Cellobiose, crystal structure

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 MM2 model resides very near the minimum 2 in the cellobiose energy map (cf. Fig. 9). (Among others, the crystal structure of methyl cellobioside-methanol complex is found in that minimum (15)). On the other hand, the PS79 model resides on the shallow saddle point between minima 2 and 3. 

The crystal structure of cellobiose [0-/3-D-glucopyranosyl-(l— 4)-/3-D-glucopyranose] (12) has been determined for the third time.10 The... 

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. 

Fig, 5. Conformation of cellobiose from X-ray crystal structure analysis as a space filling stereo plot. The reducing end is to the right... 

Tvaroska showed in a model calculation of the solvation of these conformations of P-cellobiose that in water the conformer with

crystal structure conformation. The calculated populations of the five conformers change quite a bit in going from a polar solvent (water) to an unpolar one (dioxane). 

Cellobiose octaacetate and 1,6-Anhydro-p-cellobiose hexaacetate are compared with respect to their glycosidic conformation [93, 94], For cellobiose octaacetate it was concluded that the conformation in solution is close to that one determined by X-ray crystal structure analysis to cp = 45° and i / = 16° (Fig. 5) whereas the 1,6-anhydro derivative is demonstrated by use of NOEs, relaxation data, and coupling constants 3JC>H to adopt torsional angles of = 25° and ]c = 45° respectively. 

The hydrogen bonding in cellobiose and methyl cellobioside as models for that in the celluloses. In the absence of crystal structure analyses of higher oligomers of 1 - 4-linked /5-glucopyranose, cellobiose is frequently used as a model for interpreting the X-ray fiber diffraction patterns of the celluloses, especially since the cellobiose unit is considered to be the repeating unit of the polysaccharide chain. 

Application of this approach to cellulose has confirmed31 that the Hermans conformation is the only possibility that is free from strong steric clashes while fitting the usual interpretation of the x-ray evidence by having two fold screw symmetry and a projected residue height of 5.15 A. It is also stabilized by a hydrogen bond between successive residues, as in the crystal structures of cellobiose and... 

These crystal-structure analyses are concerned with the following molecules o-D-glucopyranose monohydrate/ D-xylopyranose/ methyl yS-D-xylopyranoside, di-jS-n-fructopyranose-strontium chloride trihydrate, cellobiose (independent determination), D-glucaric acid, D-galactonic acid, methyl a-D-lyxofuranoside, /3-D-lyxopyranose, methyl 3,4,6-tri-0 - acetyl - 2 - (chloromercuri) - 2 - deoxy - j3 - d - glucopyranoside, D-glu-copyranosyl (potassium sodium phosphate) tetrahydrate," and methyl 6-bromo-6-deoxy-a-D-galactopyranoside. ... 

Figure 2 shows the energy map for cellobiose, as calculated from PFOS method the two deepest minima are indicated by arrows. These two minima correspond to crystalline conformations found for cellobiose (4) and methyl- 6-D-cellobioside (5). For these models of the two stable conformations, an Intramolecualr hydrogen bond of the type 03. ..05 is gresent, with 03. ..05 distance of about 2.60 A, compared to 2.76 A in the crystal structures. 

Crystal data on cellobiose octa-acetate and cellotriose undeca-acetate have been compared in order to analyse what information available from crystal structures of oligosaccharides can be used to deduce the three-dimensional structure of the related polysaccharide." ... 

FIGURE 9.23 Contours of iso-n and iso-h values in —ip space for model cellulose helices composed of the nonreducing residue of crystalline cellobiose [ 179] and a value of t = 116. Also shown as dots are the experimentally determined values of

from crystal structures of small molecules related to cellulose. With one exception at the bottom of the map, all of the dots correspond to extended helices with the numbers of residues per turn between 2 and 3. Geometries found in complexes of cellulose fragments and proteins are shown as triangles. They have a similar distribution but the range is expanded. Left-handed helices are indicated with negative values of n. 

Two interesting points are the number of cellobiose units per cell for cellulose triacetates I and II is 4, versus 2 for celluloses I and II and the measured density for cellulose triacetate II was 1.315 g/ cc, which is less than the calculated density of 1.348 g/cc as expected because cellulose triacetate is not 100% crystalline. The above studies on the crystalline structure of cellulose triacetate lead to the conclusion that commercial heat-treated cellulose triacetate is expected to have the cellulose triacetate II crystalline structure. Analysis of the crystal structure of cellulose triacetate continues [55]. 

Another way to learn of the likely molecular shapes for cellulose depends on extrapolation of the shapes that are found in crystal structures of molecules such as cellobiose (Chu and Jeffrey 1968), a-cellobiose complexed with Nal and HjO (Peralta-Inga et al. 2002), cellobiose octaacetate (Leung et al. 1976) and related compounds (French and Johnson 2004a). Similar, although less accurate. 

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