Cellulose allomorphs

Artificial cellulose showed the cellulose II allo-morph, a thermodynamically more stable form with an antiparallel structure, by X-ray diffraction study, when a crude celluase was employed for the enzymatic polymerization.123 The other allomorph cellulose I is a thermodynamically metastable form with a parallel structure, which living cells normally produce, but was believed impossible to be realized in vitro. Interestingly, however, the in vitro synthesis of cellulose I was successfully achieved by using a purified cellulase.125 The molecular packing of glucan chains in a crystal is affected by the purity of the enzyme as well as the enzymatic polymerization conditions. A novel concept choroselectivity was therefore proposed, which is concerned with the intermolecular relationship in packing of polymers having directionality in their chains.126... 

Cellulose III. Cellulose III results from treatment of cellulose with Hquid ammonia (ammonia mercerization) or amines. Cellulose III can be made from either Cellulose I or II. When treated with water. Cellulose III can revert to its parent stmcture. Some cellulose III preparations are much more stable than other preparations. The intensities on diffraction patterns from Cellulose III differ slightly depending on whether the Cellulose III was made from Cellulose I or II, and thus these allomorphs are called IIIj or IHjj- Workers studying III concluded, based partiy on the results of I and II, that the packings of IIIj and IIIjj are parallel and antiparallel, respectively (67). IIIjj also is thought to have hydrogen bonds between the corner and center chains. 

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. 

Isogai, A. (1994). Allomorphs of cellulose and other polysaccharides. In Cellulose Polymers Blends and Composites, Gilbert, R. D. (Ed.), p. 21. Hanser Verlag, Munich. 

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. 

One of the most special aspects of cellulose polymorphy is the transformation from I to II. The conversion of the parallel-packed cellulose I structures to an antiparallel cellulose II structure is interesting because it can occur without loss of the fibrous form. This transformation is widely thought to be irreversible, although there are several reports [231-233] of regenerated cellulose I. The observation that there are two different forms of cellulose III and of IV is also remarkable. The two subforms of each allomorph have essentially identical lattice dimensions and at least similar equatorial intensities. Other intensities are different, particularly the meridional intensities, depending on whether the structures were prepared initially from cellulose I or II. The formation of the III and IV structures is reversible and the preceding polymorph (I or II) results. 

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)... 

Figure 4.35 Hydrogen bonding and conformation of an individual glucan chain in cellulose I allomorphs.

One of the discoveries growing out of the early diffractometric studies of cellulose was that it can occur in a number of allomorphic forms in the solid state, each producing distinctive X-ray diffractometric patterns. In addition to the cellulose II form, which has been discussed extensively, two other forms have been recognized these are cellulose III and cellulose IV. It is of interest to consider them briefly because they reflect the capacity of cellulose to aggregate in a wide variety of secondary and tertiary structures and because some of the higher plant celluloses produce diffraction patterns that are not unlike those of cellulose IV. Furthermore, they reflect the tendency for some of the celluloses to retain some memory of their earlier states of aggregation in a manner not yet understood. 

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 the first detailed comparison of the Raman spectra of celluloses I and II, it was concluded that the differences between the spectra, particularly in the low frequency region, could not be accounted for in terms of chains possessing the same conformation but packed differently in the different lattices (33). As noted above, that had been the general interpretation of diffrac-tometric studies of Che two most common allomorphs. The studies of the Raman spectra led Co the proposal Chat two different stable conformations of the cellulose chains occur in the different allomorphs. 

Yet another set of interdisciplinary studies are represented by the work of Hayashi and coworkers, wherein they attempt to shed light on the questions of reversibility, or lack thereof, in transformations between the allomorphs of cellulose and its derivatives. 

In addition to their diffractometric studies reported in prior publications, they add in their contribution to the present symposium analyses of the infrared spectra as well as analyses of the CP-MAS 13c NMR spectra. Their thesis is not inconsistent with the proposals of Atalla and coworkers concerning differences between the conformations of celluloses I and II. However, Hayashi and coworkers go beyond this by proposing that the differences in conformation can be preserved in the course of heterogeneous derivati-zation reactions, and also in the process of generating the other allomorphs of cellulose, namely celluloses III and IV, from the two primary allomorphs 1 and II. 

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... 

Figure 10 indicates that (a) If tvrc) allomorphs coexist in the Cladophora cellulose, the T behavior of both allomorphs is the same, (b) If the reduction of intensity in the T ... 

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). 

IIIj and IVj ) and those of the cellulose II family (II, IHn and IVji). The trasformations among allomorphs within each family occurred reversibly, but transformations between families were irreversible. For example, IVj could be made either directly from I or through IIIi, and was transformed into I by hydrolysis. Also, IVj can be converted into IIIn and IVjj through cellulose II. When IVji, derived from IVj or other members of the cellulose I family, was transformed into II by hydrolysis, it could not be transformed back into a member of the cellulose I family. There is strong similarity between the x-ray patterns of IIIi and IIIn and between those of IVj and IVu, but the meridional intensities are quite different (3). The IR and 13C NMR spectra are clearly different and it is clear that IIIj, IIIn, IVj and IVjj are distinct allomorphs. 

When cellulose trinitrate Ij(TNG Ij) and triacetates Ij and IIj (TAG Ij and IIj) were prepared from allomorphs of the I family in the fibrous state under low-swelling conditions, they could be saponified into cellulose I. On the other hand, TNG Ijj, TAG Ijj and TAG IIu can be saponified into cellulose II (A-6). When the esters of the I family were recrystallized by heat treatment, they were transformed irreversibly into corresponded esters (for example, TAG Ij to TAG Iji). They crossed the barrier of irreversibility and were saponified into cellulose II. 

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. 

Atalla and Van der Hart (11, 12) concluded, based on their Raman and NMR spectra, that the molecules in cellulose I and II have different conformations. Based on x-ray analyses, Sarko et al. (13i H) and Blackwell et al. (15, 16) both concluded that crystal structures of cellulose I and II were based on parallel and antiparallel packing, respectively, of chains that have similar backbone conformations. Sarko (17) concluded that the allomorphs in the I and II families were based on parallel and antiparallel chains, respectively. The irreversibility may arise from the increase in entropy when parallel packing is converted to antiparallel packing. 

In the present work, we tried to determine which proposal is better using IR and solid state 13C NMR. There are many papers on the spectra of cellulose I and II (12, 18-24.), however, there are few on the other allomorphs. Mann and Marrinan (21) found differences in the OH stretching bands of IR spectra of IIIj and IIIn and of IVj and IVjj. Chidambareswaran et al. (25) reported IR spectra for several allomorphs, but their spectra lacked enough resolution for detailed discussion of chain conformation. The 13C NMR spectra of the other allomorphs have not been reported. 

Cellulose IVt and IVtt. These allomorphs were prepared from IIIi IIIii, respectively, by heating in glycerol at 260 C for 30... 

Fig. 1 X-ray diffractograms of the allomorphs in cellulose I family by the reflection method (the reflection plane is parallel to the membrane surface). A I, valonia cellulose, B IVj prepared from A through IIIi(C), C IIIi prepared from valonia.

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