Fundamental Science and Applications for Biomaterials 41
Figure 2.2A simplified representation
of the stereochemical asymmetry
present in cellulose: the existence of a
non-reducing end group (NREG)
versus an opposite reducing end
group (REG) give cellulose different
terminal chemistries.
Sometimes shown as
Cellulose
CH 2 OH
CH 2 OH
Non-reducing end Reducing end
n – 2
n – 2
CH 2 OH
HO HO HO
O
O
O
O
O
HO
OH
OH
OH
OH
CH 2 OH CH 2 OH CH^2 OH
OH OH
OH
OH OH
HO
OH OH
O
6
5
4 1
2
3
O O
whereas it can be highly crystalline inValonia[2]. Nevertheless, cellulose tends to be
rather nonreactive to chemicals other than cellulose enzymes. It is a linear polymer
that is made up ofβ-1,4-linked d-glucopyranose units.
One of the glucose units within the chain can be described as having a rotation of
180 ∘relative to its neighbor within the chain. The chemical description of a “dimer” or
two units is cellobiose unit, which technically could be used to describe the polymeric
structural pattern. Classically, a cellulose chain, in as much as its glucose monomer
makeup, presents two different terminal hydroxyl groups: the non-reducing end group
(NREG) and the reducing end group (REG). The NREG consists of a 4-hydroxyl group
(at the 4 carbon), whereas the REG is categorized in organic chemistry as a hemiacetal
linkage (or aldehyde hydrate group). Figure 2.2 shows a classical representation of the
NREG and REG.
Each glucose unit presents three reactive hydroxyl groups (C 2 ,C 3 ,andC 6 ), which
expedite the formation of both intra- and interchain hydrogen bonding. The stiffness and
compactness of the chain can almost exclusively be attributed to interchain interactions.
However, the structural uniformity of cellulose in any plant is lacking; typically, there
exist both highly ordered (crystalline) domains and amorphous (low degree of order)
domains, which as a rule depend on the raw material and the treatments it has been
subjected to. The relative robustness of cellulose is critically dependent on these features.
Cellulose generally provides the main mechanical properties of any lignocellulosics
owing to its packing and hydrogen-bonded structure; in general, it provides the overall
load-bearing capacity of wood and plants but does not typically complex with the other
biopolymers in the lignocellulosic matrix.
Cellulose is synthesized in vascular plants^1 in the plasma membrane of the rosette
terminal complexes (RTCs), a class of macromolecular protein structures approximately
25 nm in diameter. These structures contain the cellulose synthase enzymes (at least
1 Vascular plants (also known as tracheophytes or higher order plants) possess the so-called lignified
tissues – a distinguishing characteristic from avascular plants – that are primarily responsible for circulating
water, nutrients, and photosynthetic by-products within the plant.
