Cellulose I allomorphs

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

The solid-state l C-NMR spectra of the fibrous hydrocellulose also demonstrate the predominance of the cellulose I allomorph (Figure 6). All three spectra contain the sharp resonances associated with the cellulose I conformation and the broader C-4 and C-6 resonances indicative of regions of three-dimensional disorder and crystallite surfaces (16,17). The relative intensities of the sharp and broad resonances of the three spectra are similar, again demonstrating the lack of change in physical structure during degradation. 

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... 

Kudlicka K., Brown, Jr. R.M. Li L., Lee J.H., Shin H., and Kuga S. 1995. P-Glucan synthesis in the cotton fiber. IV. In vitro assembly of the cellulose I allomorph. Plant Physiol 107 111-123. Kudlicka K., Lee J.H., and Brown, Jr. R.M., 1996. A comparative analysis of in vitro cellulose synthesis from cell-free extracts of mung bean (Vigna radiata, Fabaceae) and cotton (Gossypium hirsutum, Malvaceae). Am J Bot 83 274-284. 

When compared to the classical type 1 cellulose biosynthesis operon of Glu-conacetobacter xylinus that produces the cellulose I allomorph under laboratory conditions the cellulose biosynthesis operons of Salmonella spp. and E. coli have both, homologous and unique components (Figure 7-3). As in G. xylinus bcsA, which encodes for the catalytic subunit of the cellulose synthase, and bcsB, which... 

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... 

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. 

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. 

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... 

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)... 

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. 

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. 

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. 

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.

Figure 6 shows the region from 900 to 1200 cm. The spectra showed common characteristics within each family. Two bands were assigned to stretching of the glucose ring for allomorphs in the cellulose I family. The bands were sharp and showed a weak perpendicular dichroism. 

Table IV. Glucose Ring Stretching of Allomorphs in Cellulose I Family ...

Recently, we have obtained better NMR data for the allomorphs with a Bruker instrument at 200 MHz for protons. The Cl signal of IIIj seemed to be a singlet, but its profile was broadened unsymme-trically to the lower ppm side and suggested additional weak peaks. The half-width was almost the same as that of cellulose I. The common characteristics related to half-width of the Cl signal within each families were clearly evident. 

Dudley et al. (29) suggested the observed doublets of Cl and C4. in cellulose II were due to independent chains in the unit cell. Cael et al. (30) explained the 13C NMR spectrum of cellulose I with the eight-chain unit cell using Dudley s proposal. But the proposal could not explain the similarity of the Cl and C4. signals of the allomorphs in each of the families. They should show similarity between IIIi and IIIii or IVi and IVn because their chain packing should be similar to each other. When the high resolution NMR spectra of these are measured the problem will be more clear. 

II allomorph is known by the term regenerated cellulose. Regeneration involves either preparing a solution of cellulose in an appropriate solvent or of an intermediate derivative followed by coagulation and recrystallization. This process is used to produce rayon fibers. Mercerization involves intracrystalline swelling of cellulose in concentrated aqueous sodium hydroxide (NaOH), followed by washing and recrystallization. This process is used to improve the properties of natural yams and fabrics. The transition from cellulose I to cellulose II is not reversible, and fliis implies that cellulose II is a stable form as compared with the metastable cellulose I. 

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