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. These two hydrogen bonds are absent in the surface chains, and we are evaluating NMR and FTIR evidence which suggests that the other intramolecular hydrogen bond, from O-3 to the ring oxygen, is destabilised.
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. It is an anomaly that cellulose Ia and Ib, with more intramolecular hydrogen bonding, cannot form as many hydrogen bonds from chain to chain as the surface form of cellulose, yet the chains within a crystalline unit are held together with spectacular tenacity.
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 2-3 nm across, containing very approximately 15 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. We have suggested that one of the cellulose synthase complexes ('rosettes') at the cell membrane may synthesis six crystalline units, which can either coalesce as in cotton or flax, or remain loosely associated into a microfibril 8-10 nm in diameter, or separate into single 2-3 nm strands.
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