Structure Of α-Keratin Research Articles

Nanoindentation and Hierarchy Structure of the Bovine Hoof Wall.

The bovine hoof wall with an α-keratin structure protects the bovine foot from impact loads when the cattle are running. Reduced modulus, hardness and creep behavior of the bovine hoof wall have been investigated by a nanoindentation technique. The average reduced modulus of the Transverse Direction (TD) specimens from the outside to inside wall is 3.76 and 2.05 GPa, respectively, while the average reduced modulus of the Longitudinal Direction (LD) specimens from the outside to inside wall is 4.54 and 3.22 GPa, respectively. Obviously, the orientation and the position of the bovine hoof wall have a significant influence on its mechanical properties. The use of the generalized Voigt–Kelvin model can make a good prediction of creep stage. Mechanical properties of the LD specimens are stronger than those of the TD specimens. The bovine hoof wall has a layered structure, which can effectively absorb the energy released by the crack propagation and passivate the crack tip. Therefore, a kind of structural model was designed and fabricated by three-dimensional printing technology, which has a 55% performance improvement on fracture toughness. It is believed that the reported results can be useful in the design of new bionic structure materials which may be used in motorcycle helmets and athletes’ protective equipment to achieve light weight and improved strength at the same time.

On the Strength of Hair across Species

Summary The hair of mammals varies in diameter across species from ∼60 μm in humans to over ∼400 μm in giraffes and elephants. Here, we establish that the tensile strength of hair generally decreases with increasing diameter. Although there is a commonality in the morphology of the hair in such differing species as they all possess a cortex surrounded by cuticles, there are also significant differences in the hierarchical structure that contribute to the observed differences in strength. In this work, the internal structure of the hair from different species is examined, with some unique peculiarities in javelina and capybara hair uncovered. The dimensional dependence of hair strength is compared using Weibull predictions. The good correlation of the strength of hair using such Weibull analysis suggests that the failure strength is dictated by the probability distribution of flaws within the cortex.

Single-Pot Extraction-Analysis of Dyed Wool Fibers with Ionic Liquids

Analytical capabilities to identify dyes associated with structurally robust wool fibers would critically assist crime-scene and explosion-scene forensics. Nondestructive separation of dyes from wool, removal of contaminants, and dye analysis by MALDI- or ESI-MS, were achieved in a single-pot, ionic liquid-based method. Ionic liquids (ILs) that readily denature the wool α-keratin structure have been identified and are conducive to small volume, high-throughput analysis for accelerated threat-response times. Wool dyed with commercial or natural, plant-based dyes have unique signatures that allow classification and matching of samples and identification of dyestuffs. Wool released 0.005 mg of dye per mg of dyed wool into the IL, allowing for analysis of single-thread sample sizes. The IL + dye mixture promotes sufficient ionization in MALDI-MS: addition of common MALDI matrices does not improve analysis of anionic wool dyes. An inexpensive, commercially available tetrabutylphosponium chloride IL was discovered to be capable of denaturing wool and was determined to be the most effective for this readily fieldable method.

Evidence for Control of von Willebrand Factor Multimer Size by Intramolecular Thiol-Disulfide Exchange.

Plasma von Willebrand factor (VWF) is a multimeric glycoprotein from endothelial cells and platelets that mediates adhesion of platelets to sites of vascular injury. In the shear force of flowing blood, however, only the very large VWF multimers are effective in capturing platelets. The multimeric size of VWF is controlled by proteolysis at the Tyr842-Met843 peptide bond by ADAMTS13. It is apparent, however, that VWF multimers are processed to a significant degree in the absence of ADAMTS13 in mice and humans. This finding implies that another mechanism of controlling VWF multimer size must exist. We noticed similarities between the structural control of α-keratin and VWF. Both are multimeric elongated proteins whose substrates are linked by disulfide-bonds and both proteins undergo shear-dependent conformational change. Keratin shape is controlled by thiol-mediated events and we have shown that the free thiol of thrombospondin-1 can facilitate cleavage of the disulfide-bonds that hold VWF multimers together. The structure of α-keratin is controlled by shear-dependent intramolecular thiol-disulfide exchange. Free thiols in keratin from unpaired cysteines exchange with nearby disulfides when the α-keratin fibre is exposed to shear. The thiol-disulfide exchange process continues until the shear across the fibre is resolved. We hypothesized that a similar mechanism may be operating in VWF. Intramolecular thiol-disulfide exchange is dependent on the presence of at least one unpaired cysteine thiol in the protein. Using a biotin-linked maleimde that specifically alkylates free thiols, we have shown that plasma VWF does contain unpaired cysteine residues. VWF was coated onto the interior of glass capillary tubes and exposed to shear using a precision syringe pump. VWF eluted from the tube when shear was stepped, but not when the shear was constant, and the eluted VWF was of smaller multimer size than that bound to the tube. These observations imply that VWF multimer size can be controlled by intramolecular thiol-disulfide exchange. We suggest that this mechanism may be responsible for the processing of VWF multimer size observed in the absence of ADAMTS13.

Changes in the Molecular Structure of Hair in Insulin-Dependent Diabetes

Low-angle synchrotron X-ray diffraction has revealed clear and consistent changes in the molecular structure of α-keratin of hair in insulin-dependent diabetes (IDDM) both for human IDDM subjects and for baboons with streptozocin induced diabetes. These changes in both meridional and equatorial intensity distributions are fully explained by a newly developed hexagonally packed model for keratin which locates the modification produced in hair in IDDM in the labile structure of the matrix at established intermediate filament linkage sites. The nature of the extracellular bonding suggests that the change is endogenous, occurring via the blood during the aggregation of the IFs in the follicle. The reproducibility of these changes indicate that hair may represent an easily accessible tissue for the study of how hyperglycaemia can modify extracellular matrix materials which lead to diabetic complications.

Intermediate filament structure of α-keratin in baboon hair

High-quality photographic recording of X-ray diffraction data from the hard α-keratins has mainly been limited to that from porcupine quill. Earlier diffraction patterns of hair have shown very few reflections. In the present study, extensive sets of high-quality data have been obtained using a synchrotron source and an image plate detector, revealing information about the internal structure, both axial and lateral. The diffraction patterns of hair from six different baboons have been examined. The meridional axial diffraction data reveal evidence of the superposition of three and possibly four separate lattices. Despite considerable overlapping, sufficient reflections unique to each lattice have been observed. These lattices comprise an infinite one of 46.76 nm, together with finite lattices of 19.67, 27.20 and 7.27 nm. These patterns reveal information regarding the fibrillar structure of the hair fibres, namely that it consists basically of cylindrical fibrils arranged in a disordered lattice. The mean diameters of these cylinders have been determined, together with their average separation. There is also an indication of the presence of another set of cylinders of smaller diameter, possibly comprising the microfibrils of keratin.

DIFFERENTIAL THERMAL ANALYSIS OF COMPONENT PROTEINS FROM WOOL

Thermal behavior of component proteins separated from wool keratin has been investigated in order to elucidate the endotherms in the differential thermal analysis (DTA) curves of wool fibers. For this purpose, the S-carboxymethylated keratin derived from wool was separated into the microfibrillar and the matrix proteins. The microfibrillar protein has further been fractionated into two fragments, i.e., a helix-rich and a non-helical fragment. Each protein film was prepared either by heating and/or stretching after casting from various solvents in order to vary the higher-order-structure. Three endotherms in a range of 200-300°C were observed in the DTA curves of the microfibrillar protein and the helix-rich fragment films cast from the aqueous solution. These endotherms were also found in the native wool fiber having so-called α-keratin structure. While, the matrix protein and the non-helical fragment films cast from aqueous solution exhibited only a broad endotherm in a range of 250-270°C.The DTA curves of these samples were varied from each other according as the structural change of the higher-order of proteins induced a posteriori. These variations observed in the endotherms suggest that (i) the phase-transition temperatures observed for solubilized keratin proteins are much lower than those for wool fibers, (ii) the lowest temperature endotherm at about 220°C is due to the melting of the α-helix structure of microfibrillar proteins, (iii) the melting of the β-structure of microfibrillar and matrix proteins causes the endotherm in the range of 250-270°C, and (iv) the pyrolysis of non-helical proteins (i.e., matrix proteins and non-helical fragments) occurs at much lower temperature than that of α-helical proteins.

The Structure of α-Keratin

Known structural principles (close packing, maximum hydrogen bonding, the tendency of like groups to be surrounded in like manner, and the approximate constancy of interatomic distances and bond angles) are used, with meridional and equatorial x-ray data, to deduce and check the structure pattern for alpha-keratin. Internally hydrogen-bonded polypeptide helices are grouped into "3-stacks", in which each chain is rotated and shifted vertically a distance equal to the helix pitch (5.15 A, average), relative to the other two. This shift accounts simply for the meridional x-ray reflection at this spacing. The 3-stack structure repeats after three turns, except for differences in the R groups and a slight twist, required to give satisfaction of both intrachain and interchain forces. The 3-stacks are grouped into 9-stacks and these into 27-stacks (all twisting), giving a crystallographic unit containing 81 chains, with the chain axes spaced approximately like those of close-packed cylinders. When the twisting reaches the limit of stability for good interchain contacting and cross-linking, the residue/turn ratio in each chain helix shifts to another, with twisting in the opposite direction. The twisting reversal mechanism keeps all the helix axes approximately straight, parallel, and in a close-packed arrangement. Interchain distances and orientations are suitable for cystine cross-linking. The dimensions of the 27-stacks agree well with estimates of the "effective radius" of microfibrils. X-ray measurements of spacing changes during fiber extension are explained as due to alternation of zones with much cross-linking and zones with few cross-links.

Structure of α-keratin: Structural implication of the amino acid sequences of the type I and type II chain segments

Two amino acid sequences from potentially helical fragments of low-sulphur proteins from α-keratin have been analysed computationally and periods 9.4 and 28 residues long noted in the axial disposition of charged residues. Ionic interactions between chains have also been calculated and these indicate a preference for the helical fragments to aggregate in parallel with zero shift between chains in a manner essentially identical to that found for α-tropomyosin.

Structure of α-Keratin

Structure of α-Keratin

The structure of α-keratin

High-resolution low-angle meridional and near-meridional X-ray diffraction data have been collected from specimens of α-keratin and various heavy-atom derivatives of α-keratin. The observed reciprocal spacings of the layer lines satisfy the selection rule Z=m/h+n/P+s/P d where h = 67.1 A ̊ , P = 220 A ̊ , P d = 235 A ̊ and m, n and s are integers. Taken in conjunction with observations on the lateral distribution of intensity this evidence suggests that the microfibrils in α-keratin have a helical structure with pitch = 220 A ̊ and unit height = 67.1 A ̊ which is subject to a regular axial distortion of period 235Å. Possible relationships between this helix and the coiled-coil α-helix rope segments of the constituent molecules are discussed.

Structure of α-Keratin

Structure of α-Keratin

Mammoth hair: stability of alpha-keratin structure and constituent proteins.

Hair from a woolly mammoth (Mammuthus primigenius, age about 32,000 years) retains the ordered structure characteristic of alpha-keratins, but the proteins of this hair differ in composition from and are smaller than similar proteins isolated from other keratins, for example, elephant hair. It is suggested that these changes have been caused by limited proteolysis.

Disulphide interchange reactions involving cyclocystine and their relevance to problems of alpha-keratin structure.

Disulphide interchange reactions involving cyclocystine and their relevance to problems of alpha-keratin structure.

The Mechanical Properties of Wool Fibers in Water at Temperatures Above 100° C

The mechanical properties of normal and of chemically modified wool fibers in water were examined for temperatures up to 140°C. Marked changes in the mechanical properties of normal wool fibers occurs at temperatures around 130°C. At this tempera ture the Hookean modulus of the load-extension curve approaches zero, the x-ray diffraction pattern shows that the organized α-keratin structure of the wool is replaced by a randomized β-structure, unstrained fibers supercontract and the longitudinal swelling from dry to wet increases. Chemical modification of the fibers can alter the temperature at which these physical property changes occur. The set, dry-wet longi tudinal swelling and load-extension curves in water at 20°C of fibers held extended in water at temperatures up to 140°C were also examined. It is concluded that the α-helices stabilize the structure of the wool fibers longitudi nally by opposing both extension and swelling. The disappearance of the α-keratin structure for normal fibers at around 130°C results in a loss of this longitudinal stability. During setting of normal fibers, for temperatures less than 130°C, the modification only occurs in the unfolded structural components. The maximum modification (the best setting condition) occurred at 120°C, which was the highest temperature used below the "melting" temperature of 130°C.

Effect of Formic Acid on the Structure of α-Keratin

Effect of Formic Acid on the Structure of α-Keratin

Structure of α-Keratin

IN a recent publication, Fraser, MacRae and Miller1 have given a comprehensive interpretation of the X-ray diffraction photograph of α-keratin (porcupine quill tip) and, in particular, considered the quantitative aspects of the small-angle and large-angle equatorial scatter. This problem has also been discussed by Wilson2, whose approach differs in some important respects from that of Fraser et al., although based on a microfibril model of the same type. Wilson supposes that interference effects between protofibrils within a microfibril, as well as between different microfibrils, affect the X-ray intensity at small scattering angles, whereas Fraser et al. consider only the ‘between microfibril’ interference. According to them, the small-angle scatter can be ascribed solely to a difference in effective mean electron density between the microfibrils and their surrounding matrix, and for a given microfibril-matrix texture the intensity should be simply proportional to this difference. The evidence in favour of this idea is that when heavy metals, such as silver or osmium, are incorporated in the matrix material the main effect on the diffraction is to enhance the small-angle scatter, leaving the maxima of intensity at the same scattering angles as before.

Alpha-Keratin: Molecular and Fine Structure of α-Keratin (IV)

Alpha-Keratin: Molecular and Fine Structure of α-Keratin (IV)

The coiled-coil model of α-keratin structure

Quantitative measurements of the X-ray diffraction pattern of dry porcupine quill tip are reported for both low- and high-angle regions. There is a considerable measure of agreement with the intensity transform calculated for the coiled-coil rope model proposed by Crick but precise comparison is handicapped by interference effects due to macromolecular organization. A method of allowing for these effects is devised for the equator and good agreement obtained with the observed pattern.

The molecular configuration of α-keratin

The coiled-coil configurations of the α-helix which have been suggested by Crick and Pauling & Corey as models for the structure of α-keratin have been tested by comparing their optical transforms with the observed X-ray diffraction pattern. The observed meridional low-angle reflections may be indexed as orders of an axial repeat of 197 Å and this has in the past been associated with the pitch of the coiled-coil model. Evidence is presented which suggests that coiled coils if present do not persist over sufficient lengths to justify this conclusion. The coiled-coil projections used to obtain the optical transforms were therefore restricted to short lengths (~ 60 Å) and the results obtained with the 3-strand parallel-chain rope show good agreement with the observed wide-angle X-ray pattern except in the 1 Å region of the meridian. It is shown that a modification of the coiled-coil model in which the distortion associated with coiling is localised at particular residues rather than being uniformly distributed gives a better agreement in the 1 Å region.