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.     

THE FLUORESCENCE DECAY OF REDUCED NICOTINAMIDES IN AQUEOUS SOLUTION AFTER EXCITATION WITH A UV-MODE LOCKED Ar ION LASER

Abstract— The fluorescence decay kinetics of the reduced nicotinamides NMNH, NADH and NADPH in aqueous solution were investigated using an Ar ion laser, mode locked in the UV, as source of excitation and single photon counting electronics in the detection system allowing for a time resolution in the picosecond range. Analysis of the experimental fluorescence decay showed that the dinucleotides did not follow a single exponential decay law. Good fitting was accomplished with a sum of two exponentials. The mononucleotide fluorescence decay was a single exponential for at least 95% of its amplitude.
The heterogeneity in lifetimes of the fluorescence of the dinucleotides was interpreted in terms of an exciplex mechanism.     

Volumetric determination of the U/O ratio in uranium oxides

A method is given for determining the general formula UO(2+x), of sintered uranium dioxide pellets and uranium dioxide powders. Uranium(VI) is reduced by titration with iron(II) ammonium sulphate, after which the total amount of uranium is oxidized by titration with potassium dichromate. The end-points of both titrations are detected electrometrically. Determination of x in the range 0.0001-1.00 is possible.     

Measurement uncertainty: practical problems encountered by accredited testing laboratories

Following the introduction of ISO 17025 in 1999, all accredited testing laboratories have started to implement a procedure for estimating measurement uncertainties for the analytical methods that they employ. These procedures and uncertainties are an integral part of the quality system of the accredited laboratory and are therefore reviewed during audits by the accreditation bodies. In January/February 2004 the status of measurement uncertainties available at accredited testing laboratories was evaluated. It was found that most laboratory customers do not make use nor want to make use of the available measurement uncertainties. Furthermore, the current lack of standardisation has caused the estimated measurement uncertainties to vary widely, even for the simplest determinations.     

KINETICS OF STACKING INTERACTIONS IN FLAVIN ADENINE DINUCLEOTIDE FROM TIME-RESOLVED FLAVIN FLUORESCENCE

The time dependence of the fluorescence of flavin adenine dinucleotide (FAD) was measured with a subnanosecond-resolving fluorometer. In contrast to the fluorescence decay of FMN, the decay of FAD was proved to be nonexponential. The time-dependent fluorescence of FAD can be interpreted by assuming an equilibrium between closed and open conformers in the ground state. The rate constant for folding in the excited state and the fluorescence lifetime of the intramolecular complex could be evaluated from analysis of the observed fluorescence decay. The results on FAD were compared to those on NADH obtained earlier.