Richard S. Bear

The Structure of Collagen Fibrils

Publisher Summary This chapter discusses the available information, concerning collagen and featuring in particular X-ray diffraction, electron optics and certain phases of chemical evidence developed, covering the characterization of various members of the collagen class of fibrous protein and a derivation of the principal structural features of the unitary element of connective tissues, the collagen fibril. Typical mesenchymal or mesogleal collagens are found in many animal phyla and other members are derived from secretions of epithelial tissues. The differences between collagens resemble those of isomorphous simpler substances in various collagens and there are differences in the content and distribution of amino-acid residues within similar sized frameworks. More chemical evidence regarding the collagens of lower vertebrate and invertebrate animals is needed to make the meaning of these differences clearer. Judging from the evidence drawn largely from the vertebrate animal sources, collagen fibrils are composed of thinner fundamental units, protofibrils. Collagen molecules are isolated when certain weak links occurring periodically along protofibrils are broken. The collagen molecule, like the protofibril, can become extended or randomly contracted. Interbands contain the average sized, hydroxyl-containing side-chains, as hydroxyproline and serine.

THE RESULTS OF X‐RAY DIFFRACTION STUDIES ON KERATIN FIBERS

Polypeptide Chain Configurations in Keratin Fibers. Animal hairs and other keratinous tissues have occupied a central place in the study of the fine structure of protein fibers. Much of this development is due to British investigators, who have introduced concepts and terminology widely encountered in the literature. The roles of side chains and backbones in the folding and unfolding of polypeptide chains are now almost classical parts of the thinking regarding these systems. According to the views of the British school, the polypeptide molecular chains of the better arranged (crystalline or fibrillar) portions of many fibers may exist in t h r g principal stages of contraction or extension, each characterized by a distinctive X-ray diffraction diagram (Astbury and Bell, 1939). These diagrams have been interpreted, in relation to other physical and chemical observations, as indicating that the following are the, three principal structural configurations available to the polypeptide chains (FIGURE 1). Normally, in such structures as mammalian hairs, the chains are in the somewhat contracted or a form. Under certain conditions, notably a t elevated temperatures in aqueous environments, the application of longitudinal tension results in an elongation (70 to 100 per cent), with transformation to an extended or 8 state. In similar environments (with possibly the help of reducing and/or hydrogen-bond relaxing agents), complete release of tension will eventually result in a third, or supercontracted, state, in which the fiber has contracted to considerably less than the original length (ca. 30 per cent). Early studies showed that supercontracted fibers may be in a condition exhibiting disoriented, &type dzractions. Subsequently, Rudall (1946), working largely with epidermal material, concluded that the chains are often thrown into long folds of specific type, the loops of which extend normal to the fiber axis (an exaggeration of the folds of a type shown in FIGURE la). The arms of the folds develop a structure much like the extended (parallel-8) form, except for predominant orientation of the mainchain segments normal to the fiber axis (the cross-8 configuration). Rudall found that saturated urea solution at room temperature is a particularly useful reagent for securing, under appropriate conditions of tension, reversal of the crossand parallel-8 forms back to the original a condition. Transformations of this sort, however, become more difficult if the fiber has previously been held in steam a t a given length, allowed to relax its tension spontaneously, and then permitted to become set in the new configuration. Speakman (1947), in particular, has defended the view that the processes of relaxation and set involve chiefly the interaction of side chains. During

Effective Use of Collimating Apertures in Small‐Angle X‐Ray Diffraction Cameras

The use of simple, aperture‐collimated cameras in x‐ray diffraction studies at small angles is of value in investigating the molecularly large structural features of colloidal systems, particularly when pinhole collimation is required. To make effective use of available x‐radiation one must (1) consider the initial limitations imposed on a camera by the purpose for which it is to be employed, and (2) choose remaining camera dimensions so that the registering film may receive as much radiation as possible.The initial conditions are three in number, involving the desired pattern magnification, the necessary angular or large‐spacing resolution, and the requisite guarding against beam spill‐over, which together essentially limit the specimen‐to‐film distance, the beam diameter at the film, and the diffraction area permitted to be unguarded but covered by a beam stop. Other camera dimensions are chosen so that the film center is as close to, while viewing as much of, the radiation source as possible. In accomp...

Electron microscope and X-ray diffraction studies of muscle structure.

In the study of the mechanism of muscle contraction, skeletal muscle has been the material most wideIy investigated. There are many physiological and chemical reasons for this, but the structural aspects are also important. The complex cytoarchitecture of cross-striated muscle and its alterations in various physiological states furnish visible clues regarding the contractile mechanism. The anisotropy of the alternate bands early led to a recognition of the importance of molecular orientation in contraction (Engelmann, 1875). The demonstration that myosin solutions show double refraction of flow (Muralt and Edsall, 1930) led to a localization of this protein in the anisotropic ( A ) bands. However, subsequent, more or less indirect lines of evidence indicate that the entire myofibril is composed of myosin, while the relative isotropy of the I bands was ascribed to a lack of preferred orientation of myosin in these regions (Schmidt, 1937). With the discovery of the enzymic relationship between myosin, adenosinetriphosphate (ATP) , and the mineral constituents, attention has again been directed to the localization of these substances in the muscle fibril. Explanation of contraction and relaxation is currently sought in topochemical reactions between the structural protein, myosin, and ATP activated by mineral constituents.

Periodic Statistical Distortion of Unidirectionally Ordered Diffractors, with Application to Collagen

The diffraction expected of distorted, one‐dimensionally ordered cylinders (fibrils) is considered quantitatively. The distortion involved is statistically of cylindrical symmetry about any point in the fibril, and periodic along the fibril axis. An important case is that of the ``mixed perfect and imperfect fibril, in which, at some axial levels, distortion is absent, and at others it is appreciable. For such systems it is shown that the reciprocal‐space disk corresponding to a given diffraction layer line may be regarded as composed of three sub‐disks: one of perfection, whose central intensity is maximal and whose diameter is small and independent of layer‐line index; another of longitudinal or axial imperfection, whose central intensity is also maximal; and a third, related to radial imperfection, whose intensity is noncentrally maximal. Both types of imperfection sub‐disk expand linearly in diameter with increase in index.It is demonstrated briefly that certain dry collagen specimens exhibit small‐...

Optical Diffractometer for Facilitation of X-Ray Diffraction Studies of Macromolecular Structures*

An instrument employing optical analogies to the diffraction of x-rays by macromolecular substances is described. Simple and readily obtainable components, materials and procedures are involved. Photographic methods are used to reduce the size of models for trial structures in forming masks whose optical diffraction is then compared with the x-ray diffraction of real structures. Conversely, the diffractometer can be employed to synthesize images of structures from masks simulating observed x-ray diffraction.Applications are described dealing with the investigation of helical chain molecules (polypeptide α helices, collagen and deoxyribonucleate molecules); with the study of larger sized structures exhibiting small-angle x-ray diffraction (e.g., collagen fibrils); and with image synthesis for centrosymmetric configurations (phthalocyanine).