Keratins supercontraction

This section is concerned primarily with the effects of chemical modifications of keratins on their physical properties—supercontraction, setting, swelling, load-extension characteristics, and other mechanical properties. Much of this work could be described by the term mechanochemical coined by Speakman (1947). The complexity of the cellular and sub-cellular structure of keratins necessitates the use of simplifying assumptions in the interpretation of mechanochemical experiments. 

Hence the changes which take place during supercontraction of keratin fibers are similar to those occurring when collagen or aligned crystalline polymers are converted to their elastomeric form (Mandelkern, 1959). 

It is our opinion that the structure of a-keratin, a-myosin, and similar fibrous proteins is closely represented by our 3.7-residue helix, and that this helix also constitutes an important structural feature in hemoglobin, myoglobin, and other globular proteins, as well as of synthetic polypeptides. We think that the 5.1-residue helix may be represented in nature by super-contracted keratin and supercontracted myosin. The evidence leading us to these conclusions will be presented in later papers. 

When hair or muscle is treated with hot water or steam it shortens in the direction of the fiber axis, and swells laterally. The resultant material is called supercontracted keratin. The contraction from the a state is about 30 per cent for both hair and myosin. It is possible that super-contracted keratin has the configuration of the 5.1-residue helix, but there is very little evidence to support this suggestion. The fiber-axis residue length for this helix is about 0.99 A, which corresponds to 35 per cent contraction from the a helix, with residue length 1.53 A. The agreement of this value with the experimental value for the amount of supercontraction provides some support for the suggestion that the y helix is present in supercontracted keratin. A careftd study of the x-ray diagram should permit a decision on this point to be made. 

How the contractile molecules operate is shown by Astbury (2,3,4) in his work on wool. Under exposure to steam, wool fibers become shorter by supercontraction, which means that the normal, partly coiled state of a-keratin folds up still further. A slightly stretched state is apparently the normal state of equilibrium for rubber this permits both stretching and shortening of the molecular threads. 

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