Regular folds

Figure 17. Schematic lepiesentation of a lamellar polyethylene single ciystal. (a) and (b) show enlaiged features of its stnicture in (o) the zigzag conformation of the macromolecular chain inside the crystal in (b) a hypothetical model for the regular folding, drawn according to the calculation reported in ref. 224a.

The term secondary structure refers to the local conformation of some part of a polypeptide. The discussion of secondary structure most usefully focuses on common regular folding patterns of the polypeptide backbone. A few types of secondary structure are particularly stable and occur widely in proteins. The most prominent are the a helix and /3 conformations described below. Using fundamental chemical principles and a few experimental observations, Pauling and Corey predicted the existence of these secondary structures in 1951, several years before the first complete protein structure was elucidated. 

For this cycloalkane, the Tic s of the methylene carbons in the folded sequence are estimated to be 35-39 s at room temperature and become shorter with increasing temperature [66]. Hence, the Tic s for the regularly folded methylene sequence in polyethylene, if it exists, will be appreciably shorter than 35 s at room temperature, and much shorter than ca. 220 s of the inner methylene carbons of polyethylene (cf. Table 1). Hence, if we measure the DD/CP 13C NMR spectrum of the solution-grown polyethylene sample by a single pulse sequence with a rep-... 

Figure 12 shows that the fold of the 110 sector yields stems inclined to the lamellar normal. Regular folds have been identified in solution-grown crystals by infrared spectroscopy from the detailed spectrum of the CH2... 

Fig. 14 Schematic description of how overcrowding of the amorphous phase is avoided by adjacent, regular folding...

A protein will generally have one or more random coil regions in its overall structure Random coils, as the name suggests, do not have a regular, folded structure and undergo... 

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... 

A detailed model for the single crystal was introduced by Keller in 1957 for polyethylene. The chains (Figure 4.16) are more or less regularly folded in lamellae. 

As pointed out earlier, the fluorine atoms are too large to allow planar zigzag structure, which confers rigidity on the polymer.20 The PTFE molecule has a regular folded structure, which produces a laminar crystal.16... 

There is another less direct way, however, in which heat could be the energy source of contraction. It could be consumed in an endothermic chemical reaction which in turn leads, not to a statistical coiling, but to a regular folding of threadlike molecules (Varga, 1946 Szent-... 

In the early 1930 s Astbury and his collaborators defined the principal features of the molecular structure of essentially all mammalian hard keratins (4, 6). This work introduced the concept of the regularly folded a-protein chain. Within this scheme of things the hard keratins of birds and reptiles stood out, for they gave highly characteristic diffraction patterns which could only be interpreted in terms of a special type of 0 or nonfolded chain (5). Nevertheless, much of the epidermal protein in birds and reptiles has the same a-type structure as is always found in the case of mammalian epidermal tissue. The special interest of these observations was to reveal a widespread common type of molecular structure in the principal fibrous proteins of vertebrate epidermis. This common type is not quite universal because of the seeming mutation in the hard keratins of birds and reptiles, which defines the unique relationship of these groups at the molecular level (5, 50). 

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