mechanical integrity among
