Chain arrangement

Figure 1.1 The amino acid sequence of a protein s polypeptide chain is called Its primary structure. Different regions of the sequence form local regular secondary structures, such as alpha (a) helices or beta (P) strands. The tertiary structure is formed by packing such structural elements into one or several compact globular units called domains. The final protein may contain several polypeptide chains arranged in a quaternary structure. By formation of such tertiary and quaternary structure amino acids far apart In the sequence are brought close together in three dimensions to form a functional region, an active site.

The chain arrangement of this morphology was schematically proposed as in Fig. 10. The cell of the microsphere has a hexagonal surface, and the AB diblock copolymers form a bilayer between the microspheres. From this schematic arrangement, the optimal blend ratio of the AB block copolymer in this system was calculated as 0.46. This value was very close to the blend ratio of the AB type block copolymer 0.5 at which the blend showed the hexagonal packed honeycomb-like structure. 

Fibrous protein (Section 26.9) A protein that consists of polypeptide chains arranged side by side in long threads. Such proteins are tough, insoluble in water, and used in nature for structural materials such as hair, hooves, and fingernails. 

Melting of ECC involving transition into the isotropic melt was shown by Flory to be a first-order process. It can be seen in Fig. 18 b that there occurs a transition from a complete order to a fully random chain arrangement in the isotropic melt (Fig. 16, point 4). 

Omstein [276] developed a model for a rigidly organized gel as a cubic lattice, where the lattice elements consist of the polyacrylamide chains and the intersections of the lattice elements represent the cross-links. Figure 7 shows the polymer chains arranged in a cubic lattice as in Omstein s model and several other uniform pore models for comparison. This model predicted r, the pore size, to be proportional to I/Vt, where T is the concentration of total monomer in the gel, and he found that for a 7.5% T gel the pore size was 5 nm. Although this may be more appropriate for regular media, such as zeolites, this model gives the same functional dependence on T as some other, more complex models. 

Normal hemoglobin molecules are complex, three-dimensional structures consisting of four chains of amino acids known as polypeptide chains. Two of these chains are known as alpha subunits with 141 amino acid residues each, and the remaining polypeptide chains are the beta subunits with 146 amino acid residues each. The sequences of amino acids in the alpha and beta subunits are different, but fold up via noncovalent interactions to form similar three-dimensional structures. When a polypeptide chain arranges itself in space, i.e., when it folds, amino acids that were far apart in the chain are brought closer in proximity. The final overall shape of the protein molecule is influenced by (1) the amino acids in the chain, and (2) the interactions that are possible between distant amino acids. 

We note here that all the information presently available on high molecular weight polymer crystal structures is compatible with the bundle model. While very nearly all crystalline polymer polymorphs involve all-parallel chain arrangements, even the only known exception, namely y-iPP [104,105], where chains oriented at 80° to each other coexist, is characterized by bilayers of parallel chains with opposite orientation. This structure is thus easily compatible with crystallization mechanisms involving deposition of bundles of 5-10 antiparallel stems on the growing crystal surface. Also the preferred growth... 

In case of [M(edt)2] based salts, the size of the small anion is similar to the size of the CsMes ligand of the cation and only type I structural motives (D+ A D+ A-chains) were observed. For the intermediate size anionic complexes, [M(tdx)2], [M(mnt)2] and [Ni(a-tpdt)2], the most common structural motive obtained in salts based on those anions is also of type I. For the larger anionic complexes, [M(bdx)2] and [M(dmix)2], types III and IV chain arrangements were observed. In both cases anion molecules (type IV) or face-to-face pair of anions (type III) alternate with side-by-side pairs of cations. The complexes [M(mnt)2] and [M(dmix)2] (M = Ni, Pd and Pt) frequently present dimerization in the solid state [19], and they are the only anions where the chain arrangements present face-to-face pairs of anions (structural motives II and III). The variety of structural... 

Besides the decamethylmetallocenium salts, in the compounds based on other metallocenium derivatives, mixed linear chain arrangements were only observed in the case of the salts [Fe(C5Me4SCMe3)2][M(mnt)2], M = Ni and Pt, which present type I structural motives. 

This type of chain arrangement was observed only in the case of a-[Fe(Cp )2][Pt (mnt)2]. In the crystal structure of this salt, layers of parallel type II chains, with a net charge (—) per repeat unit, [A2]2 D+, alternate with layers presenting a D+D+ A-... 

The usual alkali employed is lime. The raw material for gelatine is tropocollagen, which is present in the original hides or bones. This protein consists of three polypeptide chains arranged in a triple helix. In contrast, gelatine consists of several free or interassociated chains, ranging in molecular weight from around ten thousand to several hundred thousand. On extraction, monomers (a-chains MW 100 000), dimers (P-chains) and trimers ( -chains) and some lower order peptides are released. 

FIGURE 2.16 Spherulite structure showing the molecular-level lamellar chain-folded platelets and tie and frayed chain arrangements (a), and a more complete model of two sets of three lamellar chain-folded platelets formed from polyethylene (PE) (b). Each platelet contains about 850 ethylene units as shown here. 

FIGURE 13.4 Simple chain arrangement for a linear polymer containing both ordered and unordered structures. 

On a molecular level, partially crystalline to amorphous polymers are normally used. As the material is heated, Brownian motion occurs resulting in a more random chain arrangement. When a unidirectional force is applied to a resting polymer melt, the chains tend to move away from the applied force. If the applied force is slow enough to allow the Brownian movement to continue to keep the polymers in a somewhat random conformation, the movement of the polymer melt is proportional to the applied stress, i.e., the flow is Newtonian. 

Proteins may be structural, functional, or catalytic. Structural proteins are frequently fibrous proteins (insoluble polypeptide chains arranged side by side in long filaments)... 

It is interesting to note in Figs. 13 and 14 that the relaxation behavior of PS(OH)-18/PMMA and PS(OH)-4/PMMA is indistinguishable. However, as shown in the next section, these blends have quite different chain arrangements. This implies the limitation of routine NMR relaxation time measurements for monitoring the blend structure at the molecular level. 

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