Biomass Fundamentals

Modules 12: Cellulose & Hemicelluloses

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

Cellulose

b-D-anhydroglucopyranose units linked by (1,4)-glycosidic bonds

O O

O OO

OO

OH

CH2OH

HOHO

HOOH

CH2OHHO

OH

CH2OH

CH2OH

OHOH

HO

3'

4'n

1

2

3

4

5

6

2'5'

6'

1'

(potential aldehyde)

Non-ReducingEnd-Group

ReducingEnd-Group

Cellulose

b-D-anhydroglucopyranose units linked by (1,4)-glycosidic bonds

O O

O OO

OO

OH

CH2OH

HOHO

HOOH

CH2OHHO

OH

CH2OH

CH2OH

OHOH

HO

3'

4'n

1

2

3

4

5

6

2'5'

6'

1'

(potential aldehyde)

Non-ReducingEnd-Group

ReducingEnd-Group

Why the Cellulose Story is so Convoluted?

Polymorphism or allotropy refers to the existence of more than one crystalline forms differing in physical and chemical properties.

Four major polymorphs of cellulose have been reported– Cellulose I, – Cellulose II, – Cellulose III, – Cellulose IV

glycerol

glycerol

260o

260o

NH3 (l)

NaOHNaOH

NH3 (l)

NaOH

Cellulose I Cellulose I

Cellulose II Cellulose IIIII Cellulose IVII

Cellulose IIII Cellulose IVI

NH3 (l)

Early Models of Cellulose Morphology

Fringed Micellar StructureMicellar Structure

Before Polymers After Polymers

Blocks of material mixed with amorphous material

Alignment of chains followed by folding; entropically not very favourable

More Recent Models of Cellulose Morphology

Microfibrils exist as discrete crystalline regions of >600Å, separated by less ordered amorphous domains. No chain folding

What is Happening During Acid Hydrolysis?

The non-ordered domains of the long Microfibrils are degraded leaving behind the discrete crystallites

Packing of the Cellulose Microfibrils

in Cell Wall

Tc

Tc

Tc

Tc

R

R

R

R

or

Tc

Tc

Tc

Tc

Tc

Tc

Tc

R

R

R

R

R

R

First Reported Unit-Cell for Cellulose

Meyer-Mark-Misch- Chains run anti-Parallel within the unit cell.

Meyer et al. (1929)

Gardner & Blackwell Chains run Parallel within the unit cell.

Gardner and Blackwell (1974)

b

a

a

c

The unit cell parameters were almost double that of the Meyer-Mark and Misch unit cell.

a = 16.34 Å, b = 15.72Å, c = 10.38Å (fiber axis), angle β = 97o.

In summary for Native Cellulose I• Blackwell Unit-cell• Parallel chain

orientation• Accepted today

• Meyer-Misch Unit-cell• Anti-parallel chain

orientation

How does one go about unequivocally confirming chain

orientation?• Nature has provided a way to do so.• The fact that cellulose has two distinct

ends a reducing end group (aldehyde) and a non-reducing end group allows for some creative chemistry to be done

Staining of the Chain Ends

RNR

Hieta et al., Biopolymers, 23, 1807,1984

Elegant Proof of Chain Orientation by selective staining of the reducing end-

groups

Kuga et al. (1984) and Chanzy

For parallel model one expects the black dots to preferentially be on one end of microfibrils.

Cellulose I Intra-molecular Hydrogen-Bonding

O

O

O12

3 45

1'2'

3'4'5'

6'

6

• The intra-molecular hydrogen-bonds are responsible for the stiff and rigid nature of the cellulose molecule.

• Due to the equatorial orientation of the hydroxyl groups and its linear structure, cellulose molecules have a strong tendency to form intra- and inter-molecular hydrogen-bonds.

Cellulose I Intra-molecular Hydrogen-Bonding

O

O

O12

3 45

1'2'

3'4'5'

6'

6

• Two kinds of such bonds form within the same chain:

• C3-OH with endocyclic oxygen

• C6 –OH (primary) with the C2-OH

Cellulose I Inter-molecular Hydrogen-Bonding

However, one kind of H-bond forms between neighboring chains

• C3 OH and C6 OH

O

O

O O

O

O

12

3 45

1'2'

3'4'5'

6'

6

12

3 45

1'2'

3'4'5'

6'

6

Cellulose nanofiber bundles

6 assembly proteins (rosette) which produce cellulose nanofibers

C. Haigler & L. Blanton

Cellulose : Nature Working Across a Length Scale >1010!

~28nm

1/4

Cellulose II, Another from of Cellulose

• Created from From cellulose I–Via Mercerization - 17-20% NaOH–And Regeneration - precipitated from

solution• Two-chain unit-cell

–Anti-parallel chain orientation• Of Lower crystallinity than Cellulose I

Mercerization

Cellulose I Cellulose II

Why the Transformation?

• Mercerization; Heterogeneous alkali swelling

• Several mechanisms proposed– Conformational change

• “Bent” cellulose I to “twisted & bent” cellulose II

– Recrystallization of cellulose II on cellulose I• “shish-kebab” structure

– Chain-folding– Progressive ‘shifting’ of sheets or chains

Why the Tansformation contd.• Since the C6 hydroxyl group is involved in

two secondary valence interactions, it is precluded from interacting with molecules in neighboring planes (above and below).

• Therefore cellulose has a sheet-like structure with only weak van der Waals forces holding the sheets together . These sheets fall apart during mercerization

• New orientations and H-bonds may now form

• Along different planes C2 OH may now H-bond

Cellulose II Hydrogen-bonding

Plane 1 Plane 2 Plane 3

New H bonds between the C2 OH groups in neighbouring planes can set in after mercerization.

Hemicelluloses

Structurally, hemicelluloses are co-polymers of two or more sugars and sugar acids

– glucose, mannose, galactose, xylose, arabinose and 4-O-methylglucuronic acid

They are of low DP 120 - 200 with short branching chains, making them amorphous heteropolysaccharides

These are HeteropolysaccharidesSupporting material in cell walls that vary from plant to plant and from one plant part to another.

In woody plants, there are two basic types

• D glucomannans• D glucuronoxylans

The composition and amount of each is species dependent

• Softwood vs Hardwood

Hemicelluloses

Softwood Hemicelluloses (major)The principle hemicellulose of softwoods is the galactoglucomannans (~ 20% of woody material)

O

OHHO

OHOH

OO

OOH

HOOH

OOOH

O

OHOO

OHHO

O

OOH

HOOH

O OO

HOOCCH3

O

OHHO

OHOH

O

CH3CO

O-D-mannopyranose

-D-glucopyranose

-D-galactopyranose

(1-4)

-D-galactopyranose

(1-6)

(1-6)acetyl group

Alternating Glucose & Mannose along the main chain; Galactose branches off; Random acetates at C5 & C3 of main chain

Note β-1-4 links along the main chain

Note: Galactose C4 OH is axial

Mannose C2 OH is axial

Softwood Hemicelluloses (major)The principle hemicellulose of softwoods is the galactoglucomannans (~ 20% of woody material)

They are subdivided as :

High Galactose content: Galactose 1/Glucose 1/Mannose/4

Low Galactose content

Galactose 0.1/Glucose 1/Mannose/3

Softwood Hemicelluloses (minor)The minor hemicellulose of softwoods is the Arabinoglucuronoxylans (~5-10% of wood)

-L-arabinofuranose

(1-3)

(1-4)

4-O-methyl--D-glucuronopyranose

-D-xylopyranose (1-2)

OO

HOO

OHOHO

O

OOO

OHO

HOO

OHO

HOO

OH

OOH

CO2HOCH3

OH

OHO

HOCH2OH

O

Xylanose linked along the main chain; arabinose and glucuronic acid branches off

Note β-1-4 links along the main chain

Note: Xylan has no C6 primary OH

Note: Arabinose is a Furan; 5 member ring sugar structure with a C4 primary OH

Harwood Hemicelluloses (major)The principle hemicellulose of hardwoods is the glucuronoxylans (15-30% of woody material)

(1-4) -D-xylopyranose

(1-2)

4-O-methyl--D-glucuronopyranose

7

acetyl group

O

OO

OHO

OH

OOOOHO O O O

HOOH

O

O

OHOH

COOHOCH3

O

OH

HOO

CCH3OCCH3O

Note β-1-4 links along the main chain

Hardwood Hemicelluloses (minor)The minor hemicellulose of hardwoods is the glucomannans (2-5% of wood)

-D-glucopyranose-D-mannopyranose

(1-4)O

OO

OH

HOOH

OOOH

HO

OHOO

OHHO

OH

OOH

HO OHO O

OH

HOO

OH

Note β-1-4 links along the main chain