Crystallization of Arthrobacter sp. strain 1C N-(1-D-carboxyethyl)-L-norvaline dehydrogenase and its complex with NAD+

The novel NAD+-linked opine dehydrogenase from a soil isolate Arthrobacter sp. strain 1C belongs to an enzyme superfamily whose members exhibit quite diverse substrate specificites. Crystals of this opine dehydrogenase, obtained in the presence or absence of co-factor and substrates, have been shown to diffract to beyond 1.8 ? resolution. X-ray precession photographs have established that the crystals belong to space group P21212, with cell parameters a = 104.9, b = 80.0, c = 45.5 ? and a single subunit in the asymmetric unit. The elucidation of the three-dimensional structure of this enzyme will provide a structural framework for this novel class of dehydrogenases to enable a comparison to be made with other enzyme families and also as the basis for mutagenesis experiments directed towards the production of natural and synthetic opine-type compounds containing two chiral centres.     

Fragmentation dynamics of ammonia cluster ions after single photon ionisation

A reflecting time of flight mass spectrometer (RETOF) is used to study unimolecular and collision induced fragmentation of ammonia cluster ions. Synchrotron radiation from the BESSY electron storage ring is used in a range of photon energies from 9.08 up to 17.7 eV for single photon ionisation of neutral clusters in a supersonic beam. The threshold photoelectron photoion coincidence technique (TPEPICO) is used to define the energy initially deposited into the cluster ions. Metastable unimolecular decay (µs range) is studied using the RETOF's capacity for energy analysis. Under collision free conditions the by far most prominent metastable process is the evaporation of one neutral NH₃ monomer from protonated clusters (NH₃) _{n ? 2}NH ₄ ⁺ . Abundance of homogeneous vs. protonated cluster ions and of metastable fragments are reported as a function of photon energy and cluster size up ton=10.     

Multipliers and singular integrals on exponential growth groups

We propose a simple abstract version of Calderón–Zygmund theory, which is applicable to spaces with exponential volume growth, and then show that important specific operators can be treated within this framework. Both authors are part of the European TMR network entitled ``Harmonic Analysis'. The first author was partially supported by KBN grant 5 P03A 050 20 and by the TMR network. The second author was partially supported by his university and by the Italian M.U.R.S.T     

Structural and superconducting properties of oxygen-deficient GdBaSrCu3Ox (6.5<x<7.0)

3 O_x with both orthorhombic and tetragonal structure were synthesized using different heat treatments. From each type, a series of oxygen-depleted samples, denoted as “O” and “T”, with oxygen contents 6.5<x<7.0 were prepared by annealing in argon at different temperatures in the range 300–550 °C. For O- and T-samples, superconductivity vanishes below x≈6.8 and x≈6.6, respectively. While T-samples show a continuous variation of both T_c and the unit-cell volume with x, O-samples show coinciding anomalies in these dependences. In all cases, the oxygen loss with respect to fully oxygenated samples was determined by both weight measurements (δ_w) and iodometric titration (δ_t) With T-samples, δ_w and δ_t are in good agreement, whereas with O-samples considerable discrepancies between these values were found. These disparities in the properties of O- and T-samples may be related to differences in the oxygen sublattice. Received: 25 February 1997/Accepted: 27 February 1997