Antiparallel chains

Figure 26.6 (a) The /3-pleated sheet secondary structure of proteins is stabilized by hydrogen bonds between parallel or antiparallel chains, (b) The structure of concanavalin A, a protein with extensive regions of antiparallel / sheets, shown as flat ribbons. 

According to a recent report, the unit cell of cellotetraose hemihydrate in single crystals contains two antiparallel chains, which are conformationally distinct—especially in the sugar geometries.74 However, all hydroxymethyl groups adopt similar gt orientations. Whether this oligosaccharide morphology can be implemented for cellulose II in fibers remains to be seen. 

Fig. 8.—Packing arrangement of four symmetry-related 2-fold helices of mannan II (6). (a) Stereo view of two unit cells approximately normal to flic frc-plane. The two chains in the back (open bonds) and the two in the front (filled bonds) are linked successively by 6-0H-- 0-6 bonds. The front and back chains, both at left and right, are further connected by 0-2 -1V -0-2 bridges, (h) Projection of the unit cell along the c-axis the a-axis is down the page. This highlights the two sets of interchain hydrogen bonds between antiparallel chains, distinguished by filled and open bonds. The crossed circles are water molecules at special positions.

Extrapolation of the molecular structure of an a-maltohexaose duplex com-plexed with triiodide in single crystals leads to a left-handed, 8-fold, antiparallel double-helix for amylose.90 The pitch of this idealized helix is 18.6 A, so h is only 2.33 A. Although this model is no contender to the fiber data, in terms of biosynthesis, it is doubtful that the native amylose helix favors antiparallel chains. 

Fig. 18.—Antiparallel packing arrangement of 2-fold poly(ManA) (15) helices, (a) Stereo view of two unit cells roughly normal to the hoplane. The helix at the center (filled bonds) is antiparallel to the two in the back (open bonds). Intrachain hydrogen bonds stabilize each helix. Association of helices through direct hydrogen bonds involve the carboxylate groups for parallel chains, but involve the axial hydroxyl groups for antiparallel chains, (b) A view of the unit-cell contents down the t-axis highlights the interactions between the helices.

FlC. 32.—Antiparallel packing arrangement of the 2-fold helices of calcium chondroitin 4-sulfate (35). (a) Stereo view of two unit cells approximately normal to the he-plane. The two comer chains, drawn in filled bonds are hydrogen bonded to the antiparallel center chain (open bonds). Calcium ions (crossed circles), associating with sulfate and carboxylate groups and water molecules link adjacent antiparallel chains, which ate also directly hydrogen bonded. 

Finally, a highly complex structure is created when /ra s-Cd(4-ampy)2 (4-ampy = 4-aminopyri-dine) and Cd(4-ampy)(ame) Ag(CN)2 units (ame = 2-aminoethanol) are linked by bridging //- Ag(CN)2 groups to form a 2-D network with rhombus meshes and almost linear chains. These antiparallel chains in pairs embrace the nets.221... 

The space group is P2i2i2 . The unit cell is pseudotetragonal, with a = b = 19.17 A (1.917 nm), and c = 24.39 A (2.439 nm), with two antiparallel chains per cell. The amylose chain is a left-handed 6(—1.355) helix, with three turns per crystallographic repeat. One molecule of dimethyl sulfoxide for every three D-glucose residues is located inside the helix. An additional 4 molecules of dimethyl sulfoxide and 8 of water are located in the interstices. The interstitial dimethyl sulfoxide is the source of additional layer-lines that are not consistent with the 8.13 A (813 pm) amylose repeat. The overall R factor is 35%, and, for the layer lines with amylose contribution alone, it is 29%. 

Fig. 1. The basic arrangements of /f-strands in hydrogen-bonded /f-sheets (A) parallel chains, (B) antiparallel chains. Green spheres of different sizes denote side chain groups...

The question of parallel vs. antiparallel chain packing in cellulose I has been a controversial one practically since the first cellulose structure was proposed. A consensus appears to be forming, however, based on both diffraction analysis and other experimental evidence, that one of two possible... 

As the above results show, the gross features of the cellulose I crystal structure predicted by various methods do not differ appreciably, but the accompanying deviations in the R -factors are significant. When these predictions are used to assess, for example, whether the cellulose I crystal structure is based on parallel- or antmarallel-chains, the range in the R"-factors seen for the parallel models (cf. Table II) is comparable to that between the two different polarity models. As shown in Fig. 5, the most probable parallel- and antiparallel-chain structures of cellulose I, refined by minimizing the function O, differ in R -factors by approximately the same extent as the three predictions for the parallel model shown in Fig. 4 and Table II. 

Finally, the question of the ability of the modeling methods to predict the crystal lattice (i.e., the unit cell) from the conformation of the chain should be addressed, despite the expected computational difficulties. Based on previous work, in which the prediction of the unit cells of all four cellulose polymorphs in both parallel and antiparallel chain packing polarities was... 

A molecular structural property wherein a rotation of 180° leads to the same superimposable structure. Because proteins are made up of L-amino acid residues, neither a single subunit nor a multisubunit protein can possess a plane or point of symmetry. However, proteins do exhibit dyad symmetry. This structural property allows many DNA-binding proteins to make equivalent contacts to the two antiparallel chains of the nucleic acid. 

PVDF is mainly obtained by radical polymerisation of 1,1-difluoroethylene head to tail is the preferred mode of linking between the monomer units, but according to the polymerisation conditions, head to head or tail to tail links may appear. The inversion percentage, which depends upon the polymerisation temperature (3.5% at 20°C, around 6% at 140°C), can be quantified by F or C NMR spectroscopy [30] or FTIR spectroscopy [31], and affects the crystallinity of the polymer and its physical properties. The latter have been extensively summarised by Lovinger [30]. Upon recrystallisation from the melted state, PVDF features a spherulitic structure with a crystalline phase representing 50% of the whole material [32]. Four different crystalline phases (a, jS, y, S) may be identified, but the a phase is the most common as it is the most stable from a thermodynamic point of view. Its helical structure is composed of two antiparallel chains. The other phases may be obtained, as shown by the conversion diagram (Fig. 7), by applying a mechanical or thermal stress or an electrical polarisation. The / phase owns ferroelectric, piezoelectric and pyroelectric properties. 

The sodium salt form, at 92% r.h., crystallizes in a trigonal unit-cell, with a = b = 1.45 nra and c = 2.88 nm, with a 3(— 0.96) conformation of the chain. On conversion of the sodium form into the calcium form, the unit cell converts to the orthorhombic, with a = 0.745 nm, b = 1.781 nm, and c = 1.964 nm. The chain is a 2(0.982) helix, with the disaccharide repeat-unit. Two antiparallel chains are contained in the unit cell, along with— 30 water molecules. The space group is P2,2,21. 

Putting together much published data, Watson and Crick postulated that native DNA consists of two antiparallel chains in a right-handed double-helical arrangement. Complementary base pairs, A=T and G C, are formed by hydrogen bonding within the helix. The base... 

P-Poly(L-alanine) in antiparallel-chain pleated sheet -139 +135... 

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