Protein crystals diffraction patterns from

Harmson A, Leberman R, Schulz GE (1976) Comparison of protein crystal diffraction patterns and absolute intensities from synchrotron and conventional X-ray sources. J Molec Biol 104 311... 

The initial avalanche towards the anode plane is accompanied by induced pulses on both sets of orthogonal cathode planes. From Faruqi (1977) with permission, (b) Protein crystal diffraction pattern recorded on an MWPC (flat chamber, 1 atm gas pressure). 

Figure 12.12 X-ray diffraction pattern from crystals of a membrane-bound protein, the bacterial photosynthetic reaction center. (Courtesy of H. Michel.)...

A very narrow window produces monochromatic radiation that is still several orders of magnitude more intense than the beam from conventional rotating anode x-ray sources. Sucb beams allow crystallographers to record diffraction patterns from very small crystals of the order of 50 micrometers or smaller. In addition, the diffraction pattern extends to higher resolution and consequently more accurate structural details are obtained as described later in this chapter. The availability and use of such beams have increased enormously in recent years and have greatly facilitated the x-ray determination of protein structures. 

A Laue X-ray diffraction pattern from a protein crystal. A stationary crystal is irradiated with very intense white, multiwavelength X rays from a synchrotron source. The diffraction pattern is rich in information. A single 0.1 ms X-ray pulse may provide a pattern with enough information to determine a three-dimensional structure. 

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. 

The x-ray radiation usually employed for protein crystallographic studies is derived from the bombardment of a copper target with high-voltage (50 kV) electrons, producing characteristic copper x-rays with A = 1.54 A. Figure 2 shows, in schematic fashion, the x-ray diffraction pattern from a protein crystal. Several features about this pattern bear explanation. First, as you can see, the diffraction pattern consists of a regular lattice of spots of different intensities. The spots are due to destructive interference of waves... 

Figure 2.6 Diffraction pattern from a crystal of the MoFe (molybdenum-iron) protein of the enzyme nitrogenase from Clostridium pasteurianum. Notice that the reflections lie in a regular pattern, but their intensities (darkness of spots) are highly variable. [The hole in the middle of the pattern results from a small metal disk (beam stop) used to prevent the direct X-ray beam, most of which passes straight through the crystal, from destroying the center of the film.] Photo courtesy of Professor Jeffery Bolin.

The complete diffraction pattern from a protein crystal is not limited to a single planar array of intensities like those seen in Figures 1.13 and 1.14. These images represent, in each case only a small part of the complete diffraction pattern. Each photo corresponds to only a limited set of orientations of the crystal with respect to the X-ray beam. In order to record the entire three-dimensional X-ray diffraction pattern, a crystal must be aligned with respect to the X-ray beam in all orientations, and the resultant patterns recorded for each. From many two-dimensional arrays of reflections, corresponding to cross sections through diffraction space, the entire three-dimensional diffraction pattern composed of ten to hundreds of thousands of reflections is compiled. 

The extent of the diffraction pattern from a crystal is directly correlated with its degree of internal order. The more extensive the patterns, or the higher the resolution to which it extends, the more uniform are the molecules in the crystal and the more precise is their periodic arrangement. The level of detail to which atomic positions can be determined by a crystal structure analysis corresponds closely with the degree of crystalline order. While conventional molecular crystals often diffract almost to their theoretical limit of resolution, protein crystals, by comparison, are characterized by diffraction patterns of limited extent. 

Crystals of proteins, nucleic acids, viruses, and macromolecular complexes must be handled with considerable care because they are extremely fragile and contain a high proportion of solvent, principally water. Bernal and Crowfoot demonstrated in 1934 that diffraction patterns from protein crystals quickly degenerate upon dehydration in air. Thus it is essential... 

A good diffraction pattern from protein crystals generally requires mounting the crystal in contact with its mother liquor inside a thin-walled glass capillary tube along with a drop or two of the mother liquor so that an equilibrium atmosphere is maintained (Fig. 19). The capillary containing the crystal is then mounted on the goniometer head. Alternatively, a flow cell, in which liquid with a controlled composition is allowed to flow over a crystal, can be used to maintain the required crystal environment. 

As remarked in note (3) of Table 2.1 crystals of macromolecules cannot contain an inversion centre, a mirror or a glide plane. The diffraction pattern from a protein crystal can, however, contain an inversion centre (otherwise known as a centre of symmetry and a mirror plane. The symmetry symbols given here are for those symmetry elements seen therefore for macromolecular crystals and their diffraction patterns. 

The original synchrotron Laue diffraction patterns from protein crystals recorded at Daresbury using a broad bandpass were conducted on this instrument with the monochromator removed (see Helliwell (1984)). Some preliminary multiwavelength experiments with a silicon double crystal monochromator (Si (111) triangle removed) were conducted. The growth of the Laue and MAD experiments has led to two further stations at Daresbury (SRS-3 and SRS-4). 

Multiple isomorphous replacement allows the ab initio determination of the phases for a new protein structure. Diffraction data are collected for crystals soaked with different heavy atoms. The scattering from these atoms dominates the diffraction pattern, and a direct calculation of the relative position of the heavy atoms is possible by a direct method known as the Patterson synthesis. If a number of heavy atom derivatives are available, and... 

Seven crystal systems as described in Table 3.2 occur in the 32 point groups that can be assigned to protein crystals. For crystals with symmetry higher than triclinic, particles within the cell are repeated as a consequence of symmetry operations. The number of asymmetric units within the unit cell is related but not necessarily equal to the number of molecules in a unit cell, depending on how the molecules are related by symmetry operations. From the symmetry in the X-ray diffraction pattern and the systematic absence of specific reflections in the pattern, it is possible to deduce the space group to which the crystal belongs. 

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