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Structure of Protein Fibers(Wool)

The protein fibers are the fibers whose origin is protein. They are formed by the natural animal source through condensation of a (alpha)-amino acids to build repeating units of polyamide with many substituents on a (alpha)-carbon atom.

Structure of Protein Fibers(Wool)

The following is the note on the structures of protein fibers. However, first we have to know about the basic concept about the protein and structure of protein.

Introduction

The protein fibers are the fibers whose origin is protein. They are formed by the natural animal source through condensation of a (alpha)-amino acids to build repeating units of polyamide with many substituents on a (alpha)-carbon atom. The sequence and the type of the amino acids bonding together the on individual protein chains contribute to the overall properties and characteristics of the resultant fibers. There are two major classes of natural protein fibers exist. They are as following

  • Keratin (hair or fur)
  • Secreted (insect) fibers.

In general, protein fibers have moderate strength, resiliency, and elasticity. Their moisture absorbency and transport characteristic is excellent. They do not produce static charge. While they are resistant to acids, they are easily attacked by bases and oxidizing agents. They tend to yellow color on exposure to sunlight due to oxidative attack.

Example of protein fibers are wool, silk, mohair, cashmere etc.

Wool

Wool fiber is the natural hair fiber obtained from the sheep and certain other animals. It is protein-based textile fiber.

Wool is used from a long time ago before the history has been recorded. Perhaps primitive man first clothed himself in the skin of sheep when he hunted it for the sake of his food long ago in Stone Age of humankind. Then with the modernization of human being he changed the shape of this clothes and prepared a strong cloth that facilitates him with everything needed which no other material natural or manmade has all of its qualities. It gave him protection equally from cold and heat, from rain and wind. The thing which man did is he refined and improved it. Some of the properties of wool that made it different from hair are that it has crimped, elastic and grows in staples.

Wool fiber varies from other fibers due to its chemical structure. The texture, elasticity, staple and crimp formation of wool fiber depends on its chemical structure. Wool is consisted of more than 20 amino acids. These amino acids form protein polymers in the wool fiber structure. Wool fibers have highly complex physical and chemical compositions that protect the wearer from extremes of heat and cold.

When the Biochemistry Unit of the Wool Textile Research Laboratories was formed in the late 1940s, research began on unraveling the molecular structure of the wool fibre. Investigations were performed on three fronts: electron microscopy of the wool fibre by George Rogers; chemical studies on the number and nature of wool fibre proteins by Lennox and Gillespie, O'Donnell and Thompson, Crewther and Sparrow, Lindley and Elleman; and physical studies including X-ray and electron diffraction by Fraser, MacRae and Tulloch.

Wool, a member of the keratin family of insoluble, proteinaceous, dead tissues that is loaded with sulphur. They are widely categorized as 'hard' or 'soft' in accordance with their physical properties. The hard keratins - hair, wool, nails, beaks, claws, feathers, etc. - offer insulation, tools and weapons; the soft keratins, primarily epidermal structures, while playing a shielding role may also have other important physiological functions. Keratins have categorized supplementary as α-keratin, β-keratin or feather keratin in accordance with their respective X-ray diffraction patterns.

Chemical Structure of Wool Fiber

It estimated by experimental values that wool contains more than 170 different proteins, which are not uniformly distributed in the fiber. Proteins of different structures are present in specific parts of the fiber. This heterogeneous composition is due to the difference in physical and chemical properties of the various parts of wool fiber. The proteins in wool are made up of amino acids.

Individual amino acids vary from each other in the nature of the side groups. Wool contains 18 of the 22 naturally occurring amino acids. In wool, the amino acids are bonded together to build up long polymer chains. In wool, individual polypeptide chains are bonded together to build up proteins by a range of covalent (chemical bonds), known as cross-links and non-covalent physical interactions.

The keratin fibers are insoluble in water and more stable to chemical and physical attack than other types of proteins due to the sulphur containing disulphide bonds. These are involved in the chemical reactions that occur in the 'setting' of fabrics during finishing. In this process, disulphide crosslinks are rearranged to give wool fabrics smooth-drying properties so that ironing is not required after laundering. Another type of crosslink is the isopeptide bond. Some other types of interactions also help to stabilize the fiber under both wet and dry conditions. These ionic interactions or 'salt linkages' are also present between acidic (carboxyl) and basic (amino) side chains. The most important of the non-covalent interactions are the ionic, or 'salt linkages' between acidic (carboxyl) and basic (amino) side groups. The ionic groups handle the dyeing behavior of the wool fiber, because of their interactions with negatively charged dye molecules.

The Physical Structure of Wool Fiber

Wool fiber also has a very complex physical structure. A wool fiber is a biological material consisting of regions that are both chemically and physically different.

Australian merino wool fibers range in diameter typically from 17 to 25 micrometer. They are consisted of two types of cell: the internal cells of the cortex and external cuticle cells that form a sheath around the fiber.

Transcript

The different parts of the wool fiber structure are given below.

Cuticle

On the outside of the wool fiber is an outer covering called epidermis or cuticle. This consists of a layer of horny, irregular scales, which cover the fiber. The thickness of the scales is about 0.5-1.0 micrometer. The number of scales varies greatly depending on the fineness of the fiber. The fine fibers such as merino have as many as 2000 scales per inch. A courser fiber has upto 700 scales per inch.

The outer protective layer of scales called cuticle cells. They overlap one on another like tiles on the roof. This edge of cell faced away from the center of the fiber cause the friction between the fibers when we rub them with one another. Wool expels dirt and gives it the characteristic to be felted due to this quality. Wool is felted by alignment of fibers in the opposite direction and they are entangled.

The scales of the wool fiber have a waxy layer chemically bonded to the surface. This stops the penetration of water but allows the water vapour absorption. This help wool to repel water and resist water based stains.

Cortex

The cortex forms the main central (portion of fine wool fiber. It is built up from long, spindle-shaped cells, which provide strength and elasticity to the wool. The average length of cortical cells is about 80-110micro meter the average width 2-5micro meter and thickness is 1.2-2.6 micrometer.
The microscopic study of the cortical cells have shown that they are built up of many tiny fibrils which are in term constructed of micro fibrils which are in the constructed of micro – fibrils. The internal cells of the cortex make up 90% of the wool fiber. There are two main categories of cortical cells -ortho-cortical cells and para-cortical cells.

Each of them has different chemical composition. In finer fibers, these two types of cells form in two distinct halves. These cells expand when they absorb moisture, and make the fiber bend. Crimps are made in wool due to this. In coarser fibers, the cells expand more randomly so there is less crimp. Fiber crimp makes wool feel springy and insulates the fiber by air trapping.

Cortical cell

A cell membrane complex surrounds the cortical cells. The cell membrane complex contains proteins and waxy lipids that spread in the whole fiber. The molecules in this part of have weak intermolecular bonds, which can be broken down when exposed to continued abrasion and strong chemicals. The cell membrane complex makes it easy the uptake of dye molecules.

Macro fibril

There are long filaments inside the cortical cells called macrofibrils. There are the bundles of even finer filaments known as microfibrils that are surrounded by a matrix region.

Matrix

The matrix contains high sulphur proteins. Due to which wool is water absorbent because sulphur atoms attract water molecules. Wool can absorb water up to 30% of its weight. It can also absorb and retain large amounts of dye. This part of the fiber is also responsible for the fire-resistance and anti-static properties of wool.

Micro fibril

In matrix, there are embedded smaller units called microfibrils. The microfibrils are as the steel rods embedded in concrete to give strength and flexibility to the fiber structure. It contains pairs of twisted molecular chains.

Twisted molecular chain and helical coil

The twisted molecular chains have protein chains that are coiled in a helical shape much like a spring. This structure is hardened by hydrogen bonds and disulphide bonds in the protein chain. It helps to prevent it stretching by linking each coil of the helix. Flexibility, elasticity and resilience of the wool fiber structure are given by the helical coil, which supports wool fabric to keep its shape and remain wrinkle-free during the usage of the fabric.

IR study of the wool fiber

For the sake of investigation of fiber samples, IR absorption spectra has to be plotted in the A=f (1/λ), or T=f (1/λ) system. They provide the interpretation of changes being in the molecular and molecular structures of the surface layer. The absorption bands are corrected by determining the baseline, then their absorbance is determined.

Molecular structure of wool fiber surface layer

The results of three types of wool fibers with various thicknesses shows a clearly different molecular structure of their surface layer. The thinner fibers with a thickness of 15,5μm or 19, 5 μm show same types of the spectrum, which results from the comparable absorbance values of absorption bands correlated with the peptide group being characteristic of keratin. The lower absorbance of the band correlated with the peptide group of the fiber with a higher thickness (25,5μm) indicates a lower keratin content in its surface layer. This shows that the spectrum are obtained from a more developed cuticle layer (greater and thick scales), the structure of which contains a higher quantities of other chemical substances then keratin, e.g. lipids.

The external factor's effect used on the depolymerisation of keratin is low as shown by a low differentiation of value absorbance A in relation to the absorbance value of the absorption band of –CO-NH- group in the initial fiber. The acidic perspiration and heat at RH 65% exerts the strongest effect on the keratin molecular structure. In the case of thinner fibers, the increase in the number of -COOH and NH2 groups do not always causes the decrease in the absorbance value of the absorption band correlated with peptide group. The molecular restructuring process is more complex and concerns the groups that occur in the substituents of α-amino acids and basic character.

Electron microscopy of wool

First of all, the electron microscopy of the wool fibre was performed by George Rogers. Now we will study the structure of wool by electron microscopy method. Electron microscopy method cannot tell exactly about the structure of wool. Anyhow, some results are shown by studying the results we are enabling to tell but the structure of wool. The structure of wool fiber cannot be studied directly with electron microscope method because wool fiber is not thin enough, so replica is made. Replica is made with two thermoplastic materials. Now wool fiber is thin enough to examine in electron microscope. When sample of wool fiber is prepared it is examine under electron microscope. Electron microscope shows the details of structure of wool. By studying the details obtained from electron microscope, we are able to describe the structure of wool fiber. Wool fiber has scales on its surface. Wool fiber has crystalline and amorphous region. Due to large amorphous region, wool has high moisture regain.

Wool fiber has highest absorption capacity of all fibers. Fabric made from wool are used in winters because wool fiber has scales on its surface due to which when fabric is made from wool scales of wool fibers arranged in such a way that no or less heat is passed through fabric and fabric act as nearly insulator. Wool fiber is hard to wear and it is lightweight. Its structure is of such kind that it is resistant to dirt and it does not wrinkle easily. Now a day's new technology has been introduced. Therefore, a lot of work has done on structure of wool. Cool wool has been introduced in industry. Cool wool has structure different from ordinary wool. It can be used in summer as well. The following picture shows the microscopic study of the fiber of wool.

An Overview

The result obtained from the studies of the wool fiber by electron microscopy of the wool fibre, chemical studies on the number and nature of wool fibre proteins and physical studies including X-ray and electron diffraction told us that the wool fibre was formed of a complex mixture of proteins of two main types: low sulphur proteins which are highly helical and are extracted from the filamentous structures in the wool cell; and high-sulphur proteins which are non-helical and forms the matrix. The amino acid sequence data told us that the low sulphur proteins are specialized intermediate filaments. Intermediate filaments, along with thin filaments (actin) and microtubules are important cytoskeleton elements that effects the structural integrity, cell shape, and cell and organelle motility.

The combined results of EM, X-ray diffraction, electron diffraction and chemical data proposed a three dimensional model of the wool fibre to be assembled. The research included keratins other than wool as well as detailed X-ray diffraction studies of another fibrous protein collagen.

Muhammad Rehan Ashraf

I am a Textile Engineer, founder and editor of "Textile Trendz". Currently working in an export-oriented textile organization. I love to share my knowledge about textiles.