No abstract

Accurate ( ca .0.1Å) knowledge of the metal environment in metalloproteins is essential to understanding their function. Single crystal X-ray analysis has provided detailed descriptions of metal environments in a number of crystallizable proteins but their accuracy has often been limited by their restricted diffraction patterns. The technique of X-ray absorption fine structure (EXAFS) is not limited to crystals and can provide very accurate radial distances between metal ions and their ligands. It has therefore great potential for the study of biochemical metal-containing systems in solution. The method depends on the analysis of the oscillations in the absorption or fluorescence spectrum extending over several hundreds of electron volts above the metal absorption edge. The very intense synchrotron X-ray sources make EXAFS applicable to biological systems where the metal ion concentration is low, typically in the millimolar range. We have determined EXAFS spectra for 2Zn–insulin and a variety of Zn–ligand model compounds of known crystal structures in both the absorption and the fluorescence modes. The X-ray crystallographic refinement of 2Zn-insulin with 1.5 Å data provides estimated standard deviations for well defined atoms ranging from 0.03 to 0.06 Å which thus determine the zinc-ligand distances sufficiently accurately for comparison with those derived from EXAFS. The experimental procedures for obtaining the spectra with the use of the storage ring doris at the Deutsche Elektrononen Synchrotron (DESY) are described. The shapes of the exafs spectra of 2Zn-insulin and the zinc complexes are remarkably similar. These results emphasize one of the major weaknesses of the technique: the difficulty in distinguishing between atoms of similar atomic mass such as oxygen and nitrogen in the present instance. The small but real effect of solvent on the spectrum of one complex has important implications; it reveals the ability of EXAFS to provide evidence for structural changes in the metal coordination and that other structures possibly more relevant to function may exist. For the detailed analysis we used ab initio calculations on the model compounds. The method used is outlined and the programs are cited; the theoretical basis for these calculations is to be found in Lee & Pendry ( Phys . Rev . B 11, 2795-2811 (1975)). Its application to the 2Zn-insulin EXAFS spectrum showed that there is good agreement in both the exafs and the X-ray crystallographic methods for the CE1 and NE2 to Zn radial distance; significant discrepancies, however, exist for the other atoms in the coordinating structure. This failure stems essentially from the smaller contribution of outer atoms to the EXAFS spectrum. To resolve these correctly the amplitudes in the EXAFS spectrum need to be more accurate, and there needs to be a more adequate theory to deal with multiple scattering effects. Until these have been achieved it is probably more profitable to make use of our exact knowledge of the bonding behaviour and geometry of such coordinating groups as imidazole rings.

Bei sehr großen plastischen Verformungen von Kupfer, wie sie durch Torsion zylindrischer Vollstäbe oder durch Walzen erreicht werden können, tritt eine Aufteilung der Kristallite in zahlreiche Bereiche verschiedener Orientierung auf. Die mittlere Größe der Bereiche wird bei hohen Verformungsgraden konstant. Sie ist umso kleiner, je tiefer die Verformungstemperatur ist. Hat man durch Tieftemperaturverformung ein feines Gefüge erzeugt, so erhält man nach anschließender Verformung bei Raumtemperatur ein gröberes Gefüge. Es wird angenommen, daß das bei hohen Verformungsgraden sich einstellende Gefüge das Ergebnis eines dynamischen Gleichgewichtes ist zwischen einem infolge zunehmender Verzerrung auftretenden Kornzerkleinerungsprozeß und einem dynamisch entweder durch Rekristallisation oder durch Subkornwachstum verursachten Kornwachstumsprozeß. Bei Raumtemperatur wurde statische Rekristallisation nachgewiesen. Die Gefügeuntersuchungen wurden elektronenmikroskopisch mit dem Nickelabdruckverfahren durchgeführt.

In certain metallurgical applications of SEM, such as the observation of deformation effects, and the recovery and recrystallization processes in bulk specimens, it is of special interest to obtain information about local inhomogeneities of the dislocation densities in a sample. This can be obtained using TEM on thin foils. Previously information on distortion in cold‐worked specimens was obtained by measuring the dependence of the deterioration of the resolution of the selected area channelling patterns (SACP) on the degree of deformation. This method, however, is not applicable to highly deformed materials. Direct SEM imaging of the cell structure of the dislocation network has been achieved without resolving single dislocations by using a suitable arrangement of specimen and detector, such that only backscattered electrons (BSE) leaving the surface at a large angle to the normal contribute to the image signals. The signal‐to‐noise ratio was improved to the degree that it was possible 1) to image the cell structure of the dislocation network and 2) on heating the specimens, to record the growth of recrystallized regions with TV‐frame frequency. The imaging of the cell structure gives qualitative information about the degree of crystal deformation and the crystallographic direction of grain boundary motion during recrystallization.

Durch Torsionsversuche an zylindrischen Vollstliben wurde das Verfestigungsverhalten von vielkristallinem Kupfer bei Schubverformung bis zu Scherungen an der Probenoberflache von y = 11 (bei Raumtemperatur) bzw. y = 8 (bei 77 "K) untersucht. Es kann abgeschatzt werden, daB diese Verformung mit einem Zugversuch vergleichbar ist, bei dem die Probe auf mehr als das zehnfache ihrer Ausgangslange gedehnt wird. Aus den gemessenen Drehmoment-Drehwinkel-Kurven laBt sich die Schubspannung a n der Probenoberflache erniitteln, vorausgesetzt, daB die Schernng in einem Volumelement im Innern der Probe proportional zu seiner Entfernung von der Probenachse ist. Es konnte gezeigt werden, daf3 dies der Fall ist. Die Schubspannungs-Scherungskurven erreichten bei Raumtemperatur einon Grenzwert tm = 26,6 kp/mm2 bei y = 7,5. Die Versuclie lassen vermuten, daB dieser Grenzwert durch eine konstante FlieBspannung bedingt ist. Bei 77 "K bzw. 90 "K gelang es nicht, einen konstanten Wert der Schubspannung nachzuweisen, da die Proben vorher brachen. Aus Temperatur-bzw. Geschwindigkeitswechselversuchen ergab sich, daB t , nur von der Temperatur und der Verformungsgeschwindigkeit abhangt, nicht aber von der Vorgeschichte der Probe. b y torsion tests on cylindrical specimens, the work-hardening behaviour of polycrystalline copper is investigated for shear strains up to y = 11 (at 293 OK) and y = 8 (at 77 OK). It is estimated that for such a deformation the test is equivalent to a tensile test in which the specimen is strained t o between ten and twenty times its initial length. From the measured torque-twist curves, the shear stress at the surface of the specimens can be calculated by assuming that the shear of a volume element inside the specimen is proportional t o its distance from the axis. It is shown that this assumpion is justified.The shear stress-strain curves a t room temperature reach a limiting value of t , = 26.6 kp/mm2 a t 7' = 7.5. This limiting value is thought t o be due to a constant yield stress. At 77 and 93 "K a constant shearing stress could not be obtained, due to premature fracture of the specimens. From temperature and speed change tests it is found that the limiting value is a function of temperature and speed only, and does not depend on the previous treatment of the specimen. I . EinleitungUnter der Verfestigungskurve eines vielkristallinen Materials versteht man die Abhangigkeit der FlieBspannung von der plastischen Verformung. Dabei ist die FlieBspannungsie sei im Folgenden mit t,bezeichnetproportional der Invarianten der FlieBbedingung. Im Falle eines plastisch isotropen Materials ist sie auch proportional einer Invarianten des Spannungszustandes. Da es verschiedene mogliche FlieBbedingungen gibt, gibt es auch verschiedene mogliche Definitionen der Verfestigungskurve. Wir benutzen im Folgenden, sofern es sich urn isotropes plastisches FlieBen handelt, die FlieBbedingung v o i~ HUBER und von MISES. pbysica