Keratin mechanical properties

Our last example of the mechanical properties of a protein is that of keratin found in the top layer of skin. The stratum corneum in skin is almost exclusively made up of different keratins that have an a-helical structure. The helices do not run continuously along the molecule so the structure is not ideal. However, the stress-strain characteristics are shown in Figure 6.4 and demonstrate that at low moisture content the stress-strain curve for keratins in skin is approximately linear with a UTS of about 1.8 GPa and a modulus of about 120 MPa. These values are between the values reported for elastin and silk, which is consistent with the axial rise per amino acid being 0.15 nm for the a helix. Thus the a helix with an intermediate value of the axial rise per amino acid residue has an intermediate value of the... 

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. 

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. 

X-ray diffraction and IR dichroism studies suggest that the long-range elasticity of wool is related to a reversible molecular transformation of the alpha-keratin to an extended beta form (66). No convincing evidence supports this mechanism in stratum corneum viscoelasticity. In fact, the available evidence suggests that the elastic behavior of corneum is primarily entropic in origin. At low deformations, the mechanical properties of hydrated stratum corneum is best described as the behavior of a lightly-crosslinked rubber. 

Ma L, Yamada S, Wirtz D, Coulombe PA (2001) A hot-spot mutation alters the mechanical properties of keratin filament networks. Nat Cell Biol 3(5) 503-506... 

On the other hand, variation of pH in keratin solutions causes changes in amino-acid behavior related to the electronic charge response. Thus, if keratin solution has a pH below keratin s isoelectric point (reported at pH of 4), a cationic response can be expected, whereas higher values of pH produce an anionic performance. Therefore keratin has the capability of interact with different kind of species pollutants, including not only chromium but also other metals that could be removed. Besides the pH of pollutant solutions also plays an important role as was demonstrated ecently by Manrique-Juarez et al. In this study, the authors reported that pH has a strong influence on the morphology, mechanical properties and Cr(VI] removal efficiency, since if pH in keratin solution is 2.5, the protein separates from water solution, and therefore the polymerization reaction is affected producing a more closed cell in the membrane. The Cr(VI)... 

Electrospun SF-based fibers were prepared from aqueous regenerated silkworm silk Bombyx mon)/PEO solutions to be used as scaffolds for tissue engineering (Jin et al. 2004). PEO supplied good mechanical properties to the electrospun fibers. An MeOH posttreatment induced an amorphous to silk p-sheet conformational transition. The electrospun silk membrane was washed with water to remove PEO in order to improve the cell adhesion and proliferation. These silk fibrous membranes were nonimmunogenic, biocompatible, and capable of supporting bone marrow stromal cell (BMSC) attachment. In another work, electrospun wool keratin/silk fibroin (WK/SF) blend nanofibers exhibited higher Cu + adsorption capacity than SF nanofibrous membrane (Ki et al. 2007). 

The majority part of the interior of the fiber mass is the cortex, which, from the point of view of mechanical properties, is also the most important component. The cortex consists of elongated, spindle-shaped cells aligned in the direction of the fiber axis. Within these cells resides the major part of the keratinized protein in the form of macrofibrils, which in turn are formed by lower levels of organization, i.e., microfibrils and finally protofibrils. The latter two are low-sulfur proteins and more or less crystalline in nature with their a-helical parts as crystalline lattice components. They are embedded in a noncrystalline, nonfibrillar matrix of disulfide cross-linked, globular proteins. 

Vacuolated cells may also be present along the axis of coarser a-keratin fibres, forming the medulla. These cells generally constitute only a small percentage of the mass of hair and are believed to contribute negligibly to the mechanical properties of human hair fibres. Physically, the medulla forms the empty space of the fibre [4, 7],... 

Feughelman M (1997) Mechanical properties and structure of alpha-keratin fibres. UNSW Press, Sidney... 

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. 

The complex morphology of hair essentially consists of four components of different functionality (i) The cortex that gives the hair its mechanical properties consists of elongated, spin-shaped cells aligned in the direction of the fiber axis. The keratinized protein in the form of microfibrils resides in these cells, (ii) The medulla is located in the center of some thicker fibers and it consists of a loosely packed porous cellular structure (it does not contribute to the mechanical properties of the hair), (iii) Cell membrane complex which cements the various cells of the cuticula and the cortex and it consists of several layers, (iv) Cuticle, a multilayered structure which consists of flat cuticle cells and the most outer layer, i.e. the epicuticle (which is about 2.5 nm thick) is the most important part for deposition of surfactants and polymers in the shampoo formulation. This consists of 25 % lipids and 75 % protein, the latter having an ordered possibly p-pleated sheet structure with 12% cystine. The cystine groups are acylated by fatty acids which form the hydrophobic surface region. A schematic representation of the epicuticle is shown in Fig. 1.46. 

---