Fibrous protein mechanical properties

Biomechanical Machines. The mechanical properties of fibrous polypeptides could be put to use for the commercial production of fibers (qv) that are more elastic and resiUent than available synthetics (see Silk). The biochemical properties of proteins could also be harnessed for the conversion of mechanical energy to chemical energy (35). 

The pleural tissue is a typical connective tissue that consists mostly of matrix the fibrous proteins (collagen, elastin), and mucopolysaccharides, and a few scattered mesothelial cells, capillaries, venules, and ducts. Anatomists have defined several layers (Fig. 3.4) for each of the pleura. Layers 3 and 5 in Fig. 3.4 contain an abundance of fibrous protein, especially elastin. Both the interstitial (Layer 4) and mesothelial (1 and 2) layers contain capillaries of the vascular system and lymphatic channels. The matrix (ground substance) gives the pleura structural integrity and is responsible for its mechanical properties such as elasticity and distensibility. 

Collagen and elastin are examples of common, well-characterized fibrous proteins that serve structural functions in the body. For example, collagen and elastin are found as components of skin, connective tissue, blood vessel walls, and sclera and cornea of the eye. Each fibrous protein exhibits special mechanical properties, resulting from its unique structure, which are obtained by combining specific amino acids into reg ular, secondary structural elements. This is in contrast to globular proteins, whose shapes are the result of complex interactions between secondary, tertiary, and, sometimes, quaternary structural elements. 

In Nature, there are many examples of protein and peptide molecular self-assembly. Of the genetically engineered fibrous proteins, collagen, spider silks, and elastin have received attention due to their mechanical and biological properties which can be used for biomaterials and tissue engineering. 

Fibrous protein structure investigations applying X-ray diffraction and electron microscopy were reviewed by Blakely (31). Keratin fibers are made of three main structural components the cuticle, the cortex, and the medulla. The medulla is only present in coarse fibers. The cortex forms the bulk of the fiber. Various morphological models have been proposed to explain the mechanical properties of keratin fibers. It is generally agreed that the cortex consists of fibrils in which protein molecules exist in helical and nonhelical regions. 

In native state, proteins exist as either fibrous or globular form. Protein should be denatured and unfolded to produce an extended chain structure to form film. Extended protein chains can interact through hydrogen, ionic, and hydrophobic bonds to form a three-dimensional stmcture (24). Protein films are excellent gas barriers but poor moisture barriers because of their hydrophilic nature. Mechanical properties and gas permeability depend on the relative humidity (1). Al-ameri (25) smdied the antioxidant and mechanical properties of soy, whey and wheat protein, and carrageenan and carboxymethyl cellulose films with incorporated tertiary-butylhy-droquinone (TBHQ), butylated hydroxytoluene (BHT), fenugreek, and rosemary extracts. Armitage et al. (26) studied egg albumin film as a carrier of natural antioxidants to reduce lipid oxidation in cooked and uncooked poultry. 

Collagen [77] is the principal protein constituent of a wide range of mammalian coimective tissue. It is a fibrous protein and the interest is interpreting its mechanical properties in terms of its chemical structure. Properties such as elastic moduli and stress-strain curves depend on the interatomic force constants so vibrational spectroscopy is a necessary... 

The major structural property of a coiled coil superstructure of a-helices is its great mechanical strength. This property is applied very efficiently in both the fibrous proteins of skin and those of muscle. As you can imagine, these proteins must be very strong to carry out their functions of mechanical support and muscle contraction. 

The two major classes of proteins are the fibrous proteins and the globular proteins. Fibrous proteins are distinguished from globular proteins by their filamentous, elongated form. Most of them play structural roles in animal cells and tissues, holding things together. Fibrous proteins have amino acid sequences that favor a particular kind of secondary structure which, in turn, confer particular mechanical properties on the proteins. 

Silks are fibrous proteins produced by spiders and insects such as the silk worm Bombyx mori). There are an astonishing variety in different mechanical properties and compositions of the different silks naturally produced. Many spiders and insects have a varied tool kit of task-specific silks with divergent mechanical properties [42 -49]. Those silks seem to have evolved to match a very particular need for the creature that produces them. Furthermore, although some spiders may use silk sparingly, most make rather elaborate nests, traps, and cocoons typically using more than one type of finely timed and speciaUzed silk. Those different silks are... 

Silk is derived from the cocoon of the silkworm (JSombyx mort). It is defined as protein fibre. Fibroin chains are believed to be nearly fiill-extended, highly crystalline, and almost perfectly aligned in the fibre direction, all of which contribute to the fibre s considerable stiffiiess and strei th. Fibrous proteins, such as silk, are characterised by a highly repetitive primary sequence that leads to significant homogeneity in fine structure. Because of these structural properties it ows impressive mechanical properties and provides important material options in the field of controlled release, biomaterials and scaffolds for tissue engineering (5). 

All silks are not, however, chemically identical. Different silkworm species produce fibrous proteins that contain different sequences and proportions of amino acids. These compositional differences in turn influence the mechanical properties of the fibers. The most common form of sUk, produced by the silkworm Bombyx mori from which most silk sutures are made, has a predominant six-residue sequence Gly-Ser-Gly-Ala-Gly-Ala, which repeats itself for long distances along the chain. This sequence accounts for a large proportion of the amino acid residues that are present. 

Alternate strategies to control the mechanical properties of elastin-like proteins, via combination with amino acid sequences from different fibrous proteins, have also been explored. Combining the temperature-responsiveness... 

---