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1.
Microsc Microanal ; 4(6): 622-631, 1998 Nov.
Article in English | MEDLINE | ID: mdl-10087285

ABSTRACT

: Originally designed as position-sensitive detectors for particle tracking, silicon drift detectors (SDDs) are now used for high-count rate X-ray spectroscopy, operating close to room temperature. Their low-capacitance read-node concept places them among the fastest high-resolution detector systems. They have been used in a new spectrum of experiments in the wide field of X-ray spectroscopy: fluorescent analysis, diffractometry, materials analysis, and synchrotron experiments such as X-ray holography and element imaging in scanning electron microscopes. The fact that the detector system can be used at room temperature with good spectroscopic performance and at -10 degrees C with excellent energy resolution, avoiding liquid nitrogen for cooling and high-quality vacuum, guarantees a large variety of new applications, independent of the laboratory environment. A brief description of the device principles is followed by basics on low noise amplification. The performance results of a complete detector system are presented as well as some dedicated applications already realized, including use in a surface mapping instrument and use of a "mini-spectrometer" for the analysis of works of art. Fully depleted pn-charge-coupled devices (pn-CCDs) have been fabricated for the European X-ray Multi-Mirror mission (XMM) and the German X-ray satellite ABRIXAS, enabling high-speed, low-noise, position-resolving X-ray spectroscopy. The detector was designed and fabricated with a homogeneously sensitive area of 36 cm2. At -70 degrees C it has a noise of 4 e- rms, with a readout time of the total focal plane array of 4 msec. The maximum count rate for single photon counting was 10(5) cps under flat field conditions. In the integration mode, more than 10(9) cps can be detected at 6 keV. Its position resolution is on the order of 100 µm. The quantum efficiency is higher than 90%, ranging from carbon K X-rays (277 eV) up to 10 keV.

2.
Biochem J ; 177(1): 357-60, 1979 Jan 01.
Article in English | MEDLINE | ID: mdl-218562

ABSTRACT

On the basis of the work of Gutteridge, Tanner & Bray [Biochem. J. (1978) 175, 887-897] and of other data in the literature, a mechanism for the reaction of xanthine oxidase with reducing substrates is proposed. In the Michaelis complex, xanthine is bound to molybdenum via the N-9 nitrogen atom. Coupled transfer of two electrons to molybdenum and the C-8 proton to the enzyme yields (Enzyme)-Mo-SH. Concerted with this process, reaction of the xanthine residue with a nucleophile in the active centre yields a covalent intermediate that breaks down to give the product by alternative pathways at high and at low pH values.


Subject(s)
Xanthine Oxidase , Chemical Phenomena , Chemistry , Electron Spin Resonance Spectroscopy , Kinetics , Models, Chemical , Molybdenum
3.
Bioinorg Chem ; 7(1): 87-93, 1977.
Article in English | MEDLINE | ID: mdl-192320

ABSTRACT

DDT1 is converted to DDD by reduced myoglobin (rapidly), cytochrome c oxidase, and a haem-containing undecapeptide derived from cytochrome c. Cytochrome c itself is inactive. This demonstrates that an accessible haem site is necessary for the reaction. Spectrophotometric evidence is presented for an interaction between DDT and the undecapeptide. These results cast light on one of the biological pathways for the breakdown of DDT.


Subject(s)
DDT , Hemeproteins , Animals , Cattle , Chemical Phenomena , Chemistry , Cytochrome c Group , DDT/analysis , Electron Transport Complex IV , Myoglobin , Oxidation-Reduction , Spectrophotometry , Time Factors
4.
Eur J Biochem ; 71(2): 613-6, 1976 Dec 11.
Article in English | MEDLINE | ID: mdl-12973

ABSTRACT

The pH-dependence of the magnetic moment of a ferri-haem undecapeptide, produced by peptic digestion of cytochrome c, has been measured in aqueous solution using a nuclear magnetic resonance method. Below pH 3 the magnetic moment is consistent with the presence of hydroxo-bridged dimers of high-spin iron(III). Above pH 7 the iron(III) is low-spin, the spin crossover conforming to a simple titration curve with pK 6.3 (n=1). This transition is discussed with reference to spectrophotometric studies of ligand binding at the haem.


Subject(s)
Cytochrome c Group/analysis , Heme/analogs & derivatives , Hemin/analysis , Oligopeptides/analysis , Amino Acids/analysis , Animals , Horses , Hydrogen-Ion Concentration , Iron/analysis , Isoelectric Focusing , Magnetic Resonance Spectroscopy
5.
Nature ; 249(454): 286-7, 1974 May 17.
Article in English | MEDLINE | ID: mdl-4833251
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