Cellulose is the ultimate raw
material. There is more cellulose in the biosphere than any other substance.
Its primary structure is simple: a long chain of glucose units attached
together by b(1,4) linkages. It is the ability
of these chains to hydrogen-bond together into fibres (microfibrils)
that gives cellulose its unique properties of mechanical strength and chemical
stability.
Cellulose microfibrils
contain two crystalline forms, cellulose Ia and Ib, in which the chains are packed
slightly differently. The chain conformation in both forms is similar, a flat
ribbon with a 180o twist between successive
glucosyl residues. This chain conformation is
stabilised by two hydrogen bonds parallel to the glycosidic
linkage, one from O-3 to the ring oxygen of the preceding glucose unit and the
other from O-2 to O-6 of the next glucose unit. We have suggested that the Ia
and Ib forms
can be interconverted by bending the microfibril, so that hydrogen-bonded sheets of chains slide
over one another.
Cellulose Ia or Ib forms the core
of each crystalline unit, in the microfibrils from
higher plants, but at the surface there are chains that do not conform to
either of these crystalline allomorphs. We have shown by solid-state 13C
NMR that these surface chains do not have the same conformation at C-6 as the
core chains. In the core chains the C-6 conformation is trans-gauche. In
the surface chains there is a mixture of the gauche-gauche and gauche-trans
conformations with gauche-trans predominant when the cellulose is dry,
but less so when it is wet. The trans-gauche conformation at C-6 in the
core chains allows O-6 to hydrogen bond to the chain alongside, and also lets
it accept an intramolecular hydrogen bond from O-2 of
the preceding residue of the same chain. This hydrogen bond is absent in the
surface chains.
These differences in hydrogen
bonding mean that the surface chains have some freedom to move out of the
flat-ribbon conformation. The lack of intramolecular
hydrogen bonding in the surface chains also means that they can form more
hydrogen bonds to water or adjacent polysaccharides.
From the ratio of surface to core
chains it is possible to estimate the size of the crystalline units. In
cellulose from higher plants they are about 3 nm across, containing approximately
18-24 chains: an estimate that is in agreement with NMR spin-diffusion
experiments. Cotton and flax are an exception, with microfibrils
about 6x4 nm containing approximately 80 chains..
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