Origin polypeptide chain arrangement

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. 

The investigation was then extended to a monolayer formed from dipalmitoyl phosphatidyl choline and the same amphiphilic photochromic polypeptide XXIII.11211 When the monolayer was kept in the dark, the polypeptide molecules arranged themselves perpendicularly to the membrane (the water/air interface) and formed a bundle of helices which could be observed by atomic force microscopy as a transmembranous particle of about 4 nm in diameter. Irradiation with UV light and the consequent trans—>tis isomerization of the azobenzene moiety caused a bending of the molecular main chain, which in turn produced a destabilization and dena-turation of the bundle of helices in the monolayer. After removal of the light, the polypeptide molecules reverted to their original bundle structure. 1211... 

Denaturation is a process, or a succession of processes, in which the steric arrangement of polypeptidic chains inside the native protein molecule is changed, and simultaneously the original properties of the protein molecule are changed to those of the denatured protein. Denaturation is any modification of the secondary, tertiary, or quartary structure of the protein molecule, excluding any breakage of a covalent bond (39). 

A second form of keratin, known as fi-keratin, is produced by stretching the a-keratin in hair to about twice its original length. The X-ray diffraction pattern of -keratin is similar to that of silk fibroin (page 56) and its structure, like that of fibroin, is based on the /5-pleated sheet. There is a difference, however, in that polypeptide chains are in a parallel (Figure 5.2a) and not an antiparallel arrangement. 

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