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1.4.5. Cellulose
I.
The naturally occurring (and most abundant) form of crystalline cellulose is
called cellulose I. Key features include:
-
Fully stretched chains (1.03 nm pr. cellobiose unit)
-
Each glucose residue is rotated close to 180
°
relative to the neighbour
-
The chains are parallel
Cellulose chains thus have cellobiose as (physical) repeating unit.
The extended shape is stabilized by intramolecular hydrogen bonds (in the
chain direction):
-
O3-H
….
O5' (from H-atom of OH-group at C3 to the ring oxygen)
-
O6
….
H-O2' (from O-atom of OH-group at C6 to H-atom of OH-group at C2)
In cellulose I the chains as organized as
layers (sheet-like), with stabilizing hydrogen
bonds between the chains within the layers
as shown in the figure (left, ‘top view’)
1.4.6. Cellulose
II
If cellulose s swollen or dissolved and then
precipitated again another and
thermodynamically more stable crystal form –
Cellulose II – is formed. It differs from
cellulose I by having:
O
CH
2
OH
OH
HO
O
O
O
CH
2
OH
OH
HO
O
O
CH
2
OH
OH
HO
O
O
O
CH
2
OH
OH
HO
O
1
2
3
4
5
6
1
2
4
5
6