Cellobiose residue

A monoclinic unit-cell with a = 8.2 A (820 pm), b(fiber axis) = 10.30 A (1.030 nm), c — 7.90 A (790 pm), and /3 = 83.3° is used. The distance between the terminal oxygen atoms in the cellobiose unit is taken to be 10.3912 A (1.03912 nm). A left-handed, helical structure, with seven cellobiose residues in a pitch of 72.1 A (7.21 nm) was proposed. The packing arrangement involves the central reversed and comer chains, and a relative shift between them of 0.25 repeat length along the b axis. 

The difference in frequency between the parallel type bonds in cellulose II and III and those of cellulose I is appreciable and suggests that the molecular form of the cellobiose residue is different in cellulose I from the other two modifications. However, since the frequency of a hydrogen-bonded OH group is very sensitive to the 0 0 distance, this difference in molecular configuration may not be very great. 

The crystalline structure of cellulose has been characterized by X-ray diffraction analysis and by methods based on the absorption of polarized infrared radiation. The unit cell of native cellulose (cellulose I) consists of four glucose residues (Figs. 3-6 and 3-7). In the chain direction (c), the repeating unit is a cellobiose residue (1.03 nm), and every glucose residue is accordingly displaced 180° with respect to its neighbors, giving cellulose a... 

It seems clear today that crystallography, alone, could have resolved the problem of the inter-residue linkage between D-glucose residues in cellulose. However, the contributions of chemistry regarding the presence of cellobiose residues in cellulose, and the principal methylation-hydrolysis product, 2,3,6-tri-O-methyl-D-gIucose, were major influences in the final solution to the problem. The fact that Sponsler and Dore actually championed the incorrect view of alternate (1—>1)- and (4— 4)-linkages, on the basis of x-ray diffraction results, emphasizes the fact that is as true as ever... 

It may be argued that the twofold axis in the fiber direction of an oriented polysaccharide need not be an element of molecular symmetry, in which case the chemical evidence that the structure is based on a chain of cellobiose residues is the determining factor in making the final choice between the two chains shown in Fig. 8. However, this interpretation is not the usual one, and it is a line of thinking that has not proved useful to date. 

It is often stated that the repeating unit of cellulose is a cellobiose residue, a hydrolysis product of cellulose that is composed of two glucose residues. This acknowledges that... 

Figure 1. Conformations of cellobiose with inter-residue intramolecular hydrogen bonding. (a,b) conformations with two inter-residue bonds, (c) hydrogen bonding observed in the crystal structure (18).

Figure 2. A contour diagram of the conformational energy of p-cellobiose computed from eqn. (6) holfing constant all variables except < ), v see ref. 5 for details. The rigid glucose residue geometry was taken from ref. 23, and the valence angle p at 04 was chosen as 116 in accordance with the results of pertinent crystal structure determinations. Contours are drawn at 2,4, 6, 8,10,25, and 50 kcal/mol above the absolute minimum located near ( ), v = -20 , -30 higher energy contours are omitted.

The effects of propagated distortions of the residue are shown in Figure 4, a CA map without contouring that was prepared with the standard driver. The gtgtRR starting model of cellobiose had an energy of 31.4 kcal/mol (its conformation was < ) = 20, - -60). ... 

Figure 2. The four starting models used for the study of cellobiose (lone pairs of electrons are not shown). Convention defines the R and C notation when the residue is in a conventional orientation and is viewed from above. The least energetic structure observed in this study is gtgtRR. This Figure and Figure 5 were drawn with CHEMX, developed and distributed by Chemical Design Ltd, Oxford, England.

Cellobiose Octaacetate and Glucose Pentaacetate. The results on both compounds were similar. Samples irradiated at 253.7 and 313 mfi, both in the presence of oxygen and in vacuum, were saponified. Infrared analysis of the saponified residues showed that all samples had carbonyl absorption at 5.76 to 5.78 microns, owing to lactones, ketones,... 

All of the likely conformations of cellobiose, cellulose, and xylan are explored systematically assuming the ring conformations and IC-D-O-IC-4 ) angle for each pair of residues to be fixed and derivable from known crystal structures. The absolute van der Waals energies, but not the relative energies of different conformations, are sensitive to the choice of energy functions and atomic coordinates. The results lead to possible explanations of the known conformational stiffness of cellulose and Its solubility properties in alkali. The characteristics of xylan conformations are compared with cellulose. 

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