Keratin, swelling

FIGURE 12.1 Penetration enhancer activity, (a) Action at intercellular lipids. Some of the ways by which penetration enhancers attack and modify the well-organized intercellular lipid domain of the stratum comeum. (b) Action at desmosomes and protein structures. Such dramatic disruption by enhancers (particularly potent solvents) as they split the stratum corneum into additional squames and individual cells would be clinically unacceptable, (c) Action within comeocytes. Swelling, further keratin denaturation and vacuolation within individual horny layer cells would not be so drastic but would usually be cosmetically challenging (see Menon and Lee [69] for further details). (Reproduced from Barry, B.W., Nat. Biotechnol. 22, 165, 2004. With permission.)... 

The morphological structure of the fiber determines the pathway that dyes take during dyeing and is critical for the rate and extent of dye uptake. In some way, the dye has to penetrate the more or less hydrophobic layer on the fiber surface, formed by the epicuticle and the exocuticle. The strong swelling capacity of the intercellular cement is important for the penetration of dyes into the fiber. Only then are the sulfur-rich keratins also penetrated by the dye molecules. In general terms, Fick s law can be applied to the diffusion phenomena [46],... 

Ethanol and methanol have been used to swell other types of keratinized protein without loss of the a-helieal structure in the presence or after removal of the alcohols (46). Polar lipids such as the ceramides or sphingolipids are soluble in methanol (47). Therefore, extensive extraction periods in methanol could have provided sufficient time to swell the keratinized protein and allow the highly polar lipids entrapped within the protein fibrils to diffuse from the stratum corneum sheet. The resultant protein residue sheets retained the a-keratin conformation throughout the extensive extraction process. 

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. 

In spite of the fact that stratum corneum cells are metabolically inert, changes in keratin structure and organization occur as each cell transits through the stratum corneum prior to desquamation (28). This suggests some asymmetry in physical and chemical properties through the thickness of the corneum. One demonstration of this is the swelling of fresh frozen transverse sections of corneum in dilute acid or base. The most mature surface cells swell considerably more slowly and to a lesser extent than the lower layers of the corneum (18). Such asymmetry is of particular importance in studying the diflFusion and mechanical properties of this membrane. 

Xeropthalmia Xeropthalmia is an eye disease characterized by drying of the eyes. The cells of lachrymal glands become keratinized and stops secreting tears hence the bacteria are not washed away. The external surface of cornea becomes dry with dull appearance. The eyelids, swells and becomes sticky and there will be severe eye infection. Ulcers may develop and if not treated in time blindness results. Less severe sub clinical... 

Salicylic acid (1) is not employed internally as an analgesic due to its local irritating effect on the gastrointestinal tract. It is employed externally on the skin, where it exerts a slight antiseptic action and a marked keratolytic action. The latter property makes salicylic acid a beneficial agent in the local treatment of warts, corns, fungous infections, and certain forms of eczematoid dermatitis. Tissues cells swell, softer, and ultimately desquamate. Salicylic acid is applied as a 2 to 20% concentration in collodion, lotions, or ointments, and as a 10 to 40% concentration in plasters. Salicylic acid plaster is used for the destructive effect of salicylic acid on hardened, keratinized tissue. The so-called corn plaster are typical. 

The keratin fiber differs from that of collagen in particular due to the importance and variety of polar, salt and covalent interactions between the main chains and side groups. The water is expected to moderate the interactions in question by concentrating around the polar sites. There should subsequently be in the course of the swelling process or redistribution of the water molecules. 

The organizational level believed to control the swelling behavior of keratins is the secondary and tertiary structure of the intermediate filaments and the matrix [116, 117]. As indicated previously, the intermediate filaments consist of proteins containing alpha-helical segments embedded in the less organized matrix of high cystine content. 

Thus, it would appear that the dominant factor for determining crimp in human hair and other coarse animal hairs is the protein composition of the matrix of each cortical cell. This factor could determine the state of swelling of the cortical cells during keratinization or hardening and, therefore, the shape of the hair fiber as it emerges through the skin. 

Evans et al. [25] have confirmed these conclusions of Wickett. In addition, the observation that Japanese hair is easy to perm and that fine Caucasian hair, less than 75 pm in diameter, is more difficult to perm was also confirmed. However, these scientists were unable to identify any common characteristics such as fiber diameter or cystine content that would account for this behavior. The fact that fine hair is more difficult to perm than thick hair may be due to the larger ratio of cuticle to cortex in fine hair and the fact that cortex plays a stronger role in waving than cuticle. This explanation is consistent with the experiments by Wortmann and Kure [2], demonstrating that the cuticle does inhibit the reduction reaction. In addition to pH, other important variables that influence the rate of reduction of keratin fibers by mercaptans are temperature, hair swelling, prior history of the hair, and structure of the mercaptan. 

Above pH 7, the rate of reaction of THP appears to be controlled by diffusion of the reagent into the fibers [73] and, like the reaction of mercap-tans with hair, increases rapidly with increasing pH in the vicinity of pH 9 to 12. Presumably, this increase in reaction rate results from increased swelling of the keratin substrate with increasing pH. 

The minimum in swelling is consistent with the observation of Steinhardt and Harris [106]. In the absence of added electrolyte, there is no combination of wool hber with mineral acid or alkah from pH 5 to 10. This is in the vicinity of the isoionic point of hair. The large increase in swelhng above pH 10 is largely due to ionization of diacidic amino acid residues in the hair and partly due to keratin hydrolysis. The increase in swehing from pH 3 to 1 is due to the combination of acid with the dibasic amino acids. Brener and Prichard [57] attribute the decrease in swelhng below pH 1 to an irreversible structural change. 

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