Biomass Fundamentals

Module 13: Lignin

A capstone course for

BioSUCCEED:

Bioproducts Sustainability: a University Cooperative Center of Excellence in EDucation

The USDA Higher Education Challenge Grants program gratefully acknowledged for support

This course would not be possible without support from:

USDA

Higher Education Challenge (HEC) Grants Program

www.csrees.usda.gov/funding/rfas/hep_challenge.html

Lignin• Anselme Payen 1838: Reacts wood with nitric acid and

then sodium hydroxide. Remaining material he names Cellulose.

• Schulze 1857: Names dissolved material Lignin from the latin lignum meaning wood.

• Phenolic Polymer - The Glue that Holds the Fibers Together

• 3 Dimensional Crolinked Network• Branched Polymer/Polydisperse (Large and Small

Polymers)

Lignin Biosynthesis I

CO2 GlucoseShikimicPathway

OHO

OH

OH

HO

OHO

NH2

OHO

NH2

OH

L-Tyrosine L-Phenylalanine Shikimic Acid

Photosynthetic Pathways

The Metabolic Pathways for the Biosynthesis of Wood Components

H2O + CO2 Sugar MetabolismPool

PolysaccharidesCellulose &Hemicelluloses

PhosphoenolPyruvate (PEP)

ShikimicAcid

Acetyl CoAC6-C1

AromaticC6-C3

Aromatic

PEP

Terpenoids Fatty Acids Phenolics Tannins

Lignin

Extractives - Minor Components

Major Component

hν

Shikimic Acid Pathway

• The Primary Metabolic Pathway for Lignin

• Shikimic Acid Pathway• Occurs in all plants • Leads to Lignin –Tannins and

Extractives• The whole pathway is enzymatically

controlled

Tyrosine

Phenylalanine

D-glucose

PEP

3-dehydroquinic acid 3-dehydroshikimic acid

Shikimic acid

COOHH2O3POC

CH2

O

HOCOOH

HO

OH

H2COPO3H2

COCH2

HOCHHCOH

HCOH

COOH

+

H2COPO3H2

CHOHCOH

HCOH

COOHCOPO3H2

CH2

H2COH

CHOHCOH

HOCHHCOH

HCOH

O

HO COOH

HO

HO

HO COOH

HO

NADPH

NADP+

HO

OH

OH

HO O

P-O

OH

OH

HO O

ATPADP

PEP

P-O

OH

O

HO O

CO2H

H

Phosphochorismic acidOH

O

HO O

CO2H

Chorismic acidOH

O

HO

O CO2H

Prephenic acid

O

CO2H

OH

O

CO2H

OH

NH2

CO2H

NH2

CO2H

D-erythrose-4- P3-deoxy -D-arabinose-heptulosonic acid 7- P

The 6 Carbon sugar is initially spit into 2 units

Condensation Cyclization

Reduction of Carbonyl to OH

Activation Using ATP-

Install P at C5

Another PEP is used to extend the chain, P falls off as H3PO4

Aromaticity is finally installed followed by Transamination (NH2)

Lignin Biosynthesis II

• The Cinnamate Pathway is a secondary metabolic pathway

• Specific to Woody Plants

• Creates the Lignin Precursors …the Lignin monomers

• Cinnamyl alcohols

Lignin Biosynthesis II

OH

OH

OH

OH

OCH3

OH

OH

CH3O OCH3

-Coumaryl Alcohol Coniferyl Alcohol Sinapyl Alcohol

L-Tyrosine

OH

OHO

NH2

L-Phenylalanine

OHO

NH2Cinnamate Pathway

CH2 CH2 CH2

Dehydrogenative Polymerization of Lignin precursors

• Monolignols are NOT abundant in their free forms– exist as 4-O-b-D-glucosides

• Monolignol-glucosides

• Increase solubility - Facilitate transport

• Decrease toxicity

OCH3

OH

CH2OH

UDPHOHO

OH

OOH

HOHO

OH

OOH

O

CH2OH

OCH3+

coniferyl alcoholglucosyl transferase

ConiferinConiferyl alcohol

UDP-glucose

Lignin Nomenclature

OH

R

-Hydroxyphenyl

OH

R

OCH3

Guaiacyl Syringyl

OH

R

OCH3CH3O

Lignin Nomenclature

OH

OCH3

C

C

C

Methoxyl Group

Phenolic Hydroxyl

1

2

3

45

6

Phenylpropane UnitC9 } Common Names

Side Chain

Polymerization-Initiation Enzymatic Formation of Phenoxy Radical

CH

HC

OCH3

O H

CH2OH

H+

e-

-

-CH

HC

OCH3

O

CH2OH

.Enzyme

Peroxidases-H2O2 systems cause proton abstraction from a given cinnamyl alcohol

Creating a Phenoxy Radical

Lignin Polymerization Process

1 Dehydrogenation - formation of phenoxy radicals

2 Coupling - formation of oligolignols and quinone methide

3 Addition of water, lignols, carbohydrates and acids - formation of -hydroxyl group, -O-4 linkages and lignin-carbohydrate complex

Lignin Polymerization

• Repetitive dehydrogenation of lignols, coupling and addition to quinone methide

• Lignol is a collective name for precursors and intermediate products of lignin during lignification. (Similar to saccharides)– Monolignol - precursors– Dilignol, triligonol, tetralignol, etc.– Oligolignol