Lignin and the cell wall
Trees are the largest organisms on earth
Plants first had to adapt over millions of years
Wing and Chapple 2010, New Phytologist 187:273
Surviving pests and fire- lignin
Lignin protects wood’s sugars
Lignin has good thermal
stability*
http://commons.wikimedia.org/wiki/File:F
orest_fire_aftermath.jpg&usg
Typical composition of different plant types
J. J. Bozell, et al. U. S. DOE, Report PNNL16983, Washington, DC 2008.
In reaction wood lignin content is changed
As highlighted by Rowell et al. Cell Wall Chemistry
2005
Lignin content within the cell wall of plants
How does lignin get into the wall?
• Lignification occurs after polysaccharide scaffolding is in place.
Terashima et a. 2009 J.
Wood Sci 55:409
Monomers of lignin
Courtesy W. Glasser
Chiang, 2002 Nature
Biotech 20:557
Polymerization pathways: Free Radical Coupling
M= molecules that form covalent bonds, while
propagating reactive species (*)
I*
Initiating
reactive
species
MM M M
M MM
1. initiate
I*+ M
2. propagate
I-M* + M
3. terminate
I-M-[M]n-M
I-M*
I-M-M*
Monomers
With lignin the radical is
coupled between monomers
and then the initiator must
create a new radical. Step 1
and then step 3.
Enzyme initiated polymerization
Source ?, Courtesy W. Glasser
Monomer ratios are different base on plant
type
J. J. Bozell, et al. U. S. DOE, Report PNNL16983, Washington, DC 2008.
HW and SW have different monomers and
this results in guaiacyl (G) and syringyl (S)
lignins of different G:S ratios
•G:S ratio impacts properties
•G:S ratio changes within cell wall based on location
Whetten et al. 1998 Ann
Rev Plant Phys Plant Mol
Bio 49:585
Linkages in lignin
J. J. Bozell, et al. U. S. DOE, Report PNNL16983, Washington, DC 2008.
Proportional linkages
Saake 2007Ullmann’s
Encyclopedia of Industrial
Chemistry, Wiley-VCH
Movement of monomers to reactive sites
within wall
Lignification with different materials present
Compound middle lamella
Secondary wall
Globular modules in cell corners
Zhuo Li, 2009
Smaller modules within secondary cell wall
LCC FORMATION
Source ?, Courtesy W. Glasser
Lignin is usually cited as random network
• Lignin is optically inactive
• Lignin has a variety of linkages that could facilitate a cross-linked
network
• However, there is experimental evidence and developing theories to
the contrary- that lignin is oriented in the cell wall and branched, but
not necessarily a highly cross-linked network.
A model of a branched lignin
J. Ralph et al. 2004 Phytochemistry Reviews 3: 29-60, 2004
How big are lignin chains of isolated lignin?
• Polymer properties are always influenced by their molecular weight
(MW) and molecular weight distribution (MWD)
• Isolating lignin disrupts native lignin bonds
• Technical lignins are isolated through kraft pulping and sulfite pulping
• These technologies impact size of chains
• Break linkages and form new linkages
• Newer conversion methods for biofuels are being developed (organosolv pulping
and steam-explosion processing)
• “Gentle” isolation can be used to limit the number of broken bonds in
lignin for laboratory isolation
• Obtain 20 to 50% of the lignin using a “milled-wood” acidolysis process
Lignin MW studies
A wide
range of
sizes for the
compounds
from 3 to 4
units
bonded
together to
30 to 40
units
bonded
together.
Holtman et al. 2007 Journal of Wood
Chem Tech 27:179
Summary
• Lignin is a polymer made out of phenylpropane units
• The polymer is built by a process of radical polymerization outside of the
cytoplasm
• More than one monomer type influences lignin structure for different
plants
• There is some spatial patterning of lignin linkages
• Lignin can bond with carbohydrate creating lignin carbohydrate complexes
• Attempts to examine lignin’s structure disrupts lignin’s structure
• Progress has been made to view in situ lignin with newer analytical techniques
• HSQC 2-D NMR