Interactions with neighboring chains

Individual chains are planar with alternating cis—trans conformation along the chain, and the interaction with neighboring chains leads to a highly ordered matrix. 

Fig. 16. Sketch of selected chain in the amorphous layers of LPE, subject to localized constraints of the motion (O) due to the interaction with neighboring chains. Through these constraints the chain is divided into flexible units. A selected deuteron 2H on unit j is shown for convenience Fig. 17. Fractions of flexible units in the amorphous layers of LPE as obtained from the analysis of 2H NMR line shapes displayed in Fig. 15...

In the bulk state each polymer molecule is surrounded by other polymer molecules of the same type. Expansion of a given chain to relieve long-range intramolecular steric interactions only serves to create an equal number of intermolecular steric interactions with neighboring chains. These opposing volume exclusion effects exactly counteract each other and so in a bulk amorphous polymer the polymer molecules adopt their unperturbed dimensions (i.e., a = 1). 

The free radicals -NH—CH2— are very unstable and may be converted into the amine by interaction with neighboring chains. 

In the Gaylord-Douglas model [57,81] the chains are localized in a tube defined by the interactions with neighboring chains. The first term of the elastic free energy is the same as that of a phantom network model, while the second term accounts for the loss of degrees of freedom of the chains due to chain localization. In the dry network the cross-sectional dimension of the tube is of the order of the hard-core cross-sectional radius of the polymer chain, and the volume of the tube is comparable with the chain molecular volume. The tube volume is considered to be invariant with macroscopic strain, since the molecular volume of the chains is independent of the deformation. The elastic free energy is given by... 

The globular protein is unfolded during texturization by breaking the intramolecular binding forces. The resultant extended protein chains are stabilized through interaction with neighboring chains. In practice, texturization is achieved in one of two ways ... 

After having discussed the adsorption behavior of a single chain, a word of caution is in order. Experimentally, one never looks at single chains adsorbed to a siuface. First, this is because one always works with polymer solutions, where there is a large number of polymer chains contained in the bulk reservoir, even when the bulk monomer (or polymer) concentration is quite low. Second, even if the bulk polymer concentration is very low, and in fact so low that polymers in solution rarely interact with each other, the siuface concentration of polymer is enhanced relative to that in the bulk. Therefore, adsorbed polymers at the smface usually do interact with neighboring chains, owing to the higher polymer concentration at the siuface [34]. 

In the case of polymerization of multifunctional monomers (with two double bonds or at least three functional groups) one obtains in the first reaction step linear polymers, containing reactive groups, able to interact with neighboring chains and form cross linkages between polymer chains. As result one obtains a spatially cross-linked polymer, forming a very large 3D macromolecule (a block polymer can be just one macromolecule). Such pol mers are usually insoluble and do not melt. 

Another example of crystal lattice affected by disorder due to isomorphic substitution of different chains is provided by the a form of iPP (Fig. 2.15) [29,101,102]. As shown in Figure 2.25a, in the crystal the threefold helical chains of iPP of a given chirality may be up or down depending on the orientation of the bond vector b that connects the backbone methine carbon atoms to the lateral methyl carbon atoms, with respect to the c-axis. Isomorphous up and down (anticlined) [16] chains have a similar external steric hindrance because the methyl groups may assume the same position (Fig. 2.25b). Therefore, disorder in the positioning of up and down chains may be present in the crystals of the a form because if isomorphic and anticlined helices of iPP substitute each other in the same site of the lattice, the same steric interactions with neighboring chains are involved. 

Very often, however, these dyads are not isolated in the solid state and interact with neighboring ones at least along one preferential direction. In this case, we can distinguish two important situations the alternated spin chain and the spin ladder. As shown in Scheme 5, the alternated spin chain is characterized with two different magnetic interactions, noted / and aJ with 0 < a < 1. Note that if a = 0, one recovers the singlet-triplet behavior while if a = 1, we are in the presence of a uniform spin chain. The spin ladder is also characterized by two J values, noted J// and J in the following. 

It was reported that such systems exhibit cooperative stabilization of the complex, which means that the state and behaviour of every unit mainly depends on its interactions with neighboring units. Moreover, a minimal chain length is required for stable complex formation a polybase, which forms a complex with a long chain of polyacid, must have a degree of polymerization larger than a critical number corresponding on the minimum chain length (5-7. ... 

For all the types of side chains considered, with various lengths and flexibilities, the interactions of a side chain with neighboring chain units lead to a gread hindrance to intragroup motions (rint. m.c.)- Simultaneously, a distinct effect of the change in the side chain structure or the transition to a more mobOe chain segment on the parameters of IMM is observed both for high-frequency motions and for slow relaxation processes. 

The n-3 PUFAs are important components of cell membranes throughout the body, as they are incorporated into the phospholipids that form cell membranes. Each phospholipid molecule is comprised of a headgroup to which fatty acid esters are bound. There are two binding positions, termed 5 -l and sn-l. The acyl chains that bind to these headgroups interact with other chains in neighboring phospholipid molecules within the bilayer, and the level of chain interaction determines the biophysical properties of the membrane. The sn-l position is usually occupied by a saturated fatty acid the sn-2 position is usually occupied by a relatively unsaturated fatty acid chain. 

Each of the histone proteins making up the nucleosome core contains a flexible amino terminus of 11-37 residues extending from the fixed structure of the nucleosome these termini are called histone tails. Each H2A also contains a flexible C-termlnal tall (see Figure 10-20b). The histone tails are required for chromatin to condense from the beads-on-a-string conformation Into the 30-nm fiber. Several positively charged lysine side chains In the histone tails may interact with linker DNA, and the tails of one nucleosome likely interact with neighboring nucleosomes. The histone tail lysines, especially those In H3 and H4, undergo reversible acetylation and deacetylation by enzymes that act on specific lysines In the N-termlnl. In the acetylated form, the positive... 

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