Antiparallel packing arrangement

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)—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). 

Fig. 30. — Packing arrangement of 4-fold antiparallel double helices of potassium hyaluronate (32). (a) Stereo view of a unit cell approximately normal to the line of separation of the two helices. The two chains in each duplex, drawn in open and filled bonds for distinction, are linked by not only direct hydrogen bonds, but also water bridges. Inter double-helix hydrogen bonds are mediated between hydroxymethyl and iV-acetyl groups. Potassium ions (crossed circles) at special positions have only a passive role in the association of hyaluronate chains.

Fig. 37. (continued)—(b) An axial view projected along the r-axis shows the packing arrangement of three welan double helices in the trigonal unit cell. The helix drawn in solid bonds is antiparallel to the remaining helices (open bonds). Note that calcium ions are positioned between the helices and each water molecule (large open circle) shown here is connected to all three surrounding helices. The interstitial space is occupied by several other ordered water molecules (not shown). 

Cholesterol esters form crystalline structures that are similar to those formed by other lipids, consisting of alternating infinite lamellae, so that the hydrocarbon chains form close-packed sheets segregated from layers of cholesterol skeletons. There are three t) s of such structures [6]. One such can be represented by the chiral molecule cholesterol oleate, where pairs of cholesterol skeletons are arranged in an antiparallel packing in one layer, with the hydrocarbon chains in the adjacent layer. The cross-sectional area of the cholesterol molecule is about 40 A2 (derived from pressure-area monolayer curves), corresponding to the cross-sectional area of two hydrocarbon chains. The chains therefore form an interpenetrating layer. 

Studies with other poly(/3-i -aspartate)s demonstrate that these polymers not only adopt conformational patterns that are similar to poly(a-amino acids), but that they exhibit greater conformational versatility. The range of conformations now include extended chain structures, arranged as antiparallel packings that come about by stretching poly(a-methyl-/3-L-aspartate) films in boiling water.209 In solution, the helix—coil conformational transition is a phenomenon common to the whole family of poly-(a-alkyl-/3-L-aspartates).210 The ordered conformation is responsive to environmental factors such as temperature and solvent in much the same way as for poly(a-peptides). 

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