42 Introduction to Renewable Biomaterials
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(a) Cellulose fibers
(b) Macrofibril
(c) Microfibril
(d) Chains of
cellulose
molecules
Scheme 2.1A simplified pictorial summary of the contextual development of cellulose fibers: the
evolution of cellulose chains into microfibrils, an elementary unit in cellulose, which contribute to the
higher order elementary structures (e.g., the macrofibril is composed of a bundle of microfibrils).
Reprinted with permission from http://alevelnotes.com/Carbohydrate-polymers/65.
three different sythases are involved that are encoded by CesA^2 genes) that are
responsible for synthesizing the individual cellulose chains. The RTCs are able to spin
a bundle of cellulose chains known as a microfibril into the cell wall. Scheme 2.1 shows
a simplified representation of the production of microfibrils leading to cellulose fibers.
Native cellulose whose production is demonstrated in Scheme 2.1 is known from
crystallographic data to have two distinct crystalline phases: Iαand Iβ[3]. Recently,
exact representations of the principal phases of cellulose (Iαand Iβ) were obtained using
atomic-resolution synchrotron and neutron diffraction data. The resulting structure of
Iαwas a one-chain triclinic unit cell (shown in Figure 2.3 with the principal axes: a, b,
and c). The conformation of the glucosidic linkages and hydroxymethyl groups is iden-
tical, which is not the case for Iβ.InIβ, there are two conformationally distinct chains
in the monoclinic unit cell (corner and center chains) that requires adjacent glucosyl
residues (in the same chain) to be the same. Both triclinic and monoclinic crystal systems
derive from the seven crystal systems that are available to describe highly ordered
systems.
The triclinic system can be best described as a crystal whose vector descriptors possess
unequal length AND are not mutually orthogonal perpendicular to each other as in a
classic three-dimensional Cartesian coordinate system. Similarly, a monoclinic system
2 CesAgenes are likely responsible for encoding the catalytic subunit of cellulose synthase.
