Structural unit

Molecular structure
Cellulose is an insoluble molecule consisting of between 2000 - 14000 residues with some preparations being somewhat shorter. It forms crystals (cellulose Iα) where intra-molecular (O3-HO5' and O6H-O2') and intra-strand (O6-HO3') hydrogen bonds holds the network flat allowing the more hydrophobic ribbon faces to stack. Each residue is oriented 180° to the next with the chain synthesized two residues at a time. Although individual strand of cellulose are intrinsically no less hydrophilic, or no more hydrophobic, than some other soluble polysaccharides (such as amylose) this tendency to form crystals utilizing extensive intra- and intermolecular hydrogen bonding makes it completely insoluble in normal aqueous solutions (although it is soluble in more exotic solvents such as aqueous N-methylmorpholine-N-oxide (NMNO, , ~0.8 mol water/mol, then up to 30% by wt cellulose at 100°C [1060]), CdO/ethylenediamine (cadoxen), LiCl/N,N'-dimethylacetamide or near-supercritical water [1070]). It is thought that water molecules catalyze the formation of the natural cellulose crystals by helping to align the chains through hydrogen-bonded bridging.

Part of a cellulose preparation is amorphous between these crystalline sections. The overall structure is of aggregated particles with extensive pores capable of holding relatively large amounts of water by capillarity.

The natural crystal is made up from metastable Cellulose I with all the cellulose strands parallel and no inter-sheet hydrogen bonding. This cellulose I (that is, natural cellulose) contains two coexisting phases cellulose Iα (triclinic) and cellulose Iβ (monoclinic) in varying proportions dependent on its origin; Iα being found more in algae and bacteria whilst Iβ is the major form in higher plants.

Cellulose Iα and cellulose Iβ have the same fibre repeat distance (1.043 nm for the repeat dimer interior to the crystal, 1.029 nm on the surface [721]) but differing displacements of the sheets relative to one another. The neighboring sheets of cellulose Iα (consisting of identical chains with two alternating glucose conformers) are regularly displaced from each other in the same direction whereas sheets of cellulose Iβ (consisting of two conformationally distinct alternating sheets, (as shown right where the 2-OH and 6-OH groups both change orientations so altering the hydrogen bonding pattern) each made up of crystallographically identical glucose conformers) are staggered [559]. It has been found that cellulose (Iβ) significantly alters the water structuring at its surface out to about 10 ?, which may affect its enzymatic digestion [905].

Cellulose Iα and cellulose Iβ are interconverted by bending during microfibril formation [418] and metastable cellulose Iα converts to cellulose Iβ on annealing.

If it can be recrystalized (for example, from base or CS2) cellulose I gives the thermodynamically more stable Cellulose II structure with an antiparallel arrangement of the strands and some inter-sheet hydrogen-bonding. Cellulose II contains two different types of anhydroglucose (A and B) with different backbone structures; the chains consisting of -A-A- or -B-B- repeat units [627]. Cellulose III is formed from cellulose mercerized in ammonia and is similar cellulose II but with the chains parallel, as in cellulose Iα and celluloseIβ [753]. For a review of cellulose structure, see [288] or the Centre de recherches sur les macromolécules végétales web site.

Functionality
Cellulose has many uses as an anticake agent, emulsifier, stabilizer, dispersing agent, thickener, and gelling agent but these are generally subsidiary to its most important use of holding on to water. Water cannot penetrate crystalline cellulose but dry amorphous cellulose absorbs water becoming soft and flexible. Some of this water is non-freezing but most is simply trapped. Less water is bound by direct hydrogen bonding if the cellulose has high crystallinity but some fibrous cellulose products can hold on to considerable water in pores and its typically straw-like cavities; water holding ability correlating well with the amorphous (surface area effect) and void fraction (that is, the porosity). As such water is supercoolable, this effect may protect against ice damage. Cellulose can give improved volume and texture particularly as a fat replacer in sauces and dressings but its insolubility means that all products will be cloudy.

Swelled bacterial cellulose (ex. Acetobacter xylinum), in its never-dried state with much smaller fibrils (~1%) than from plants, exhibits pseudoplastic viscosity like xanthan gels but this viscosity is not lost at high temperatures and low shear rates as the cellulose can retain its structure. Where individual cellulose strands are surrounded by water they are flexible and do not present contiguous hydrophobic surfaces. Bacterial cells may be removed by hot alkali and the clean wet cellulose used as a substrate for immobilizing biomolecules [843] or for covering wounds [844]. On drying the properties of bacterial cellulose irreversibly lose their hydrated properties and tend to those of plant cellulose.

About a third of the world's production of purified cellulose is used as the base material for a number of water-soluble derivatives with pre-designed and wide-ranging properties dependent on groups involved and the degree of derivatization (for an extensive review see [287]). Derivatizing cellulose interferes with the orderly crystal-forming hydrogen bonding, described above, so that even hydrophobic derivatives may increase the apparent solubility in water. Methyl cellulose (E461) [231] (made by methylating about 30% of the hydroxyl groups) is thermogelling, forming gels above a critical temperature due to hydrophobic interactions between high-substituted regions and consequentially stabilized intermolecular hydrogen bonding. Such gels break down on cooling In a manner similar to that causing the solubility minimum for non-polar gases; hydrophobic saccharides becoming less soluble as the temperature increases [187]. This property is useful in forming films as barriers to water loss and for holding on to small gas bubbles. Hydroxypropylmethylcellulose (HPMC, E464) has similar properties and uses but with added water interaction and surface activity [1292]. Both methylcellulose and HPMC may be used in gluten-free bakery products as gluten substitutes. Hydroxypropyl cellulose (E463) possesses good surface activity but does not gel as it forms open helical coils. It is a water-soluble thickener, emulsifier and film-former often used in tablet coating. Another important derivative of cellulose is carboxymethylcellulose.