Keratine, crystalline structure

Another popular crystalline structure for protein is the structure formed by extended protein molecules. A pleated sheet structure is formed by hydrogen bonds between adjacent extended molecules aligned in parallel. Piezoelectricily in B-form protein was first observed by Fukada in 1956 for silk fibroin [3]. Since there is no intrinsic pt -zation in the B structure, pyroelectricity and ten piezoelectricity are not observed. However, the piezoelectricity due to shear is observed. If the shear is applied such as to cause slip between oriented molecules, polarizatioo is induced in the direction perpendicular to the plane of the shear. Shear piezoelectricity is observed for most natural biopolymers, not only keratin but abo trumy other proteins. 

Some polymorphic modifications can be converted from one to another by a change in temperature. Phase transitions can be also induced by an external stress field. Phase transitions under tensile stress can be observed in natural rubber when it orients and crystallizes under tension and reverts to its original amorphous state by relaxation (Mandelkem, 1964). Stress-induced transitions are also observed in some crystalline polymers, e.g. PBT (Jakeways etal., 1975 Yokouchi etal., 1976) and its block copolymers with polyftetramethylene oxide) (PTMO) (Tashiro et al, 1986), PEO (Takahashi et al., 1973 Tashiro Tadokoro, 1978), polyoxacyclobutane (Takahashi et al., 1980), PA6 (Miyasaka Ishikawa, 1968), PVF2 (Lando et al, 1966 Hasegawa et al, 1972), polypivalolactone (Prud homme Marchessault, 1974), keratin (Astbury Woods, 1933 Hearle et al, 1971), and others. These stress-induced phase transitions are either reversible, i.e. the crystal structure reverts to the original structure on relaxation, or irreversible, i.e. the newly formed structure does not revert after relaxation. Examples of the former include PBT, PEO and keratin. 

Keratin has a crystallinity of about 30 Z. Since 1950,X-ray diffraction and electron microscopy have led to a model in which the structural elements are (13,14) the a-helix with a diameter of 10, the elementary fibril containing two to three helicoidal chains of 20 diameter, the microfibril or association of ten elementary fibrils through the amorphous regions (of 80 A diameter) and finally the fibrils or structures of several microfibrils within an amorphous matrix. 

The fiber diffraction technique has been used to determine the structures of a wide variety of synthetic and biological molecules including structural proteins such as collagen and keratin, a range of helical conformations of the nucleic acids, DNA and RNA, and polysaccharides. In the case of the B form of DNA, early fiber diffraction patterns, which were not fully crystalline, still provided sufficient information to show... 

The peptide arrangement in protein fibre has been investigated since the first half of the twentieth century. Astbury used X-rays to demonstrate the nature of a crystalline phase in hair. The X-ray dilfraction pattern of animal hairs shows a meridian reflection at 0.51 mn and an equatorial reflection at 0.98 nm. Interpreting these results Pauling et al. proposed the a-helix structure to give account of the secondary structure of the keratin fibre, shown in Figure 9.6.5. 

Pleated sheet structures ifi structures) have little stretchability, but high tensile strength. In pleated sheets the peptide chains lie in a plane, either parallel to each other as in j -keratin of bird feathers or antiparallel as in the more highly crystalline silks. 

Structural proteins, skeletal proteins, seleropro-teins, fibrous proteins simple animal proteins with structure and support functions. They are generally insoluble in water and salt solutions. The best known S.p. are the cystine-rich Keratins (see). Others are Collagen (see), Elastin (see), Crystallins (see), silk fibroin, chondrin, spongin, etc. They are subdivided on the basis of chain conformation into ... 

There are some 20 amino acids in nature, see Figure 14.29. These are organized into an a-helix in the fibrous proteins, which in turn are combined to form protofibrils as shown. In addition to being crystalline, the fibrous proteins are cross-linked though disulfide bonds contained in the cystine amino acid mer, which is especially high in keratin. Animal tendons, composed of collagen, another fibrous protein, have also been shown to have a surprisingly complex hierarchical structure (166). 

Animal hair fibers consists of a protein known as keratin, which has a composition similar to human hair. Animal tendons consist of collagen, a fibrous protein with a complex hierarchical structure. Keratin proteins are actually crystalline copwlymers of nylon, where amino acids are the repeating units. They get cross-linked through disulfide bonds present in the cystine amino acid [13]. 

Ordered secondary structures like a-helixes and p-sheets occurring in ther-moplastically processed proteins are effectively forms of crystallinity. In analysis of extruded feather keratin, which used sodium sulfite to break disulfide bridges, a shift from low crystallinity to higher crystallinity was seen between DSC scans of samples with 4% sodium sulfite and samples with 5% sodium sulfite.Proteins may also contribute to the formation of ordered regions in polymers/protein blends. DSC analysis of crystal formation in soy protein/ poly(butylene succinate) (PBS) blends showed soy protein both induced and accelerated PBS crystallization. ... 

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