Native cellulose allomorphs

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... 

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... 

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. ... 

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). 

The preparation of cellulose III is realized with ammonia treatment and allows to obtain the form cellulose IIIi or the form IIIn from to cellulose I and cellulose II, respectively. Cellulose III can be transformed into cellulose IV after treatment at high temperature in glycerol cellulose IVj and IVn obtained from cellulose IIIi and HIn, respectively. After relative recent studies by electron diffraction [47], it is now accepted that cellulose IViis a disordered form of cellulose I (native cellulose form). This allomorph, at native state, may be observed in some mushrooms but also in primary cell walls of cotton [48]. 

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. 

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