Ordering antiparallel

A special case is if the methylene units in the even-even PA are in the diamine as long as in the diacid (as in PA-4,6, -6,8, and -8,10) so that an ordering in both a parallel and an antiparallel fashion is possible (Figs. 3.4a, b) 1" Because of this feature, these polymers have the highest order. 

Three welan helices pass through the unit cell, as shown in Fig. 37b. Two of them, I and II, at (2A A0) and ( A2A0), are antiparallel as in gellan. A third helix, III, at (000) is new and parallel to the first. They are equilaterally 12.0 A apart from each other, 2.9 A farther than in 41. The unit cell contains a total of nine pentasaccharides surrounded by ordered guest molecules, which are accounted by six calcium ions and 75 water molecules. The two up and down helices interact via side chains, calcium ions, and/or water molecules. Specifically, helices I and II are linked only by side chain side chain hydrogen bonds... 

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). 

The frustration effects are implicit in many physical systems, as different as spin glass magnets, adsorbed monomolecular films and liquid crystals [32, 54, 55], In the case of polar mesogens the dipolar frustrations may be modelled by a spin system on a triangular lattice (Fig, 5), The corresponding Hamiltonian consists of a two particle dipolar potential that has competing parallel dipole and antiparallel dipole interactions [321, The system is analyzed in terms of dimers and trimers of dipoles. When the dipolar forces between two of them cancel, the third dipole experiences no overall interaction. It is free to permeate out of the layer, thus frustrating smectic order. 

A sequence, in general, is the relative order of base pairs, whether in a fragment of a protein, DNA, a gene, a chromosome, or an entire genome. DNA is composed of two antiparallel strands of deoxynucleotides held together by hydrogen bonds between purine (adenine, A and guanine, G) and pyrimidine (thymidine, T uracil, U and cytosine, C) bases. 

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