Keratins chemical structure

VI. Relationship between the Physical Properties and Chemical Structure of Keratin... 

Fig. 2 Supramolecular natural polymeric hydrogels discussed in this chapter, (a) Chemical structure of the most-repeated sequence in collagen, forming the a-chain that folds in a three-stranded superhelix [135]. These superhelices bundle to fram the collagen fiber, (b) Representative chemical structure of fibroin and the antiparallel p-sheet formation connected by hydrophilic linkers, (c) Chemical structure of alginic acid, cross-linked by calcium ions (highlighted), (d) Left Top view of two a-helixes of keratin forming a coiled coll by hydrophobic interactions. Right Overview of subsequent formation of the fibril. The left part is adapted from [57] with permission of The Royal Society of Chemistry...

Chemical Acne Many chemical compounds induce skin lesions that are similar to acne. Oils, tar, creosote, and several cosmetic products induce chemical acne. These compounds induce keratinization of the sebaceous glands of the skin, obstruction of the glands, and formation of acne. Chloracne is a specific skin lesion that is induced by chemical compounds that are structurally similar to 2,5,7,8-tet-rachloro dibenzo-p-dioxin (TCDD). Chloracne is slow to heal and difficult to... 

It is a lipophilic compound which removes intercellular lipids that are covalently linked to the cornified envelope surrounding epithelial cells [3]. It also enhances penetration of other agents. Resorcinol (m-dihydroxy benzene) is structurally and chemically similar to phenol. It disrupts the weak hydrogen bonds of keratin [4]. Lactic acid is an alpha hydroxy acid which causes corneocyte detachment and subsequent desquamation of the stratum corneum [5]. 

These processes provide a complete destruction of the structure of the hair keratin by the complete hydrolysis of proteins that compose it. Hydrolysis can occur enzymatically or chemically. 

The keratin structure is destroyed through the use of proteolytic enzymes such as pronase and proteinase K. They are often used with chemical agents such as urea and thioglycolic to cleave the disulfide bonds and increase the dissolution rate of enzyme activity. The extracting procedures, using enzymatic digestion, last about 4-6 h and must be conducted at constant temperature and pH for providing maximum enzyme activity [155],... 

The hair treatment with concentrated solutions of sodium hydroxide allows the complete dissolution of the keratin structure by chemical hydrolysis of proteins in about 1 h. Under these conditions, some drugs such as amphetamines are volatile and thus there may be losses of the analyte. In more basic solutions occurs the complete hydrolysis of molecules such as cocaine, heroin, and 6-MAM [51]. Concentrated solutions of hydrochloric acid are also used it eliminates the problem of volatile basic compounds, but increases the time of dissolution. 

The DMSO, NaOH, and SLS are useful chemical probes because of the different properties of the substances and the different mechanisms by which they induce irritation. Due to its amphiphilic nature DMSO rapidly penetrates the SC and induces whealing and erythema. The aqueous NaOH solution dissolves the keratin layers, thereby introducing structural defects in the horny layer. The mechanism by which an aqueous solution of SLS induces irritation is still unknown, but SLS penetrates the barrier like DMSO, although much slower. Combined, tests with these and other chemical probes provide valuable information on the barrier properties of the SC. 

The epidermis consists of five principal layers and is an area of both intense biochemical activity and differentiation. These layers are the stratum comeum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. The stratum corneum (horny layer) is the uppermost layer of the epidermis and the skin. The stratum corneum is composed of dead keratinocytes, which are called corneocytes, and has an abundance of keratin and lipid structures [8], The stratum comeum is considered the rate-limiting barrier for the diffusion of chemical compounds across the skin. The stratum lucidum (clear layer) is composed of two to three layers of dead flattened keratinocytes which appear translucent under a microscope and are present only in thick glabrous skin. 

The diversity in primary, secondary, tertiary, and quaternary structures of proteins means that few generalisations can be made concerning their chemical properties. Some fulfil structural roles, such as the collagens (found in bone) and keratin (found in claws and beaks), and are insoluble in all solvents. Others, such as albumins or globulins of plasma, are very soluble in water. Still others, which form part of membranes of cells, are partly hydrophilic ( water-loving , hence water-soluble) and partly lipophilic ( lipid-loving , hence fat-soluble). 

The structure, organization, and ratio of matrix and fibrous proteins contribute to the physiochemical properties of keratinous tissues. For example, a primary difference between hair and nails is the arrangement of fibrous proteins and the concentration of matrix proteins present in each tissue. In cells destined to form the cortex of hair, fibrous proteins are oriented to form filaments which cluster to form fibrils. In the keratogenous zone, fibrils undergo lateral fusion to ultimately produce the cortex. The medulla also contains keratin which has been characterized as a collection of irregular fibrous proteins. Fibrous proteins form a trabecular framework comprising 95% of the medulla, and medullary proteins are less resistant to chemical degradation than proteins in the cortex. The cell membrane complex. 

Cystine, which contains a disulfide bond, is reported to be the most numerous and reactive amino acid present in hair keratin. Disulfide bonds in cystine are reduced by mercaptans and phosphines, and oxidized by perborates, bromates, and bleach. These reactions result in structural rearrangements within keratin which may affect the physiochemical properties of hair, since disulfide bonds in cystine contribute to the stability of hair. For example, hydrogen peroxide bleaching of hair is an oxidative process which occurs readily in an alkaline medium. This results in the formation of perhydroxy anions which have been proposed to react with cystine to form cysteic acid residues. The process of bleaching results in the loss of approximately 15% of the cystine bonds originally present in keratin and may explain the increased permeability of bleached hair to chemicals. - ... 

This section discusses two families of structural proteins, namely, extracellular keratins and collagens. Each example represents a family of closely related chemical entities, which means dealing with complex protein mixtures. We consider two types of chemical modifications modifications arising from increased levels of glucose in body fluids (obviously related to diabetes) and those derived from acetaldehyde, the first metabolic product of ethanol ingestion. As noted, a common feature of the latter nonenzymatic modification reactions is the presence of an aldehydic functionality in the nonprotein reactant (Jelfnkovd et al 1995 Deyl and Mikgik, 1995). 

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. 

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