Native cellulose study

The products formed by the decomposition of sodiocellulose by aqueous reagents and alcohols were studied. For the sodiocellulose prepared from native cellulose by aqueous treatment, the spacing of the 101 reflection varied considerably with the experimental conditions (the greatest being 9.24 A 924 pm). Although the 101 and 002 reflections did not vary in spacing, their relative intensities were af-... 

The considerations involving comparisons of the structures of cellobiose and / -methylcellobioside with the structures of mercerized and native cellulose, respectively, when taken together with the additional observation that the basic repeat unit derived from the diffractometric studies is 10.3 A rather than 5.15 A, require that data relating to the structure of cellulose be reexamined with the constraint that the anhydro-cellobiose unit, rather than the anhydroglucose unit, is the basic repeat unit. To the author s knowledge, no efforts have been made to interpret... 

Hphe study of the synergism shown by enzymes of the cellulase complex in solubilizing highly ordered native cellulose has now progressed to a stage where it is possible to state with some certainty the principal enzyme components involved. Not everyone agrees, however. 

Aqueous suspensions of cellulose microcrystalhtes obtained by acid hydrolysis of native cellulose fibers can also produce a cholesteric mesophase [ 194]. Sulfuric acid, usually employed for the hydrolysis, sulfates the surface of the micro crystallites and therefore they are actually negatively charged. Dong et al. performed some basic studies on the ordered-phase formation in colloidal suspensions of such charged rod-like cellulose crystallites (from cotton filter paper) to evaluate the effects of addition of electrolytes [195,196]. One of their findings was a decrease in the chiral nematic pitch P of the anisotropic phase, with an increase in concentration of the trace electrolyte (KC1, NaCl, or HC1 of < 2.5 mM) added. They assumed that the electric double layer on... 

Fungal cellulase enzyme systems capable of efficiently catalyzing the hydrolytic degradation of crystalline cellulose are typically composed of endo-acting cellulases (EGs), exo-acting cellulases (CBHs), and at least one cellobiase (1-6). The CBHs are typically the predominant enzymes, on a mole fraction basis, in such systems (7). Consequently, the CBHs have been the focus of many studies (8). The three-dimensional structure of prototypical CBHs is known (9-12) and their specificities are, in general, well characterized (13,14). However, mechanism-based kinetic analyses of CBH-catalyzed cellulose saccharification are rather limited (15,16). Studies of this latter type are particularly difficult owing to the inherent complexity of native cellulose substrates. 

No critical discussion of biosynthesis can be attempted without consideration of the information available on the structure of native cellulose. As for studies on biosynthesis, much controversy has existed in the field of structural analyses of cellulose. There seem to be only two points on which all workers are in agreement, namely, that (a) native cellulose is a composite of linearly extended chains of (l->4)-j6-D-glu-can (1), and (b) strong intrachain and interchain hydrogen-bonding between D-glucosyl residues occurs in such a way as to create a highly insoluble, and partially crystalline, fibrillar structure. 

Cellulose is made up of saccharide units polymerized in chains containing many O—H groups and —O— links. X-ray diffraction and IR spectra show H bonds in these chains. Mark (1337) describes the evidence for H bonds in the ab plane of native cellulose ( Cellulose I ). The crystal modification called regenerated or hydrated cellulose ( Cellulose II ) is shown to have a different arrangement of H bonds. The two forms may be interconverted. There are further modifications, some hydrated, which are less well studied but which apparently are minor variations of Celluloses I and II (961). 

Atalla RH and VanderHart DL (1999) The role of solid state C-13 NMR spectroscopy in studies of the nature of native celluloses. Solid State Nuclear Magnetic Resonance, 75(1) 1-19... 

B. G. Ranby, Inst. Phys. Chem., Univ. Uppsala, Arkiv Kemi, 4, 241 (1952). Fine structure and reactions of native cellulose. Electron microscope study of morphology of celluloses from wood, cotton, bacteria, tunicates, and algae behavior of samples on swelling in sodium hydroxide and hydrolytic degradation. 

In addition to the disallowed reflections in the electron diffraction patterns that placed the crystallographic models in question, new spectral evidence was developed pointing to the need for further refinement of structural models, particularly for native celluloses. The models derived from the crystallographic studies could not rationalize many features of the spectral data known to be quite sensitive to structural variations. 

In an effort to further validate the proposal that the Iq, and forms were the primary constituents of native celluloses, VanderHart and Atalla undertook another extensive study to exclude the possibility that experimental artifacts contributed to the key spectral features assigned to the two forms. A number of possible sources of distinctive spectral features were explored. The first was the question whether the surface layers associated with crystalline domains within particular morphological features in the native celluloses could give rise to features other than those of the core crystalline domains. The second was whether variations in the anisotropic... 

Figure 7 where it is clear that even in the spectrum representative of the Iq form the upfield wings of the C4 and C6 resonances are reduced to a minimum. With the completion of this study hy VanderHart and Atalla, most of the questions about the possibility that the spectral features were the results of artifacts were put to rest, and the hypothesis that all native celluloses belong to one or to a combination of these forms was generally accepted. 

With the wide acceptance of the proposal of the two crystalline forms (I and I/3) came the challenge of understanding the differences between them and their relationship to each other within the morphology of native cellulosic tissues. A number of complementary approaches were pursued by different investigators in the search for answers. Some were based on further application of solid-state C NMR to the study of different celluloses as well as to celluloses that had been subjected to different modifying treatments. Others were based on the application of Raman and infrared spectroscopy to new classes of cellulosic samples. Others were still based on the refinement of electron microscopic and diffractometric methods. The results of these investigations will be presented in summary. 

In summary then, the Raman and infrared spectral studies undertaken after the discovery of the composite nature of native celluloses point to the conclusion that the only difference between the two forms is in the pattern of hydrogen bonding between chains that possess identical conformations. Yet electron diffractometric studies have been interpreted to indicate that the two forms represent two crystalline phases with different crystal habits." More recently, diffractometric studies by Nishiyama have also been interpreted along... 

The studies undertaken on the basis of further examination of the solid-state NMR spectra of celluloses are in a number of categories. The first group is focused on further examination of the spectra of different native celluloses, in part aided by mathematical procedures for deconvolution of the spectra or for resolution enhancement. Another group relies on exploring the spectral manifestations of native celluloses that have been modified in different ways. Yet a third approach is based on investigation of celluloses subjected to different but well-known procedures for inducing structural transformations in the solid-aggregated state of cellulose. 

In the early studies by Horii etal., all of the upheld wing of the C4 resonance was attributed to molecules in noncrystalline domains. On this basis, the line shape analysis of the C4 resonance of different native celluloses did not seem consistent with the model proposed by VanderHart and Atalla with respect to the composite... 

In later studies, the Raman spectra and corresponding infrared spectra indicated that the primary differences between the I and I/j forms of native cellulose were in the pattern of hydrogen bonding. Furthermore, the Raman spectra of the two forms raise questions as to whether the structures can possess more than one molecule per unit cell since there is no evidence of any correlation field splittings of any of the bands in the spectra of the two forms. 

The possibility of the coexistence of the Iq, and I forms within a superlattice structure has been suggested in the context of studies intended to mimic the conditions of biogenesis. These need to be examined in greater detail in relation to the discussions of native celluloses and of their biogenesis. 

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