Interchain hydrogen bonding

The copolymers are insoluble in water unless they are neutralized to some extent with base. They are soluble, however, in various ratios of alcohol and water, suggesting appHcations where deUvery from hydroalcohoHc solutions (149) but subsequent insolubiUty in water is desired, such as in low volatile organic compound (VOC) hair-fixative formulations or tablet coatings. Unneutralized, their Ts are higher than expected, indicating interchain hydrogen bonding (150). 

It must be pointed out that deviations from such a simple relationship do occur. For example, since random copolymerisation tends to promote disorder, reduce molecular packing and also reduce the interchain forces of attraction, the Tg of copolymers is often lower than would be predicted by the linear relationship. Examples are also known where the Tg of the copolymer is higher than predicted. This could occur where hydrogen bonding or dipole attraction is possible between dissimilar comonomer residues in the chain but not between similar residues, i.e. special interchain forces exist with the copolymers. 

Polyamides such as nylon 6, nylon 66, nylon 610, nylon 11 and nylon 12 exhibit properties which are largely due to their high molecular order and the high degree of interchain attraction which is a result of their ability to undergo hydrogen bonding. 

If the protein of interest is a heteromultimer (composed of more than one type of polypeptide chain), then the protein must be dissociated and its component polypeptide subunits must be separated from one another and sequenced individually. Subunit associations in multimeric proteins are typically maintained solely by noncovalent forces, and therefore most multimeric proteins can usually be dissociated by exposure to pEI extremes, 8 M urea, 6 M guanidinium hydrochloride, or high salt concentrations. (All of these treatments disrupt polar interactions such as hydrogen bonds both within the protein molecule and between the protein and the aqueous solvent.) Once dissociated, the individual polypeptides can be isolated from one another on the basis of differences in size and/or charge. Occasionally, heteromultimers are linked together by interchain S—S bridges. In such instances, these cross-links must be cleaved prior to dissociation and isolation of the individual chains. The methods described under step 2 are applicable for this purpose. 

FIGURE 7.27 The structure of cellulose, showing the hydrogen bonds (blue) between the sheets, which strengthen the structure. Intrachain hydrogen bonds are in red and interchain hydrogen bonds are in green. 

The DNA isolated from different cells and viruses characteristically consists of two polynucleotide strands wound together to form a long, slender, helical molecule, the DNA double helix. The strands run in opposite directions that is, they are antiparallel and are held together in the double helical structure through interchain hydrogen bonds (Eigure 11.19). These H bonds pair the bases of nucleotides in one chain to complementary bases in the other, a phenomenon called base pairing. 

Fig. 5.—Parallel packing arrangement of the 2-fold helices of chitin I (3). (a) Stereo view of two unit cells approximately normal to the hc-plane. The two comer chains (open bonds), separated by b, in the back are hydrogen bonded to the comer chains (tilled bonds) in the front, (b) Projection of the unit cell along the c-axis with a down and b across the page. The diagonal orientation of the sugar rings facilitates interchain hydrogen bonds involving the JV-acetyl moieties along the a-axis.

Fig. 21.—Structure of the 6-fold anhydrous curdlan III (19) helix, (a) Stereo view of a full turn of the parallel triple helix. The three strands are distinguished by thin bonds, open bonds, and filled bonds, respectively. In addition to intrachain hydrogen bonds, the triplex shows a triad of 2-OH - 0-2 interchain hydrogen bonds around the helix axis (vertical line) at intervals of 2.94 A. (b) A c-axis projection of the unit cell contents illustrates how the 6-0H - 0-4 hydrogen bonds between triple helices stabilize the crystalline lattice.

Fig. 24.—(a) Stereo view of slightly over a turn of the 3-fold double helix of i-carrageenan (23). The two chains are distinguished by open and filled bonds for clarity. The vertical line is the helix axis. Six interchain hydrogen bonds per turn among the galactose residues stabilize the double helix. The sulfate groups lined up near the periphery are crucial for intermolecular interactions. 

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