ALBERT

Notes: Easy and convenient techniques have been developed to handle air-sensitive compounds in terms of measuring their single-crystal X-ray diffraction intensities. These methods have also been fruitfully applied in collecting X-ray data of several highly air-sensitive inorganic and organometallic compounds without using a cold stream of nitrogen.

Notes: Analysis of the X-ray powder diffraction profiles of an isomorphous series of inorganic solids has been carried out for two series of complex oxides: La3LnBaCu5O13 + δ (Ln = Y, Nd or Gd) and LnBa2Cu3O7 + δ (Ln = Y, Nd, Pr, Gd or Dy). In this method, profile and structure refinements of a reference compound are followed by a difference Fourier synthesis to locate the positional parameters of a substituent atom. A linear correlation exists between the Fourier peak height in the difference Fourier map and the atomic number of the substituent atom. By an analysis of relative peak height in the correlation plot, the extent of cation disorder, if any, can be determined.

Notes: Integrated intensities of Bragg reflections have been measured at room temperature for Ni3Al alloys containing various amounts of boron, hafnium, zirconium and titanium and the data have been used to evaluate the mean Debye–Waller factors (〈B〉), Debye temperatures (ΘM) and long-range-order parameters (S). The Debye temperatures are observed to show a slight decrease with the decrease of aluminium content within the Ni3Al phase. The results for the Debye–Waller factors indicate that additions of the ternary elements hafnium, zirconium and titanium cause the mean Debye–Waller factor to increase linearly, while boron addition results in a nonlinear increase. The long-range-order parameter of stoichiometric Ni3Al is 0.97, which decreases with decreasing aluminium concentration from the stoichiometric composition. The long-range-order parameter of Ni3Al increases until 0.75 at.% B has been added, beyond that it remains constant. The long-range-order parameter of Ni3Al increases initially with the addition of hafnium, zirconium and titanium up to 0.50 at.% Hf and 0.52 at.%Zr, with further addition of hafnium and zirconium, it decreases. The results are discussed in relation to the crystal structure and chemical bonding in Ni3Al.

Notes: The line profiles from a sample containing small spherical particles, non-uniform strain and instrumental broadening can be described exactly by using error functions with complex arguments. Consequently, the development by Houska & Smith [J. Appl. Phys. (1981). 52, 748-754] has been revised in terms of these functions. This calculation has been extended, by the use of error functions with complex arguments, to include a more general distribution of particle size or column heights than that obtained from a single sphere. The latter extension is applied to profiles obtained from a partially stabilized zirconia wear debris. It is found, in this example, that a column-height variation coefficient that is greater than that from a single sphere gives a somewhat better fit of the experimental line profiles. We find that if the single-sphere model is used to fit the profiles the particle size and root-mean-square strain differ by about 12 and 5% respectively.

Notes: This paper provides an exact solution for the X-ray diffraction line shape from a bent crystal with linear strain. Such crystals may be found along concentration gradients in a diffusion or ion-implanted zone, thin films and cold-worked materials. The solution simplifies into a sum of squares of two pairs of Fresnel integrals, which can be evaluated with very little computer time. The limiting cases of both pure particle-size broadening and pure strain broadening are also considered. Instrumental broadening is introduced by a numerical convolution with a Pearson VII function. A method of determining the linear strain and crystal size from experimental data is discussed.

Notes: The group-theoretical method of reducing the polar (axial) tensor representation of the full rotation group has been explored and extended to obtain the second-, the third- and the fourth-order elastic coefficients for the 18 polychromatic crystal classes. The elastic coefficients obtained for all the classes are tabulated and the results obtained are briefly discussed.

Notes: The structure of chicken skeletal muscle troponin C (TnC) has been refined to an R value of 0.168, using 14 788 reflections, in the resolution range 8.0–1.78 Å. Our earlier 2 Å resolution structure [Satyshur, Rao, Pyzalska, Drendel, Greaser & Sundaralingam (1988). J. Biol. Chem. 263, 1628–1647] served as the starting model. The refined model includes atoms for all protein residues (1–162), 2 Ca2+ ions, 169 water molecules and one sulfate ion. The high-resolution refinement shows more clearly the details of the protein and water structure. The side chains Glu63, Cysl01, Arg123, Aspl40 and Asp152 adopt two discretely ordered conformations. The long central helix is only slightly curved/bent (7.9°) and all the central helix NH...O=C hydrogen bonds are intact. Seven of the nine carbonyl O atoms of the mid segment of this helix, including the D/E linker region, are hydrogen bonded to water molecules which weakens the helix hydrogen bonds. In contrast, in each of the protected upper and lower thirds of the long central helix, only two carbonyl O atoms are hydrogen bonded to water molecules. The hydrogen-bonding patterns displayed by some of the carbonyl O atoms of NT and A helices of the N-terminal domain and the F and H helices of the C-terminal domain, which are on the exposed surface of the protein, are similar. The B helix of the calcium-free site I is kinked, with the local helix axes at either end making an angle of 39°, by two inserted water molecules between N—H and O=C groups, breaking the adjacent helix hydrogen bonds. A sulfate ion from the crystallization buffer is also trapped in the B helix between the guanidinium group of Arg47 and these two inserted water molecules. The C helix of site II is devoid of similar hydration and is probably responsible for the different interhelical angles A/B at site I (134°) and C/D at site II (149°). Extensive interhelix hydrogen bonds occur between the side chains of the C and D helices of the `apo' site II: Gln51–Asp89, Asn52–Asp89, Glu57–Gln85, Glu57–Glu88 and Glu64–Arg84, which apparently are disrupted upon Ca uptake and the resulting rearrangement of the helices expose the side chains, lining the palm of the N-(and C-) terminal domains, for interaction with specific peptide fragment of troponin I (Tnl) during muscle contraction. The dominant crystal packing motif involves a head-to-tail interaction between the N-terminal domain A helix of one molecule and the palm of the C-terminal domain of the 32-related molecule, in a manner similar to that which can be expected for the TnC–TnI complex. Similar interactions may also be responsible for the dimerization of TnC at low pH.