Irregular folds

The irregularly folded segments of polypeptide chain that connect the helixes and sheets—that is, the loops—are usually exposed to solvent and are often short. Globular proteins thus have hydrophobic cores with their charged groups on the outside they are waxy on the inside and soapy on the outside. 

As discussed earlier, solid polymers can be distinguished into amorphous and the semicrystalline categories. Amorphous solid polymers are either in the glassy state, or - with chain cross linking - in the rubbery state. The usual model of the macromolecule in the amorphous state is the "random coil". Also in polymer melts the "random coil" is the usual model. The fact, however, that melts of semi-crystalline molecules, although very viscous, show rapid crystallisation when cooled, might be an indication that the conformation of a polymer molecule in such a melt is more nearly an irregularly folded molecule than it is a completely random coil. 

It is now generally accepted that the morphology of a polymer depends on the contributions of three different macro-conformations (a) the random coil or irregularly folded molecule as found in the glassy state, (b) the folded chain, as found in lamellar structures and (c) the extended chain. The fringed micelle (d) may be seen as mixture of (a), (b) and (c) (see Fig. 2.12) with paracrystallinity as an extreme. 

On upper GI series, loss of distensibility, distortion of the normal surface pattern of the stomach with thickened irregular folds and mucosal nodularity or irregular narrowing with or without ulceration (the ulcer is located intraluminally in contrast to a benign ulcer which is located beyond the expected border of the stomach) can be seen anywhere in the stomach (Fig. 3.22). 

At higher degrees of super cooling, the fold surface, proposed in model B (Figure 8.14), is more likely. The evidence for irregular folding with adjacent re-entry is based on ... 

Structure. In Chapter 5, it was identified that small distortions of the lattice can be observed. Further, measurement of any crystalline polymer will indicate that the density is less that that from the predictions for the perfect crystalline material, indicating the presence of amorphous material. It is proposed that part of the amorphous content arises from disorder irregular folding of the polymer chains (Figure 6.3). 

Figure 6.3 Schematic of a folded surface showing regular folds, irregular folds and bridging tie molecules.

Fig. 10. Interlocking of loops in irregular fold surfaces following folded-chain crystallization from suitably high molecular weight polymer solutions over a limited concentration range to form junctions in a network.

P. H. Lindenmeyer Highly coiled or irregularly folded conformational model, limited chain interpenetration (e)... 

Amorphous material also appears on the surfaces of homopolymer single crystals. As will be developed below, causes of this amorphous material range from chain-end cilia to irregular folding. 

Figure 1.12 Crystallites with folded lamellar crystals of thickness f in the direction of the c axis for (a) regular folding and (b) irregular folding of the chain molecules.

Fig. 3.36a-c. Scirrhous gastric carcinoma. Gastroscopy revealed irregular mucosal folds in the body of the stomach (a). Biopsies were negative and follow-up examinations with a barium meal (b) and ultrasound (c) revealed narrowing of the lumen, typical irregular folds, and a thickened gastric wall... 

It offered a derivation and equation showing that fold length in the irregularly folded polymer was proportioned to Wl/2 where N is the number of chain atoms in a macromolecule. This theme will be treated in Section VI along wth the similar concepts of Lindenmeyer and Yeh. 

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