Cellulose crystalline allomorphs

Cellulose crystallinity is not uniform. A simple experiment of immersing cellulose in cold concentrated alkali, a process used for enhancing the dye-absorbing quality of cotton fabrics called mercerization, was found already in the 1930s on the basis of X-ray diffraction to produce a cellulosic allomorph with different unit cell dimensions [13,14]. This was given the... 

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)... 

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). 

Peeling is inhibited to similar extents by the crystalline order of both cellulose 1 and II allomorphs, while chemical stopping is significantly more inhibited in the cellulose I allomorph. This is consistent with the higher ratio of the rate of chemical stopping to that of peeling typically reported for mercerized cellulose in comparison to native cellulose... 

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. 

Utilizing the characteristics of the polymerization that produces synthetic cellulose via a one-step reaction, the in situ polymerization system by TEM was directly observed. Polymerization of y6-CF by a crude cellulase catalyst in acetonitrile/buffer (5 1) solution yielded the product of irregular rodlets characteristic of a cellulose II allomorph [39]. Using a partially purified cellulase as catalyst, the assembly of synthetic cellulose I was accomplished in an optimized acetonitrile/buffer ratio (2 1) via choroselective polymerization as illustrated in Fig. 7. The cellulose I allomorph was characterized by TEM (Fig. 8), electron diffraction, and cellobiohydrolase I-colloidal gold binding [40]. Formation of a crystalline synthetic cellulose I allomorph was... 

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. 

In the present work, we extend the method to compensate for the hydrogen bonds present in carbohydrates. The hydroxylated character of carbohydrate polymers influences between-chain interactions through networks of hydrogen bonds that occur during crystallization. Frequently, several possible attractive interactions exist that lead to different packing arrangements, and several allomorphic crystalline forms have been observed for polysaccharides such as cellulose, chitin, mannan and amylose. The situation is even more complex when water or other guest molecules are present in the crystalline domains. Another complication is that polysaccharide polymorphism includes different helix shapes as well. 

Detailed structures of many crystalline materials can be determined by diffraction methods. However, because of the complex hierarchy of the cotton fiber and its very small crystallites, diffraction experiments on cotton fibers cannot provide fine details of molecular structure. Instead, the best data on cellulose structure comes from other sources. One of the major points of interest is the finding that cellulose has many different crystalline forms, or polymorphs, depending on the sources and subsequent treatments. Historically, there are four polymorphs or allomorphs, I to IV, and subclasses have been identified for all but cellulose II. 

Quite early in the x-ray diffractometric studies of cellulose it was recognized that its crystallinity is polymorphic. It was established that native cellulose, on the one hand, and both regenrated and mercerized celluloses, on the other, represent two distinct crystallographic allomorphs (14). Little has transpired... 

In previous publications (1-3) we have proposed, principally on the basis of fj R evidence, that native celluloses cire composites of two crystalline forms occurring in different proportions. These allomorphic forms were designated and I The solid-state spectra proposed for the I and I... 

Modeling studies have established that the two crystalline arrangements correspond to the two low-energy structures that could arise firom parallel associations of cellulose chains. Within the framework of these studies, three-dimensional models have also been proposed that allow comparison of the similarities and the differences that characterize the two allomorphs of native cellulose. ... 

I provides a speclnim that differs from that of the native form. Eleetron microscopy shows that cellulose I complexed wifli EDA is composed of nonuniform crystalline domains, whereas die IIIi allomorph is characterized by well-defined crystalline zones. The conformational changes observed for die primary hydroxyl groups are of interest, as they provide possible markers for study of die various conformational transitions associated with cellulosic systems. 

There are several polymorphs of crystalline cellulose-I, II, III, and IV. Each has been extensively studied [4]. Crystalline cellulose that is naturally produced by a variety of organisms, it is sometimes referred to as "natural cellulose. Cellulose-I has two polymorphs, a triclinic structure [ ) and a monoclinic structure [I ], which coexist in various proportions depending on the cellulose source. The Iq structure is the dominate polymorph for most algae and bacteria, whereas is the dominant polymorph for higher plant cell wall cellulose and in tunicates [5-7]. Allomorph ratios are species specific, and this gives rise to natural structural variations in cellulose crystals. However, the mechanisms contributing to crystal formation remain unknown [8]. 

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