Epifluorescence microscopy study of a quadruple node of triple junctions of grain boundaries in a Eu2+ -doped Kcl:Kbr solid solution by using the doping ion as a fluorochrome

A typical quadruple node (QN) of triple junctions (TJs) of grain boundaries (GBs) in a Eu²⁺ -doped KCl_0.52 Br_0.48 solid solution is investigated from the geometrical point of view by epifluorescence microscopy using the doping ion as a fluorochrome. The excitation and fluorescence optical properties of the fluorochrome were previously characterised by spectrophotometry whereas the structural nature of the studied material as well as its Bravais lattice type, unit cell size and long-range translational order degree was determined by X-ray diffraction. A three-dimensional reconstruction was built from the microscopy images of different optical cross-sections of the studied arrangement of crystal defects. In the close vicinity of the QN, the studied arrangement of crystal defects adopts the geometry of a collapsed tristetrahedron which, centred at the QN, has its legs along the TJs and, hence, has its faces as collapsed in pairs into the GBs. The angles defined by different TJ couples as well as the dihedral angles defined by the different GB couples meeting in every TJ were measured at the QN site. All, the image recording and stacking as well as the measuring procedures are carefully described. The measured TJ angles (97°, 117°, 95°, 117°, 99° and 130° ± 2°) depart from the characteristic angle (109.47°) of a tetrahedron whereas the measured GB angles (101°, 119°, 140°; 125°, 127°, 108°; 133°, 109°, 119°; 129°, 99° and 132° ± 2°) depart from the angular argument (120°) of a 3-fold symmetry rotation indicating that, in the close neighbourhood of the QN, the studied arrangement of crystal defects is structurally unstable. Such an instability is associated with an observed mismatch in orientation (by angles of 20°, 15°, 33° and 30° ± 2°) between the TJs and some <111> zone axis matrix lattice crystallographic directions ([1¯1¯1¯], [111¯], [11¯1] and [1¯11]), respectively). Local variations in anionic composition, existing within the solid solution matrix, are discussed to be responsible for this mismatching and, therefore, for the observed structural instability.

Geometry and spatial orientation of a quadruple node of triple junctions in a europium-doped KI crystal as determined by epifluorescence microscopy

The geometry and spatial orientation of a typical arrangement of four triple junctions and six grain boundaries sharing a common quadruple node in a Eu²⁺ -doped KI crystal are investigated by epifluorescence microscopy using the proper doping ion as a fluorochrome. To achieve this, an electronic three-dimensional reconstruction of the studied arrangement of crystal defects was built from microscopy images of different optical cross-sections of this arrangement. Previously, the doping ions were induced, by subjecting the crystal to a long annealing treatment, to form europium precipitates into the crystal grain boundaries. The optical properties of these precipitates were characterized by fluorescence spectrophotometry and used to tailor properly the microscope fluorescence mirror unit, whereas the single-crystal character of the microscope samples was tested by X-ray diffraction. By inspecting the reconstruction under handling, the dihedral angles between the grain boundaries that meet at a common triple junction as well as the angles between the triple junctions sharing the quadruple node were successfully measured at the quadruple node site. The measuring procedures are carefully described. The resulting values (132º, 109º, 119º, 125º, 111º, 124º, 124º, 111º, 125º, 129º, 109º and 122º ± 2º) for the dihedral angles depart for some few degrees from the characteristic angle (120º) of a 3-fold symmetry rotation, whereas the resulting values (104º, 111º, 117º, 103º, 100º and 121º ± 2º) for the triple junction angles are not far from the characteristic angle (109.47º) between the legs of a tetrahedron. These results, indicating that in the close neighbourhood of the quadruple node the studied arrangement of crystal defects deviates from a state of full structural stability, allow this arrangement to be fairly modelled in such a neighbourhood by a distorted tetrahedron. The angles between the studied triple junctions and the host lattice directions [11¯1], [111¯], [1¯11] and [1¯1¯1¯] were also measured at the quadruple node site, and the resulting values (8º, 7º, 6º and 8º ± 2º, respectively) indicate that a symmetry mismatching exists between the tetrahedral model of the studied Eu²⁺ -decorated arrangement of crystal defects and the KI matrix cubic crystal lattice. This symmetry mismatching is discussed to be responsible for the observed deviation from structural stability.

Epifluorescence microscopy: a sensitive tool for studying the morphology and oriented growth of europium precipitates in KI single-crystal hosts

The morphology and oriented growth of europium precipitates in well-annealed Eu²⁺-doped KI single crystals are investigated by epifluorescence microscopy using the proper doping ions as fluorochromes. To make this, electronic spatial reconstructions of some fields of precipitates and of some individual precipitates were built from epifluorescence microscope images of different optical cross-sections of these objects. The building procedures are carefully explained. Previously, the KI:Eu²⁺ system was characterized by fluorescence spectrophotometry and the KI-host long-range translational order was tested by single-plate X-ray diffraction. Precipitates are shaped as plates, with their broad faces being parallel to host lattice planes of either {100}- or {110}-forms (the {100}- or {110}-plates, respectively) and as rods lying along host lattice <100>-directions. The {100}-plates have rhomboidal broad faces with a side lying along a <100>-direction, an internal angle of about 45°, as measured on the corresponding {100}-plane, and, consequently, another side (the {100}<110>-side) lying along a <110> direction on this plane. The {110}-plates have rectangular broad faces with a side lying along a <100>-direction and with another side (the {110}<110>-side) lying along a <110>-direction on the corresponding {110}-plane. Spatial reconstructions of a typical precipitate field, a typical {100}-plate, a typical {110}-plate and a typical rod are described in detail. Precipitates were measured in their different dimensions and the measuring procedures are explained. The plate thicknesses and rod diameters are into a common narrow range of values (0.5-0.2 μm) which contains also the inferior limits of the obtained length ranges for the {100}<110>- and {110}<110>-sides (5.1-0.3 and 4.9-0.3 μm, respectively). It is discussed that that three different europium precipitation states are responsible for the studied precipitation and that plates grew from rods during annealing.

An X-ray-crystallographic study of beta-lactamase II from Bacillus cereus at 0.35 nm resolution

Crystals of beta-lactamase II (EC 3.5.2.6., 'penicillinase') from Bacillus cereus were grown with Cd(II) in place of the natural Zn(II) cofactor and stabilized by cross-linking with glutaraldehyde. Their space group is C2, the cell dimensions are a = 5.44 nm, b = 6.38 nm, c = 7.09 nm and beta = 93.6 degrees, and there is one molecule in the asymmetric unit. Diffraction data were collected from cross-linked crystals of the Cd(II)-enzyme, the apoenzyme and six heavy-atom derivatives. The electron-density map calculated at 0.35 nm resolution reveals the essential Cd(II) ion surrounded by three histidine residues and one cysteine residue. The position of a glutamic acid residue, modification of which destroys activity [Little, Emanuel, Gagnon & Waley (1986) Biochem. J. 233, 465-469], suggests the probable location of the active site of the enzyme. Two minor Cd(II) sites not essential for activity were also located. The structure of the apoenzyme at this resolution appears to differ from that of the Cd(II)-enzyme only in the orientation of two of the histidine residues and the cysteine residue that surround the metal ion.