Chain polarity cellulose

Since its introduction several years ago, the virtual bond, constrained optimization method has proved very useful in studies of polysaccharide crystal structure. Notable among the successes that can be ascribed to it are the structural determinations of the double-helical amylose (.11), the cellulose polymorphs of different chain polarities (.12, 13), and of a number of other polysaccharides and their derivatives. As described in a review of amylose structures elsewhere in this volume, the use of this refinement method has produced structural detail that has previously been unavailable (ll). These results have provided much-needed... 

Typically, this approach leads to a large number of possible structures, because Jones allowed both the 0-3 -0-5 and 0-2-0-6 type of intra molecular hydrogen bond. Comparison of the most plausible structures for cellulose I and II with observed equatorial and meridional intensity led Jones to conclude that several structures, including that of Meyer and Misch, are fair approximations to the actual structure, but none of them show especially good agreement with the x-ray intensities observed. As a possible solution, Jones suggested a statistical structure, in which there is randomness of chain polarity, but in which adjacent chains have one characteristic shift along the b axis when they have parallel orientation, and a different one for the antiparallel situation. 

The allomorphs and derivatives prepared from cellulose I and II in solid state could be transformed into cellulose I and II, respectively. The memory phenomenon of the original crystal structure should be due to a structural characteristic (chain conformation, chain polarity or others) of an individual chain that is common within each family and kept through the change of crystal structure. There were direct irreversible conversions between corresponding cellulose esters, Na-cellulose and cellulose IV prepared from cellulose I and II just like that between I and II. Accordingly, the structural characteristic should be the cause of the structural irreversibility between the I and II families. 

Accordingly, cellulose has an average molecular weight in the range of 300 000-500 000. One of the most interesting eharacteristics is that eellulose eonsists of several crystal polymorphs, with the possibility of eonversion from one form to another. Its six different polymorphs differ in unit cell dimensions and chain polarity, and are the principle component of all plant cell walls. The natural cellulose I has two different structures, lot and 1(3, while cellulose II is another important crystalline form of cellulose. The transformation of cellulose I to cellulose II is generally considered to be irreversible, because cellulose II is more stable than cellulose I. With proper chemical treatments, it is possible to produce cellulose III and cellulose IV. 

This cycUc hemiacetal function is in an equUihrium in which a small proportion is an aldehyde which gives rise to reducing properties at this end of the chain the cellulose chain has a chemical polarity. Determination of the relative orientation of cellulose chains in the three-dimensional structure has been and remains one of the major problems in the study of cellulose. So, in the cellulose crystal, two arrangements are possible either an organisation in parallel chains with reducing chain end placed in the same side or an organisation in antiparallel chains with alternate position. 

Thus, because of the acetamido groups, there are more intra- and inter-chain hydrogen bonds in 3 and 4 than in 1 and 2. Because of the polarity problem, the chitin I— II transition is also irreversible, as in the case of cellulose. 

Linear polymers, polystyrene and cellulose triacetate exhibit differences in hydrodynamic behavior in solution. Cellulose and its derivatives are known to have highly extended and stiff chain molecules below a Dp of about 300, but as the Dp Increases above 300 the chain tends to assume the character of a random coll (27,28). The assumption that hydrodynamic volume control fractionation in GPC may not be true for polystyrene and cellulose triacetate, though it has been found satisfactory for non-polar polymers in good solvents (29). 

As the above results show, the gross features of the cellulose I crystal structure predicted by various methods do not differ appreciably, but the accompanying deviations in the R -factors are significant. When these predictions are used to assess, for example, whether the cellulose I crystal structure is based on parallel- or antmarallel-chains, the range in the R"-factors seen for the parallel models (cf. Table II) is comparable to that between the two different polarity models. As shown in Fig. 5, the most probable parallel- and antiparallel-chain structures of cellulose I, refined by minimizing the function O, differ in R -factors by approximately the same extent as the three predictions for the parallel model shown in Fig. 4 and Table II. 

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