Lignin
Lignin (sometimes "
lignen") is a
chemical compound that is most commonly derived from
wood and is an integral part of the
cell walls of
plants, especially in
tracheids,
xylem fibres and
sclereids. It is the second most abundant
organic compound on earth after
cellulose. Lignin makes up about one-quarter to one-third of the dry mass of
wood.
Lignin fills the spaces in the
cell wall between
cellulose,
hemicellulose and
pectin components. It confers mechanical strength to the
cell wall and therefore the entire plant. It is particularly abundant in
compression wood, but curiously scarce in
tension wood.
Lignin plays a crucial part in conducting water in
plant stems. The
polysaccharide components of plant
cell walls are highly
hydrophilic and thus permeable to water. Lignin makes it possible to form vessels which conduct water efficiently.
Lignin is difficult to degrade and is therefore an efficient physical barrier against
pathogens which would invade plant tissues. For example an infection by a
fungus causes the plant to deposit more lignin near the infection site.
Highly lignified
wood is durable and therefore a good raw material for many applications. It is also an excellent fuel, since lignin yields more energy when burned than
cellulose. However, lignin is detrimental to
paper manufacture and must be removed from
pulp before paper can be manufactured. This is costly both in terms of energy and environment.
In the
sulfite and
sulfate (also called kraft)
chemical pulping processes, lignin is removed from wood pulp as
sulphates. These materials have several uses:
*
Dispersants in high performance
cement applications,
water treatment formulations and textile
dyes
* Additives in specialty
oil field applications and agricultural chemicals
* Raw materials for several chemicals, such as
vanillin,
DMSO,
ethanol,
torula yeast,
xylitol sugar and
humic acid* Environmentally friendly dust supression agent for roads
The first investigations into commercial use of lignin were done by
Marathon Corporation in
Rothschild,
Wisconsin (
USA), starting in
1927. The first class of products which showed promise were
leather tanning agents. The lignin chemical business of Marathon is now known as LignoTech USA, Inc., and is owned by the
Norwegian company,
Borregaard.
|
Structure of a small piece of lignin polymer |
|
Structures of the three commonly occurring monolignols |
Lignin is a large
macromolecule with
molecular mass in excess of 10,000
amu. It is
hydrophobic and
aromatic in nature. The
molecule consists of various types of substructures which repeat in random manner.
Lignin
biosynthesis begins with the synthesis of
monolignols. The starting material is the
amino acid phenylalanine. The first reactions in the
biosynthesis are shared with the
phenylpropanoid pathway, and monolignols are considered to be a part of this group of compounds. There are three types of monolignols:
coniferyl alcohol,
sinapyl alcohol and
paracoumaryl alcohol. Different
plants use different monolignols. For example,
Norway spruce lignin is almost entirely
coniferyl alcohol while paracoumaryl alcohol is found almost exclusively in
grasses.
Monolignols are synthetised in the
cytosol as
glucosides. The
glucose is added to the monolignol to make them water soluble and to reduce their toxicity. The
glucosides are transported through the
cell membrane to the
apoplast. The
glucose is then removed and the monolignols are polymerised into lignin.
The
polymerisation step is
catalysed by
oxidative enzymes. Both
peroxidase and
laccase enzymes are present in the
plant cell walls, and it is not known whether one or both of these groups participates in the
polymerisation. The oxidative enzyme
catalyses the formation of monolignol
radicals. These radicals then undergo chemical coupling to form the lignin
polymer. The details of this final step are being debated, since it is not known how the abundance of various possible
bond types between monolignols in controlled. Some theories favour pure chemical coupling, while other state that
dirigent proteins control this step.
Pyrolysis of lignin, eg. during
combustion, yields a range of products, of which the most characteristic ones are
methoxy phenols. Of those, the most important are
guaiacol and
syringol and their derivates; their presence can be used to trace a
smoke source to a wood fire. In
cooking, lignin in the form of
hardwood is an important source of these two chemicals which impart the characteristic aroma and taste to
smoked foods.
*
Biosynthesis pathway of lignin*
The Lignin Institute
