ALBERT

Notes: A combination of X-ray powder diffraction and single-crystal Laue photography was used to determine the unit-cell parameters for letovicite. μr = 247.25. The unit cell is triclinic, P1 or P{\bar 1}, and has dimensions a0 = 5.87(1), b0 = 10.17(3), c0 = 8.27(1) Å, α = 101.1(4), β = 111.1(1), γ = 89.9(2)°, V = 450.7 Å3, Z = 2, Dx = 1.82 Mg m−3. The M(20) cell reliability is 23.8. Many weak powder spectra were observed, which preclude the possibility of monoclinic cell symmetry previously reported. Projection of the letovicite Laue data along [001] reveals the striking pseudo-hexagonal symmetry observed optically. The JCPDS Diffraction File No. for ammonium hydrogen sulfate is 35-1500.

Notes: Crystallographic data for three different protein crystals (glycogen phosphorylase b to 2 Å resolution, β-lactamase I to 2.5 Å resolution and troponin C to 6 Å resolution) have been recorded using the intense synchrotron radiation beam emitted by the DCI storage ring at LURE and the DORIS storage ring at DESY/EMBL. Reduction in exposure times of approximately 50-fold and an increase in crystal lifetime of at least fivefold are observed when data recorded at LURE are compared with those recorded with a conventional rotating-anode source. These factors have made possible data collection which otherwise would have been impossible. For large crystals of phosphorylase b a greater reduction in exposure time ( × 125) is made possible by the focusing geometry of the synchrotron-monochromator system which allowed irradiation of a larger volume of the crystal (collimator size increased from 0.3 to 1.0 mm) without significant increase in spot overlap on the film. The data processing statistics for phosphorylase b and β-lactamase compare favourably with those from data recorded on a conventional source (improvements in merging R values of between 1 and 4%). For phosphorylase b, but not for β-lactamase or troponin C, significant thermal diffuse scatter is observed on photographs recorded with synchrotron radiation. The possible origin of this phenomenon and its effect on data processing are discussed.

Notes: By means of several low temperature devices the solid hydrous phases of the AgI–NaI–H2O system have been extensively studied by X-ray diffraction, transmission Laue photography, and polarizing microscopy. The density, composition and unit-cell parameters have been determined for three distinct AgI–NaI hydrates for which the stability regions correspond to phases C, E, and F previously reported by Davis [J. Atmos. Sci. (1969), 26, 1042]. The cell assignments are based on determinations of axial ratios from the Laue photographs and interplanar spacing analyses utilizing the Hesse–Lipson technique, Cohen cell-parameter refinement and the de Wolff cell-reliability criteria. All phases are of the composition AgI–NaI–nH2O with phase C at 25 °C having n = 4, ρ = 3.0 g.cm−3 (measured and calculated) and indexing on the basis of a monoclinic cell with parameters a0 = 7.573, b0 = 9.454, c0 = 12.403 Å, and β = 92.43° with Z = 4. Phase F at −10 °C is also monoclinic with a0 = 5.757, b0 = 13.563 and c0 = 12.541 Å and β = 103.05° with Z = 4, and n = 4. Phase E appears to consist of two structures, very similar but differing in cell volume by 2.8%. We believe this to be due to varying water content, the n = 3 phase being metastable. The cell reliability factor is less than 10 for both cells and indexing is considered to be tentative.

Notes: An X-ray diffraction study of the AgI–KI–H2O chemical system was conducted by injecting an acetonic solution of a 2:1 mole ratio of AgI:KI directly upon an inert substrate thermally strapped to a thermoelectric cold stage. The atmosphere over the stage was controlled from 0.6 mb water vapor pressure to saturation while the temperature was varied from −22 to 25°C. Under these conditions four distinct crystalline phases are formed. Crystals of phase AK (KAg4I5) appear as the acetone evaporates. Moisture available from the ambient atmosphere functions as a catalyst as the newly formed AK transforms to phase BK (K2AgI3). At higher vapor pressures a second transformation results in phase CK. All phases break down and form an aqueous solution of KI plus equant AgI crystals (complex DK) at saturation vapor pressures. Another hydrous phase EK occurs below 0°C. The most probable composition of phases CK and EK is KAg2I3.H2O and KAg3I4.8H2O respectively. Nucleation studies show that the KI system is a poorer artificial nucleant of ice than the NH4I–AgI system above −10°C. Kinetics of the system suggest an absence of any free AgI for aerosols released from airborne platforms or from ground seeding stations.

Notes: Recently discovered errors in sample preparation for the original study by Davis & Johnson [J. Appl. Cryst. (1984). 17, 331–333] have revealed that the reference intensity ratio data of Table 1 are incorrect. The reference intensity ratios for the three sets of reflections given in Table 1 have been remeasured and are as follows: 020 + 1{\bar 1}0 = 1.92; 021 + 002 = 1.16; and 1{\bar 3}0 = 0.88. The resulting average reference intensity ratio for the letovicite 111 reflection (the strongest of the pattern) is 1.15(5). Fortunately, the reference constant study was completed independently of the powder diffraction and Laue study and hence the cell constants, diffraction intensities and spacings presented in this paper are not affected by these errors.

Notes: Several inhibitors of the large regulatory enzyme glycogen phosphorylase (GP) have been studied in crystallographic and kinetic experiments. GP catalyses the first step in the phosphorylysis of glycogen to glucose-l-phosphate, which is utilized via glycolysis to provide energy to sustain muscle contraction and in the liver is converted to glucose. α-D-Glucose is a weak inhibitor of glycogen phosphorylase form b (GPb, Ki = 1.7 mM) and acts as a physiological regulator of hepatic glycogen metabolism. Glucose binds to phosphorylase at the catalytic site and results in a conformational change that stabilizes the inactive T state of the enzyme, promoting the action of protein phosphatase 1 and stimulating glycogen synthase. It has been suggested that in the liver, glucose analogues with greater affinity for glycogen phosphorylase may result in a more effective regulatory agent. Several N-acetyl glucopyranosylamine derivatives have been synthesized and tested in a series of crystallographic and kinetic binding studies with GPb. The structural results of the bound enzyme–ligand complexes have been analysed together with the resulting affinities in an effort to understand and exploit the molecular interactions that might give rise to a better inhibitor. Comparison of the N-methylacetyl glucopyranosylamine (N-methylamide, Ki = 0.032 mM) with the analogous β-methylamide derivative (C-methylamide, Ki = 0.16 mM) illustrate the importance of forming good hydrogen bonds and obtaining complementarity of van der Waals interactions. These studies also have shown that the binding modes can be unpredictable but may be rationalized with the benefit of structural data and that a buried and mixed polar/non-polar catalytic site poses problems for the systematic addition of functional groups. Together with previous studies of glucose analogue inhibitors of GPb, this work forms the basis of a training set suitable for three-dimensional quantitative structure–activity relationship studies. The molecules in the training set are void of problems and potential errors arising from the alignment and bound conformations of each of the ligands since the coordinates were those determined experimentally from the X-ray crystallographic refined ligand–enzyme complexes. The computational procedure described in this work involves the use of the program GRID to describe the molecular structures and the progam GOLPE to obtain the partial least squares regression model with the highest prediction ability. The GRID/GOLPE procedure performed using 51 glucose analogue inhibitors of GPb has good overall predictivity [standard deviation of error predictions (SDEP) = 0.98 and Q2 = 0.76] and has shown good agreement with the crystallographic and kinetic results by reliably selecting regions that are known to affect the binding affinity.

Notes: The density, composition, X-ray powder data, unit-cell parameters, and stability fields are reported for five phases of the AgI–NH4I–H2O chemical system. The determinations were made by X-ray powder diffraction but with some aid from polarizing optical microscopy. Provisional cell assignments are: phase An (4AgI.NH4I) is cubic, phase Bn (3AgI.NH4I.6H2O) monoclinic, phase Cn (AgI.2NH4I) orthorhombic, phase E〈inf/〉n (AgI.2NH4I.H2O) orthorhombic, and phase Fn (2AgI.NH4I.2H2O) monoclinic. Upon combustion of an acetone solution of silver iodide and ammonium iodide, phase An forms and, when injected into the atmosphere, transforms to phases Fn or Bn as determined by the available moisture and temperature.