Civil Engineering Reference
In-Depth Information
called disaccharide cellobiose. h ese cellobiose units form long chains which are inter-
linked by hydrogen bonds to form 3-dimensional periodic structures. h e intra- and
inter-molecular hydrogen bonds are shown as blue and red lines in Figure 17.1b. Lateral
bonding between the molecular chains leads to the formation of crystalline microi brils
of size ~ 3.5 to 10 nm width. h e crystal structure of these microi brils was found to be
similar to cotton, a plant cellulose belonging to cellulose-I, an allomorph of cellulose
having a monoclinic crystal structure with cellobiose units oriented along the c-axis of
the unit cell [4-6]. Even though the monoclinic structure of cellulose-I was determined
by X-ray dif raction, alternate molecular arrangement of cellobiose units within the
unit cell were reported. High-resolution solid-state
13
C NMR, combined with infrared
and electron dif raction studies coni rmed that cellulose-I is indeed a composite mix-
ture of triclinic cellulose-I
α
and monoclinic cellulose-I
β
( i gure 17.1c) forms in varying
proportion [7,8]. Bacterial cellulose, however, was found to consist of predominantly
cellulose-I
α
form, a structurally pure form compared to plant cellulose [9].
17.1.1
Synthesis Mechanism of Bacterial Cellulose
Cellulose is synthesized by a wide variety of aerobic bacterial genera such as aceto-
bacter, acrobacter, rizobium and so on. However, the reasons for the synthesis of cel-
lulose by bacteria, independent of the genera, are not clearly known and a plausible
mechanism has been understood. It has been hypothesized that bacteria produces cel-
lulose in order to maintain their spatial position at the interface of air-culture medium
so that it can take oxygen from the air and nutrients from the culture medium for
OH
OH
OH
OH
OH
HO
HO
OH
HO
6
3
6
O
3
O
2
5
5
O
O
2
5
4
1
O
O
4
O
1
4
O
O
O
4
1
O
2
O
1
HO
3
5
2
HO
6
OH
OH
3
HO
OH
OH
OH
6
OH
N
OH
OH
HO
OH
HO
OH
O
O
O
O
O
AL
O
O
AL
HO
HO
OH
OH
(
a
)
N
OH
OH
(
b
)
c
a
c
b
a
a & b
Figure 17.1
(a) Cellulobiose unit formed by two glucopyranose molecules covalaently connected
between C1 and C4 by acetal function with a 180° rotation of adjacent glucopyranose molecule; Here the
word 'AL' is used to indicate the orientation of each glucopyranose molecule with respect to adjacent
one; (b) repeating units of cellulosbiose to form cellulose chain with intermolecular (red solid lines)
and intramolecular (blue solid lines) hydrogen bonds; (c) arrangement of cellulobiose units within
monoclinic (black line) and triclinic (blue line) unit cells.
(
c
)
