Epidermal appendages such as hair and nail contain a complex mixture of proteins known as α-keratin. Their chemical inertness, and many of their physical characteristics, are governed by the high content of disulphide linkages between the protein chains. Sufficient data on the amino acid sequences of the constituent proteins are now available to provide insights into the nature and distrubution of these linkages in the three-dimensional structure of the α-keratin complex. From stereochemical considerations constraints on the formation of disulphide linkages in and between two-strand coiled-coil ropes were identified. In earlier studies certain staggers between rod domain segments were shown to be favoured on the basis of ionic interactions and in the present studies one of these was also found to have a high potential for disulphide bond formation.

A model for the surface lattice of the intermediate filaments (IF) in α-keratin is developed on the basis of quantitative measurements of the distribution of intensity in the X-ray diffraction pattern. A key feature of the model is the presence of a well-defined helical dislocation in an otherwise regular surface lattice. A similar feature has recently been reported to be present in microtubules. The diffraction pattern predicted by the model for α-keratin IF was calculated and compared with the observed pattern and found to explain many of the salient features. Attempts to model the distribution of scattering matter on the lattice were hampered by the paucity of data but some key features of the distribution were uncovered using trial and error methods.

Previous studies of the X-ray diffraction pattern of the crystalline regions of type I collagen fibrils yielded information on the unit cell parameters and also the orientation of the pseudo-hexagonally packed molecular segments in the overlap region. The absence of Bragg reflections at high angles attributable to the molecular segments in the gap region led to the suggestion that these segments were more mobile than those in the overlap region. We report a study of the low-angle Bragg reflections in a search for information about the nature of the orientation and packing of the molecular segments in the gap region. We conclude that the (m = 0, n = 0) helix layer plane of the molecular segments in the overlap region makes little or no contribution to the Bragg reflections at low angles, and identify three possible origins for the observed low-angle reflections in the electron density contrast associated with: (1) the "hole" created by the missing molecular segment in the gap region; (2) the telopeptides; or (3) the axial regularities in amino acid residues of a particular type, with periodicities of D/5 or D/6. Sufficient information is available to investigate the first two of these possibilities, and the results obtained suggest specific arrangements for the molecular segments in the overlap and gap regions, and specific connectivities between the molecular segments in successive overlap regions. In addition, we have examined the amino acid sequence and identified features related to the mobility of the molecular segments in the gap region and to the regions where it is thought that molecules are kinked.

Previous x-ray diffraction studies on the alpha-keratins of hair and wool have revealed that the intermediate filaments (IF) have a helical structure rendered imperfect by a precisely defined dislocation. It has also been possible to deduce a surface lattice for the IF and to determine the number of IF molecules associated with each lattice point. In this work this information is combined with data on the ionic interactions between the coiled-coil rope segments of the IF molecules to provide a plausible model for the pattern of interactions that stabilize the framework of the IF in the "hard" alpha-keratins. Similar interaction studies of the proteins from the IF in the so-called "soft" alpha-keratin from the stratum corneum layer of the skin suggest that they are likely to have an essentially similar pattern.

Molecules of intermediate filament (IF) proteins contain a central rod domain in which the two constituent chains have a predominantly α-helical conformation and are coiled around one another to form segments of two-strand rope. Possible interactions between the two long segments, termed 1B and 2 were investigated by a technique successfully employed in studies of the modes of association of collagen molecules by Miller and coworkers. Prominent maxima were found in all of the six possible modes of association between the rod domain segments in individual IF proteins and certain maxima were found to be common to all IF. The surface lattice of the IF from α-keratin has been determined and possible bonding arrangements between the rod-domain segments are catalogued. A systematic search was carried out for combinations of interaction maxima which were consistent with the dimensions of the surface lattice. By the further application of stereochemical constraints, models for the topological arrangement of the rod-domain segments on the surface lattice were derived and these are illustrated and discussed.

A knowledge of the structure of collagen fibrils is important for any rational discussion of the occurrence and treatment of fibrosis. The different genetic types of collagen, and the structure of the triple-helical molecule as refined from X-ray fibre diffraction data, are described. The problem of determining molecular arrangement in native tissues is discussed. The various models proposed for the molecular arrangement of type I collagen are compared and an account is given of the quasi-hexagonal model. A detailed analysis of the X-ray diffraction patterns from native type I collagen fibres is used to provide a quantitative description of the quasi-hexagonal model. Parameters such as molecular positions, azimuthal orientation and axial shift can be estimated from the diffraction patterns. These parameters refer to the helix main-chain. Side-chain conformations can then be built in by molecular graphics and the predicted X-ray pattern for the complete model compared with the observed pattern.

In a previous communication (Biosci. Rep. 3, 517–525, 1993) we described quantitative X-ray diffraction studies of α-keratin which were shown to be consistent with the presence of finite arrays of repeating units, successive arrays being set down at axial intervals of 470 Å. In addition the axial interval between repeating units in an array was shown to be 197.9 Å. It was suggested that this could most readily be explained by supposing that a surfacelattice was present which contained a dislocation along a helical path with unit height h = 470 Å and unit twist |t| = 49.1°. The number of repeating units was shown to be in the range 7–9. With 7 repeats the mismatch of the lattice along the dislocation is small and this choice was used to develop a detailed model for the filament. Subsequent studies of molecular interactions have shown however that the coiled-coil rope segments in the rod domain of the molecule are most probably oriented parallel to the dislocation, and so minimization of lattice mismatch may be less important than originally supposed. In the present communication it is shown that the choice of 8, rather than 7, for the number of repeating units yields a model which is more compatible with estimates of the linear density and also provides the basis for a general model for polymorphism in intermediate filament lattices.

New X-ray diffraction data have been collected from specimens of tendon collagen stained with phosphotungstic acid. Measurements of the positions of the Bragg reflections associated with the crystalline lattice provide, for the first time, a complete description of the unit cell. A strong band of intensity in the molecular transform associated with the pitch of the molecular helix can be identified and a detailed analysis of the intensities and positions of the Bragg reflections in this band has been carried out. The principal conclusions are that the portions of the collagen molecule that contribute to these reflections have a common direction; that they have a length very much less than that of a complete molecule that the paths of the individual portions through the crystal are incompatible with a completely straight molecule, and that the molecule is therefore crimped. No evidence was obtained for a second series of Bragg reflections attributable to a second set of molecular portions linking the first set, and it is concluded that the linking set is more mobile and subject to positional variation from cell to cell. The most plausible explanation of our finding is that the first set corresponds to the portions of the molecules in the overlap zone and the second set to the portions in the gap zone. A detailed analysis of the Bragg reflections in the strong band of intensity associated with the pitch of the molecular helix has provided information about the relative azimuthal orientations and the lateral positions in the unit cell of the five molecular segments in the overlap zone. None of the existing models for fibril structure accounts satisfactorily for all the results obtained in the present studies and alternative models are developed and tested.

X-ray diffraction patterns have been recorded from native rat-tail tendon and from rat-tail tendon treated with phosphotungstic acid. The reciprocal space coordinates of a number of Bragg reflections were determined and used to determine both the unit cell of the lattice and its orientation with respect to the fibre axis. The unit cell was found to be triclinic and to contain only one molecule. The results obtained are discussed in terms of the microfibril and of the quasi-hexagonal models for molecular packing. The unit cell dimensions are consistent with the latter model and values are derived for the molecular tilt and azimuth.

Keratinized epidermal appendages have mechanical properties which are typical of those expected for composite materials with a filament-matrix texture. At 100% relative humidity the matrix is highly hydrated, mechanically weak and exhibits viscoelastic behaviour. As the water content is reduced the matrix becomes progressively stiffer until at 0% relative humidity its properties approach those of the filaments. In the normally encountered range of relative humidities the advantages of such a texture are that stress is evenly distributed over the filaments, which constitute the load-bearing elements, thus preventing the propagation of cracks from local imperfections. Based on studies of synthetic filament-matrix composites potentialities for the adaptation of the mechanical properties to specific functions are to be found in variation of the properties of the filaments and the matrix and also of the cross-linking. Since keratin is a filament-matrix composite at the molecular level the great diversity of molecular species presumably has its origin in such adaptation. Superimposed on filament and matrix composition and properties are the variables of filament orientation and proportion and there is abundant evidence that many of the subtler properties of keratinized appendages stem from variations in these factors.

New data have been collected on the crystalline structure of collagen fibrils in tendon. The unit cell in decrimped tendon has been determined by measurements of the Bragg reflections in the X-ray diffraction pattern. The results are consistent with a triclinic cell with b = 75.5 Å, β = 93 °, a = bsinβ, a = 90 °, c = n × 668 Å, where n is probably 4 and γ = 90 °. A selection rule observed for prominent reflections is explicable either in terms of a specific orientation of the microfibrils on the lattice, or by a helical distortion of the microfibril axis. The cell parameter β can be varied by changing the ionic envirionment.

The amino acid sequences of sulphur-rich proteins derived from the matrix substance of wool keratin have been analysed for internal and external homologies, and the nature of the repeating patterns residues has been investigated in the proteins termed B2A and BIIIA3. The Fourier transform method was used to identify preferred positions in the pentapeptide periodicity that exists in these materials and the structural and functional implications of the results are discussed.

Quantitative X-ray diffraction data have been collected from stretched kangaroo tail tendon and used to test models for the conformation of the polypeptide chains in the collagen molecule. The magnitude of the unit twist of the molecular helix was estimated to be 107.1 ° ± 0.6 °, which is close to the value expected for a helix with ten units in three turns. The intensity data were used to carry out a linked-atom least-squares refinement of models based on two possible interchain hydrogen bonding schemes suggested by Rich & Crick (1955, 1961). No stereochemically acceptable solution could be found for the hydrogen bonding scheme of model I, but a stereochemically satisfactory solution was found for the scheme of model II which gave a crystallographic R factor of 0.272.

Operculin is a glycine-rich protein present as the major component of gastropod operculae. X-ray and infrared studies of operculins lead to the conclusion that operculae contain antiparallel-chain pleated sheets oriented so that the plane of the pleated sheet is parallel to the plane of the operculum, which is a flat ovoidal or circular sheet. Partial hydrolysis gives evidence of repeating Asp-Gly-Asp and Asp-Ala-Asp sequences as well as of regions rich in Gly.