Chain packing polarity

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... 

When crystalline cellulose I is treated with aqueous alkali solutions of sufficient strength, a process known as mercerization takes place. As a result of it, cellulose I is converted to cellulose II, the most stable or the four crystalline cellulose polymorphs. The conversion proceeds in the solid state, without apparent destruction or change in the fibrous morphology of the cellulose. As our diffraction analysis indicates, however, it is accompanied by a reversal of the chain packing polarity—from the parallel-chain cellulose I to the... 

The major difference between these two crystal structures resides In the chain packing polarity. As expected from the conversion studies and the Irreversibility of the cellulose I to Na-cellulose I transformation, the crystal structure of Na-cellulose I Is based on antiparallel chains (cf. Fig. 3). Because of the presence of Na Ions, which apparently form secondary bonds with the cel-... 

The solubility selectivity of a membrane for a specific gas pair could be increased (in principle) by inducing specific interactions between the polymer and the more soluble component of the gas pair. For example, the substitution of certain polar groups in some rubbery polymers has been found to increase their solubility selectivity for CO2 relative to CH4 (Story and Koros, 1991 Koros, 1985). Unfortunately, the increase in the polarity of a polymer also tends to increase its chain packing density, and as a result, decreases the gas diffusivity in membranes made from that polymer. 

Crystals of most polar lipids can swell in the presence of water. The corresponding phases, gel-phases, with lamellarly packed lipid, and water layers, are sometimes thermodynamically stable (Larsson, 1994, p. 41). Also, the hydrocarbon chain packing of gel-phases usually show some axial rotational disorder. The alkyl chain cross-sectional area is close to 20 A2 in a plane perpendicular... 

Most (soluble) folded proteins have a hydrophobic core in which side-chain packing stabilises the folded state, and charged or polar side chains are placed on the solvent-exposed surface, where they interact with surrounding water molecules. It is generally accepted that rninirnising the number of hydrophobic side chains exposed to water is the principal driving force behind the folding process. 

Figure 1.8. Some residues near the surface of another protein. CRID is used to elucidate why the polar arginine and nonpolar tryptophan side chains pack so closely together. See text.

This domain consists of a bundle of a-helices packed in pairs against each other. The most common arrangement is illustrated in Fig. 6.1. The space between the four helices is occupied by hydrophobic side chains, whereas polar side chains are directed towards the surrounding solution. The a-helices are twisted with respect to each other, and their arrangement is similar to that of a fragment of a blue phase (c/. Figure 5.5). In both cases, the chirality of the structural units leads to hyperbolic curvature within the aggregate. 

In Figure 6, one can see, as with amorphous materials, that the more unbalanced the biaxial orientation is the greater are the reductions in permeability. Interpretations of the transport data for biaxially and uniaxially drawn PET samples can be explained by observing conformational changes in the polymer backbone itself. Apparently, the chain packing efficiency of the amorphous phase improves as the number of trans isomers in the ethylene glycol unit increases. Polarized infrared analysis of uniaxially and biaxially oriented systems indicates that the fraction of... 

In aqueous solution, they aggregate with their hydrocarbon chains packed inside, surrounded by the polar head groups. Cellular membranes are constructed this way, with a pair of layers of side-byside lipids facing each other. 

---