Type of chain folding

We would further expect, in analogy with crystalline polymers, different types of chain folding in atactic polymers, as follows ... 

Figure 9 Types of chain ordering and folding which are possible within and between lamellae and between ribbon surfaces. In real, well-crystallized polymers, these variations he relatively far apart and the forms SC". CF, SB. and A predominate. (From Kef, 132.)...

The secondary level of structure in a protein is the regular folding of regions of the polypeptide chain. The two most common types of protein fold are the a-helix and the P-pleated sheet. In the rod-like a-helix, the amino acids arrange... 

Upon release of the two types of chain, a folding of the polypeptides occurs to obtain a stable physicochemical configuration. The assembly of an ajS unit rather than an 2 or 2 dimer is also based on favorable steric arrangements. There is evidence that the heme-globin association occurs after the release of the completed chain from the ribosomes and probably after the formation of the dimer. The formation of a stable tetramer follows soon after the heme-protein association has taken place. 

FIGURE 1.1 Schematic representation of the structure of plastics, showing the three major types of macromolecular arrangements. Approximately 1 000000 times actual size and greatly simplified. (Crystallites can also occur as the result of chain folding.)... 

Finally, it should be noted that, although the kinetic theories can explain much observed experimental data, other theories are also under consideration, including some that postulate that crystallisation may take place via metastable phases, i.e. types of unit cell that occur only in the early stages of crystallisation and are subsequently converted into the observed final type of unit cell. These theories may have implications for the xmderstanding of chain folding. 

At low degrees of crystallinity in the bulk state, a fringed micelle type of model (admitting the possibility of chain folding) may be appropriate, whereas at higher degrees of crystallinity, a paracrystalline type of model such as is depicted in Figure 1.7 may better reflect reality. 

Before applying the above analysis to experimental results, there are several important factors than need to be kept in mind. As was pointed out above, except for constant factors the value of AG is independent of the chain conformation. Thus, any type of chain structure that is used to analyze the temperature coefficient of growth is merely an assumption. This is true, irrespective of whether the chains in the nucleus are bundle-like, regularly folded, or, in fact, of any other type. Put another way, a definitive chain structure, or chain conformation, within the nuclei cannot be deduced safely from an analysis of the temperature coefficient of growth. 

Figure 7.31 Simple model of lamellar stack showing two adjacent crystal lamellae and the two types of chain present random tie chain and tight, (regular) fold.

It is possible to derive an expression by which the fraction of tight folds can be calculated under the simple assumption that only two types of chain leave the crystals, namely perfectly tight folds which occupy no space in the amorphous interlayer and chains performing a random walk in the amorphous interlayer ending in the adjacent crystal (Fig. 7.31). The number of bonds n) in a typical Gaussian amorphous chain sequence is given by ... 

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