Antiparallel packing

Fig. 4.—Antiparallel packing arrangement of the 2-fold helices of cellulose II (2). (a) Stereo view of two units cells approximately normal to the ac-plane. The two comer chains (open bonds) in the back form a hydrogen-bonded sheet. The center chain (filled bonds) is linked to the comer chains by hydrogen bonds, (b) Projection of the unit cell along the c-axis and a is down the page. 

Fig. 9. — Antiparallel packing arrangement of the 3-fold helices of (1— 4)-(3-D-xylan (7). (a) Stereo view of two unit cells roughly normal to the helix axis and along the short diagonal of the ab-plane. The two helices, distinguished by filled and open bonds, are connected via water (crossed circles) bridges. Cellulose type 3-0H-0-5 hydrogen bonds stabilize each helix, (b) A view of the unit cell projected along the r-axis highlights that the closeness of the water molecules to the helix axis enables them to link adjacent helices.

Fig. 14.—Antiparallel packing arrangement of extended, 4-fold, 2,3,6-tri-O-ethylamylose (12) helices, (a) Stereo view of two unit cells approximately normal to the lie-plane. The helix at the center (filled bonds) is antiparallel to the two helices (open bonds) at the comers in the back. There is no intra- or inter-chain hydrogen bond, and only van der Waals forces stabilize the helices, (b) A e-axis projection of the unit cell shows that the ethyl groups extend into the medium in radial directions. 

Fig. 28.—Antiparallel packing arrangement of 4-fold helices of sodium hyaluronate (26). (a) Stereo view of a unit cell approximately normal to the hc-plane. The two comer chains in the front (filled bonds) are linked directly by hydrogen bonds. The chain at the center (open bonds) interacts with die comer chains via sodium ions (crosses circles) and hydrogen bonds.

Fig. 35. (continued)—(b) Antiparallel packing arrangement of two double helices, drawn in open and filled bonds, in the trigonal unit-cell projected along the c-axis. 

Fig. 39. (continued)—antiparallel packing arrangement of helices in the orthorhombic unit cell viewed down the c-axis shows considerable interdigitation. Hydrogen bonds (not shown) connect adjacent chains. 

In spite of the alteration due to deacetylation, chitosan from crab tendon possesses a crystal structure showing an orthorhombic unit cell with dimensions a = 0.828, b = 0.862 and c = 1.043 nm (fiber axis). The unit cell comprises four glucosamine units two chains pass through the unit cell with an antiparallel packing arrangement. The main hydrogen bonds are 03 05 (intramolecular) and N2 06 (intermolecular) [82]. This material has also found medical uses (below). 

The unit cell is pseudohexagonal, with a = 10.87 A (1.087 nm), b = 18.83 A (1.883 nm), and c = 52.53 A (5.253 nm). The chain contains 14 monomers in three turns of the left-handed helix. Antiparallel packing of the chains yields the best fit with the X-ray data and the least number of close contacts. The overall R factor is 58%, and it is 30% for the zero, third, and sixth layer reflections alone. 

Using the two-chain unit-cell,3 with a = 0.817 nm, b = 0.785 nm, c = 1.034 nm, andy = 96.38°, the modified intensity-data of Mann and coworkers,37 and several residue-geometries, the structure of native ramie cellulose was refined. The resulting R factors were 15.8%, 18.5%, and 17.5% for, the antiparallel, parallel-up, and parallel-down models, respectively. A temperature factor of 0.23 nm2 was necessary in order to obtain a good fit with the observed data. It was suggested that the antiparallel packing of the chains cannot be discounted for cotton and ramie celluloses. 

Almost all unit cells shown in Table I are either orthorhombic or pseudo-orthorhombic, with a majority of space groups P2] 2] 2] and P2. Only a few structures exhibit higher symmetry and none shows lower symmetry. All structures have an antiparallel packing of chains (however, see A- and B-amyloses). On the other hand, a large variety of helix characteristics are evident, in addition to the variability in the unit cell dimensions. Some of the features useful for classifying amylose structures are shown in Table II. The distance between the two nearest antiparallel-... 

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