Intermediate filaments composition

Mechanical Properties. Although wool has a compHcated hierarchical stmcture (see Fig. 1), the mechanical properties of the fiber are largely understood in terms of a two-phase composite model (27—29). In these models, water-impenetrable crystalline regions (generally associated with the intermediate filaments) oriented parallel to the fiber axis are embedded in a water-sensitive matrix to form a semicrystalline biopolymer. The parallel arrangement of these filaments produces a fiber that is highly anisotropic. Whereas the longitudinal modulus of the fiber decreases by a factor of 3 from dry to wet, the torsional modulus, a measure of the matrix stiffness, decreases by a factor of 10 (30). 

Electron microscopy reveals several types of protein filaments crisscrossing the eukaryotic cell, forming an interlocking three-dimensional meshwork, the cytoskeleton. There are three general types of cytoplasmic filaments— actin filaments, microtubules, and intermediate filaments (Fig. 1-9)—differing in width (from about 6 to 22 nm), composition, and specific function. All types provide structure and organization to the cytoplasm and shape to the cell. Actin filaments and microtubules also help to produce the motion of organelles or of the whole cell. 

Eukaryotic cells have an internal scaffolding called the cytoskeleton or cytomatrix that maintains their cellular morphology and enables them to migrate, undergo shape changes, and transport vesicles. Microfilaments, made of actin, intermediate filaments, which are composed of laminin and other proteins, and microtubules, formed from the protein tubulin, along with many different accessory proteins, comprise the cytoskeleton. Both the microfilaments and the microtubules can assemble and disassemble rapidly in the cell, whereas disassembly of intermediate filaments may require their destruction. Although much is known about the molecular composition of the cytoskeleton, the molecular events involved in most cell movements are still unknown. 

Eukaryotes have various DNA molecules, arranged in linear fibers which are repeatedly coiled and folded to produce highly organised chromosomes, and a composite cytoplasm which is divided into distinct compartments and houses a variety of cell organelles (mitochondria, chloroplasts, lysosomes, the endoplasmic reticulum, etc.) the form of the cell is due to an internal cytoskeleton which is made of three different types of filaments (microtubules, microfilaments and intermediate filaments). 

Keratins - ot-Keratins are the major proteins of hair and fingernails and a compose a major fraction of animal skin, oi-keratins are classified in the broad group of intermediate filament proteins, which play important structural roles in nuclei, cytoplasm, and cell surfaces. Their secondary structure is composed predominantly of -helices. Figure 6.11 shows the coiled-coil structure of the ot-keratin in hair. The chemical composition of the cysteine residues in ot-keratin affects its macromolecular structure and function. For example, hair has relatively few cysteine cross-links, whereas fingernails have many such cross-links, / -keratins, on the other hand, contain much more pleated sheet secondary strucure than ot-keratins and are found in feathers and scales. 

The intermediate filaments in different tissues appear similar in form (see Chapter 1), but they differ considerably in their exact composition. The common structural feature among this class of proteins is the central helical rod [111]. On the other hand, the primary differences are in the amino and carboxyl domains, and these vary in both amino acid sequences and size... 

The q toskeleton of eukaryotic cells is generally considered to be a meshwork of protein filaments that spans the space between the nucleus and the plasma membrane. In many cell types, the three-dimensional (3D) composite network of actin filaments, microtubules (MTs), and intermediate filaments (IPs) in the cytoplasm interfaces with two-dimensional networks composed largely of spectrins that line the plasma membrane and nuclear lamins that line the inner surface of the nuclear membrane. A few eukaryotic cell types contain an entirely different cytoskeleton that powers their locomotion and which is constmcted from the cationic major sperm protein instead of actin. The three cytoskeletal proteins, acdn, tubulin, and IF subunits, constitute a significant fraction of... 

Figure 2 Composite structure of hair at various length-scales (a) filament protein with alternating helical/linker sections (helical section is shown atttie bottom) (b) coiled coil of filament proteins (in a polar environment two polypeptide chains naturally coil around each other in parallel, as this leaves the hydrophobic groups shielded in the centre) (c) intermediate filament with 16 coils (d) filament embedded in matrix (e) macrofibril (f) cortical call enclosed by cell-membrane complex.

A large number of biomaterials are made up of intermediate filaments and therefore their mechanical properties are of great interests. Elastic moduli of these filaments depend upon the particular composition of the filament and the state of hydration. Bending modulus of hydrated intermediate filaments is found to be much less compared to the dry filament. E.g., Young s modulus of horsehair keratin is 6.8 GPa and 2.4 GPa respectively. For most of the cases, in vitro measurement of stiffness parameters agree quite well with the in vivo data and therefore can be directly implemented in the model. 

Recently we observed the effect which supports the conclusion about the substantial role of the radical reaction outside of the catalyst grains. When a very efficient OCM oxide catalyst (10% Nd/MgO) was placed into the reactor together with an inactive metal filament (Ni-based alloy) the sharp increase of conversion accompanied by the selectivity shift from oxidative coupling to the formation of CO and H2 was observed [19]. Since the metal component has a low activity also with respect to ethane oxidation, this behavior is not due to successive oxidation or decomposition of C2 hydrocarbons on the metal surface, but should be attributed to the reactions of methane oxidation intermediates. Almost total disappearance of ethane (which is a product of CH3 radicals recombination) and acceleration of the apparent reaction rate by the addition of an "inert material indicate that the efficiency of methane oxidative transformations can be substantially increased if the radicals have a chance to react outside the zone where they formed and the role of reaction (-1) decreases. Although the second (metal) surface is not active enough to conduct the reaction of saturated hydrocarbon molecules (methane and ethane), the radicals generated by the oxide can react further on the metal surface. As a result, the fraction of the products formed from methane activated in the reaction (1) increases, and the formation of the final reaction mixture of different composition takes place. 

All sheet structures made directly from fibers and filaments without intermediate steps are classified under the term textile composites. In the English-speaking world, a general distinction is made between nonwoven fabrics made from stable fiber and spun-bonded sheet products manufactured from filaments. 

For the production of interleaf-repressing materials, textile fabrics made of aramid filament yarns are alternately arranged in layers with intermediate layers of thermoplastics or elastomers. In a next step, they are pressed into a composite. In doing so, it is not necessary to obtain a homogeneous mixing of fiber and matrix material as is necessary with fiber-reinforced materials. Quite the contrary is desired the aim is the reduction of energy. 

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