Unit cell cellulose

Figure 6. Schematic representation of the ceUuiose -p model, units are in angstroms. The dimensions of the cellulose unit cell are displayed in two views top (or bottom) view (left) show the distance along the cellulosic axle (10.511 A) and the distance between cellulose chains at the same lattice (8.240 A), meanwhile the side view displays the thickness of all three lattices (8.189 A) of the cellulose unit cell.

Figure 4.17 Structure of cellulose unit cell (Cellulose I). Source. Reprinted with permission from Dyer J, Daul GC, Rayon Fibers, Lewin M and Pearce EM eds., Handbook of Fiber Chemistry, Marcel Dekker, New York, 775, 1998. Copyright 1998, CRC Press, Boca Raton, Florida.

One of the first structures to be determined was the natural polysaccharide cellulose. In this case the repeat unit is cellobiose, composed of two glucoside rings. In the 1980s, NMR experiments established that native cellulose is actually a composite of a triclinic parallel-packed unit cell called cellulose / , and a monoclinic parallel-packed unit cell called cellulose I. Experimentally, the structures are only difficultly distinguishable via X-ray analysis (7a, 7b). Figure 6.1 (3) illustrates the general form of the cellulose unit cell. 

Fig. 3. A cross-section of a nearly square cellulose microfibril, with the individual molecular chains shown as rectangles. Also shown are the one- and two-chain unit cells of la and ip. This view of the microfibril is parallel to the long axis. The chains are arranged so that the edges of the crystal correspond...

Unit cells of pure cellulose fall into five different classes, I—IV and x. This organization, with recent subclasses, is used here, but Cellulose x is not discussed because there has been no recent work on it. Crystalline complexes with alkaU (50), water (51), or amines (ethylenediamine, diaminopropane, and hydrazine) (52), and crystalline cellulose derivatives also exist. Those stmctures provide models for the interactions of various agents with cellulose, as well as additional information on the cellulose backbone itself. Usually, as shown in Eigure la, there are two residues in the repeated distance. However, in one of the alkah complexes (53), the backbone takes a three-fold hehcal shape. Nitrocellulose [9004-70-0] heUces have 2.5 residues per turn, with the repeat observed after two turns (54). 

The superimposition of diffraction spots from both phases gives the previously reported pattern that was thought to require an eight-chain unit cell. In the la stmcture, because of its one-chain unit cell, all chains must have parallel packing. Since the la and ip stmctures exist in the same microfibril of cellulose, the chains in the ip stmcture should also be parallel. 

The dimensions of the unit cells deduced for Cellulose I seem to depend on the kind of plant that is the source of the cellulose wide ranges are reported (64). Since most Cellulose I s are mixtures of two or more stmctures, with mostiy overlapping diffraction spots, the various observed dimensions could result from different amounts of the la, ip, and possibly other forms. 

Fig. 4. Comparisons of the unit cells proposed for Cellulose I—IV. In all cells, the c dimension (perpendicular to the plane of the drawing) is ca 1.034 nm.

Similar models for the crystal stmcture of Fortisan Cellulose II came from two separate studies despite quite different measured values of the diffraction intensities (66,70). Both studies concluded that the two chains in the unit cell were packed antiparallel. Hydrogen bonding between chains at the corners and the centers of the unit cells, not found in Cellulose I, was proposed to account for the increased stabiUty of Cellulose II. The same model, with... 

The pattern of variation of the multiplets differ among the samples. The relative intensities are not constant and they are not in the ratios of small numbers as would be expected if they arose from different points within a single unit cell. The spectral intensities are also not consistent within a single unit cell. The spectral intensities are also not consistent with the possibility of three independent crystal forms. According to Atalla therefore a model based on two independent crystalline forms seems most possible. In Fig. 5, the spectrum of pure cellulose II is given. 

Fig. 3.—Parallel packing arrangement of the 2-fold helices of cellulose I (1). (a) Stereo view of two unit cells approximately normal to the ac-plane. The two comer chains (open bonds) in the back, separated by a, form a hydrogen-bonded sheet. The center chain is drawn in filled bonds. All hydrogen bonds are drawn in dashed lines in this and the remaining diagrams, (b) Projection of the unit cell along the c-axis, with a down and b across the page. No hydrogen bonds are present between the comer and center chains.

The second choice is a simpler solution. According to Sarko and Muggli,66 all 39 observed reflections in the Valonia X-ray pattern are indexable by a two-chain triclinic unit cell with a = 9.41, b =8.15 and c = 10.34 A, a = 90°, 3 = 57.5°, and y = 96.2°. Ramie cellulose, on the other hand, is completely consistent with the two-chain monoclinic unit cell. Also, there are significant differences between their high-resolution solid-state l3C NMR spectra, indicating that Valonia and ramie celluloses, the two most crystalline forms, reflect two distinct families of biosynthesis. On this basis, the Valonia triclinic and the ramie monoclinic forms are classified69 as Ia and Ip, respectively. It has been shown from a systematic analysis of the NMR spectra by these authors, and from electron-dif-... 

According to a recent report, the unit cell of cellotetraose hemihydrate in single crystals contains two antiparallel chains, which are conformationally distinct—especially in the sugar geometries.74 However, all hydroxymethyl groups adopt similar gt orientations. Whether this oligosaccharide morphology can be implemented for cellulose II in fibers remains to be seen. 

Fig. 9. — Antiparallel packing arrangement of the 3-fold helices of (1— 4)-(3-D-xylan (7). (a) Stereo view of two unit cells roughly normal to the helix axis and along the short diagonal of the ab-plane. The two helices, distinguished by filled and open bonds, are connected via water (crossed circles) bridges. Cellulose type 3-0H-0-5 hydrogen bonds stabilize each helix, (b) A view of the unit cell projected along the r-axis highlights that the closeness of the water molecules to the helix axis enables them to link adjacent helices.

Carbon, ring oxygen replacement by, 141-143 2-Carboxy-5-(2-hydroxymethyl)-4-methylthiazole, synthesis and transformations, 284-286 Carrageenans, 366-368,418-419 Cellotetraose hemihydrate, 331 Cellulose, 326, 329-332 alternate unit cells, 329-330 derivatives, 332... 

The slightly galactosylated mannans are essentially linear polymers. As a result of their cellulose-like (1 4)-/3-D-mannan backbone, they tend towards self-association, insolubility, and crystallinity. Crystallographic study of C. spectabilis seed GaM [180] with a Man Gal ratio 2.65 1 suggested an orthorhombic unit cell with lattice constants of a = 9.12, b = 25.63, and c = 10.28 the dimension b was shown to be sensitive to the degree of galactose substitution and the hydration conditions [180 and references therein, [191]]. 

In cellulose II with a chain modulus of 88 GPa the likely shear planes are the 110 and 020 lattice planes, both with a spacing of dc=0.41 nm [26]. The periodic spacing of the force centres in the shear direction along the chain axis is the distance between the interchain hydrogen bonds p=c/2=0.51 nm (c chain axis). There are four monomers in the unit cell with a volume Vcen=68-10-30 m3. The activation energy for creep of rayon yarns has been determined by Halsey et al. [37]. They found at a relative humidity (RH) of 57% that Wa=86.6 kj mole-1, at an RH of 4% Wa =97.5 kj mole 1 and at an RH of <0.5% Wa= 102.5 kj mole-1. Extrapolation to an RH of 65% gives Wa=86 kj mole-1 (the molar volume of cellulose taken by Halsey in his model for creep is equal to the volume of the unit cell instead of one fourth thereof). 

The unit cell of cellulose from Chaetomorpha melagonium is monoclinic, with a = 16.43 A (1.643 nm), b(fiber axis) = 10.33 A (1.033 nm), c = 15.70 A (1.570 nm), and /3 = 96.97°. In base-plane projection, each of the Meyer-Misch subcells that make up the super-lattice are identical. All equatorial reflections can be indexed by using a one-chain unit-cell, meaning that every single chain has... 

Lamellar, single crystals of cellulose triacetate, precipitated from nitromethane with butyl alcohol, were studied by X-ray and electron diffraction. Only the crystals containing the mother liquor, or moistened with nitromethane, showed rich diffraction details. From stretched and annealed fibers, it was found that the unit cell is tetragonal, with a = fe = 21.15A (2.115 nm), and c = 41.36 A (4.136 nm). 

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