Crystalline allomorph

The elucidation of a great number of helical structures of biological polysaccharides by fiber X-ray diffraction has been reviewed. Cellulose, an example of a structural plant cell wall polysaccharide based on P-1 —> 4 linked D-glucopyra-noside residues (Scheme 11), is known to occur in various crystalline allomorphs, I, II, III, and IV. Cellulose I and II consist of extended twofold helices with low diameter and high pitch,which run parallel and antiparallel, respectively. Both forms have intramolecular hydrogen bonding networks (3-OH. . . 05)... 

Three crystalline allomorphs have been described, a, p and y, with structures available only for the first two. These are based on X-ray and electron diffraction, so that hydrogen atoms are not located. 

The hypothesis (1-3) that all native celluloses are a composite of two crystalline allomorphs, designated and Ig, has been further tested using C solid state NMR. In particular, two alternate origins of sharp resonance features were considered in addition to the usual origin, the crystalline unit cell. The first source is ordered layers on crystal surfaces the second is possible anistropic bulk magnetic susceptibility (ABMS) shifts associated with well defined fibril patterns (tertiary morphology). 

The crystalline allomorphs of cellulose differ from each other also by the shapes of the crystalline unit cells. The projection of the Cl-unit cell has a parallelepiped shape, CIV-unit cell has a square shape, while unit cells of Cll and CIII have a rhombic shape. 

Current data suggest that cellulose biosynthesis is a bacterial invention and that eukaryotes acquired the process via multiple lateral gene transfers. Bacteria and eukaryota have independently evolved regulatory mechanisms and molecular structures to utilize the p-1,4-homopolymer synthesized by the catalytic activity of homologous cellulose synthase enzymes. The differences in accessory enzymes probably reflect not only convergent evolution to produce a cellulose I crystalline allomorph, but also inventions of alternative products such as cellulose II, noncrystalline cellulose, or nematic ordered cellulose. 

The structure of cellulose has been studied since the 19th century, when Carl von Nageli proposed the idea that natural cellulose contains ciystalline micelles—small crystallites (Wilkie, 1961 Zugenmaier, 2009). Only 70 years later, this idea was confirmed by X-ray diffraction, and as a result, the first model of monoclinic unit cell for crystalline structure of native cellulose Cl was developed by Mayer and Mish (Mayer et al., 1937). The model of Mayer and Mish with antiparallel arrangement of chains existed 30 years, whereupon it was replaced by a more accurate model with parallel arrangement of cellulose chains within crystallites (Gardner et al., 1974). Later it was discovered that in addition to crystalline structure of native cellulose Cl, there are also other crystalline allomorphs, CII, CIII, and CIV (O Sullivan, 1997). 

To characterize the supermolecular structure of cellulose, the primary structural parameters (type of crystalline allomorph, crystallinity, paracrystallinity and amorphicity, and orientation of nanofibrils, nanocrystallites, and nanoscale non-crystalline domains, as well as porosity of cellulose) should be determined. These structural parameters can affect physicochemical, chemical, biochemical, physical and mechanical properties of cellulose materials. 

The Segal method gives comparative index of crystallinity various natural celluloses, e.g., wood pulp, cotton, and flax fibers and celluloses, micro crystalline and powder celluloses, etc. However, this method is not intended for modified celluloses having CII, CHI, and CIV crystalline allomorphs. 

Solid-state cross polarization magic angle spinning carbon-13 magnetic resonance (CP/MAS C-NMR) spectroscopy has yielded information on the crystallinity, allomorph composition and surface properties of microfibrils [5, 39, 47-49]. Both... 

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