Axial packing 3-sheets

Fig. 22.—Antiparallel packing arrangement of the 2-fold helices of (1— 3)-a-D-glucan (21). (a) Stereo view of two unit cells approximately normal to the aoplane. The two chains in the back (open bonds) are antiparallel and so are the chains in the front (filled bonds). Each helix is stabilized by 2-OH 0-4 hydrogen bonds across the bridge oxygen atoms. Interchain hydrogen bonds are formed in sheets along the a direction, (b) An axial projection of the unit cell shows that the sheets in the front and back are also joined by hydrogen bonds.

The sheets of hydrogen-bonded cellulose chains (lying in the a-c plane of Figure 2.3d) stack on top of one another in the b-direction to form a three-dimensional crystalline structure (Figure 2.3c). In the b-direction of the unit cell the atoms that project out axially are hydrogen atoms. Fortunately, these atoms are very small so the cellulose layers can pack very close (c. 0.39 nm between layers) so close that the van der Waals forces stabilize this tight packing. Also, there is the further... 

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