Fibrous diffraction patterns

The structures of the basic building blocks of the architecture of proteins were determined by Linus Pauling and R. B. Corey many years before the solution of the structures of globular proteins.13 They solved the structures of crystalline small peptides to find the dimensions and geometry of the peptide bond. Then, by constructing very precise models, they found structures that could fit the x-ray diffraction patterns of fibrous proteins. The diffraction patterns of fibers do not consist of the lattice of points found from crystals, but a series of lines corresponding to the repeat distances between constantly recurring elements of structure. 

Following their investigations on amino acid and peptide crystals Pauling and Corey turned their attention to the x-ray diffraction patterns of a number of fibrous proteins. A... 

Fibrous proteins may achieve two-dimensional order, but they usually do not achieve three-dimensional order. Therefore, the diffraction pattern of fibrous proteins gives information about the regularly repeating elements along the long axis of the fibers but tells us very little about the orientation of amino acid side chains. 

The two-stranded a-helical coiled coil is now recognized as one of natures favorite ways of creating a dimerization motif and has been predicted to occur in a diverse group of over 200 proteins.111 This structure consists of two amphipathic, right-handed a-helices that adopt a left-handed supercoil, analogous to a two-stranded rope where the nonpolar face of each a-helix is continually adjacent to that of the other helix. 2 This structure was first postulated by Crick to explain the X-ray diffraction pattern of a-keratin in the absence of sequence information.Pl The coiled-coil dimerization motif is natures way of creating a rod-like molecule that perhaps plays only a structural role in many fibrous proteins, such as the kmef (keratin, myosin, epidermis, fibrinogen) class 3,4 and the intermediate filament proteins)5 6 ... 

Like crystals, fibers are composed of molecules in an ordered form. When irradiated by an X-ray beam perpendicular to the fiber axis, fibers produce distinctive diffraction patterns that reveal their dimensions at the molecular level. Because many fibrous materials are polymeric and of known chemical composition and sequence, their molecular dimensions are sometimes all that is needed to build a feasible model of their structure. 

A simple and frequently occurring structural element in fibrous materials is the helix. 1 will use the relationship between the dimensions of simple helices and that of their diffraction patterns to illustrate how diffraction can reveal structural information. As a further simplification, 1 will assume that the helix axis is parallel to the fiber axis. As in all diffraction methods, the diffraction pattern is a Fourier transform of the object in the X-ray beam, averaged over all the orientations present in the sample. In the case of fibers, this means that the transform is averaged cylindrically, around the molecular axis parallel to the fiber axis. 

Fibrinogen is a fibrous protein that was first classified with keratin, myosin, and epidermin based on its 5.1 A repeat in wide-angle X-ray diffraction patterns (Bailey et al., 1943), which was later discovered to be associated with the Q-helical coiled-coil structure. It is a glycoprotein normally present in human blood plasma at a concentration of about 2.5 g/L and is essential for hemostasis, wound healing, inflammation, angiogenesis, and other biological functions. It is a soluble macromolecule, but forms a clot or insoluble gel on conversion to fibrin by the action of the... 

Comment. All the less- or more-disordered packing modes introduced above are frequently encountered for arrays of helical molecules. The problem of disorder results in an additional (compared with most single crystal analyses) deconvolution problem when X-ray diffraction patterns of such systems are being interpreted. Although complications from disorder effects are not unique to fibrous systems, they are more frequently encountered there. I suspect that this has... 

Many polymeric materials have a fibrous texture in which elongated particles with an ordered internal structure are preferentially aligned parallel to a particular direction termed the fiber axis. Diffraction patterns obtained from such materials contain information about both the particles and the matrix in which they are embedded. This matrix may consist of amorphous polymer of the same or different composition to the particle or may be a liquid. 

Experimental results. Some carbon fibre specimens reveal several orders of 001 particularly in electron diffraction patterns Figure 15 shows a plot of (3 against l2, equation (3), for an electron diffraction pattern from the skin region of a high-modulus material. L(oOl)> usually referred to as Lc, is 3.5 nm and a = 2%. A full description of electron-diffraction analysis in several similarly heterogeneous carbon fibres has been published (23). Figure 15 also includes a plot from the 001 electron diffraction profiles of a carbon whisker, an exceptionally perfect graphite material. This specimen, with an Lc of 10 nm, has zero distortion, and represents the only case where we have found no distortion in a fibrous specimen. 

New methods of computational analysis will be sought in order to provide simultaneous measures of crystallinity, crystallite-size and lattice distortion in fibrous polymers from X-ray diffraction patterns, but these must pass the test of application to fibres which can be partly characterized by means of electron-microscopy. 

Figure 4.5 X-ray diffraction patterns of fibrous collagen (upper pattern) and hemoglobin crystal (lower pattern). Positions of smudges or dots may be related to molecular parameters of the protein molecule. (Reproduced with permission from Kartha G. Picture of proteins by x-ray diffraction. Acct Chem Res 1 374-381, 1968.)...

The diffraction patterns displayed by helical synthetic polypeptides are similar to those first discovered by Astbury for fibrous proteins (Astbury and Street, 1931 Astbury and Woods, 1933), with the exception that a meridional spacing here occurs at 5.1 A instead of 5.4 A. This particular pattern, which Astbury designated the a-pattern, may be reconciled with the a-helix if the helices are themselves coiled to form a cablelike structure, as suggested independently by Crick (1952, 1953) and by Pauling and Corey (1953). 

William Thomas Astbury found that fibrous proteins, such as keratin, could behave as if they had more than one structure. Mammalian keratin, such as wool, normally gives a diffraction pattern referred to as the a-keratin X-ray pattern with repeat distances of 5.1 A. On stretching, however, a different diflfraction pattern, the y9-keratin pattern, is obtained. He found that it is possible to reconvert /3-keratin (stretched)... 

From the above results we can propose the following mechanism for hexagonal MCM-41 synthesis. At low crystallization temperature or short crystallization time a fibrous agglomerate structure is often observed by SEM on intermediate samples. The 100 and 200 reflections are not detected by XRD and the value of the specific surface area is low. This reflects the initial step of synthesis which is generally referred to the nucleation step in zeolite synthesis. After this step, the 100 and 200 reflections are present on the XRD diffraction pattern. The value of the specific surface area is between 700 and 900 m /g. The fibrous agglomerate structure disappears and crystals of MCM-41 appear. This corresponds to the crystallization step. Finally if both the synthesis temperature and time are continuously raised, a triphasic mixture MCM-41, MCM-50 and amorphous phase is identified by XRD. The... 

In the period around 1920, x-ray diffraction patterns of silk, hair, muscle, tendon, and other fibrous proteins were first made, and the surmise (later verified) was advanced that in silk fibroin the protein molecules are in an extended configuration, as shown in the preceding drawing. 

Because of the complications introduced by the special treatments and the interconversion of the ring and polymer fractions during the preparation the elucidation of the structure was a long-standing puzzle. An important contribution to the determination of the molecular structure came from Prins et al. in the late 1950s [31, 110]. The authors showed that the diffraction patterns of filaments of sulfur can be understood assuming a superposition of the patterns of two constituents, namely cafe/ia-polysulfur and monochnic y-Ss- From then on the true fibrous sulfur was termed S. Prins... 

Data have been derived from pardy stretched samples. Strong extended fibers gave diffraction pattern of fibrous sulfur,... 

From the coincidence of the X-ray layer lines of Sa,i and Sa,2 with those of Sf Tuinstra concluded that these polymers have the same molecular conformation. Since the diffraction patterns of Sa,i and Sa,2 were much poorer than that of the exact positions of the atoms of the helices remained uncertain [54, 115]. However, the gross arrangement of the helices in the units of Sa,i and S b2 could be determined. Accordingly, the structure of Sa,i is an orthorhombic lattice which has nearly the same size as the unit cell of on a pseudo-orthorhombic setting (Table 20). In these structures the helices are arranged parallel to each other. Since the densities of both were found to be the same the unit cell was expected to be build by four helices as in the case of Sf. A maximum of interlocking of the helices was estimated for the direction of the h-axis which consists of alternating helices with opposite turns (see Fig. 18). Because of the similarity with the fibrous sulfur allotrope S, ... 

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