Diffusion coefficient of DNA molecules during free solution electrophoresis

The free-draining properties of DNA normally make it impossible to separate nucleic acids by free-flow electrophoresis. However, little is known, either theoretically or experimentally, about the diffusion coefficient of DNA molecules during free-flow electrophoresis. In fact, many authors simply assume that the Nernst-Einstein relation between the mobility and the diffusion coefficient still holds under such conditions. In this paper, we present an experimental study of the diffusion coefficient of both ssDNA and dsDNA molecules during free-flow electrophoresis. Our results unequivocally show that a simplistic use of Nernst-Einstein's relation fails, and that the electric field actually has no effect on the thermal diffusion process. Finally, we compare the dependence of the diffusion coefficient upon DNA molecular size to results obtained previously by other groups and to Zimm's theory.     

Analysis of chemical warfare agents. I. Use of aliphatic thiols in the trace level determination of Lewisite compounds in complex matrices

A series of normal aliphatic thiols have been used to derivatise the chemical warfare agents Lewisites I and II (LI and LII) in hydrocarbon matrices. Varying the chain length of the thiol allowed adjustment of derivative tR by 5.9 min for Lewisite I and 5.3 min for Lewisite II. Linear regression analysis of the chain length of the thiol derivatives of the Lewisite species, and that of a series of normal alkanes against tR, allowed regression models to be developed for each set of compounds. Application of the models allowed thiol reagents to be chosen to give derivatives of Lewisites I and II that eluted before and after the major hydrocarbon contaminant. Limits of detection were comparable for all thiol derivatives analysed by GC-MS in the selection ion monitoring mode (all below 1 microg ml(-1)). The robustness of this approach was illustrated by successful identification of Lewisite I in samples from the Sixth Proficiency Test (organised by the Organisation for the Prohibition of Chemical Weapons, OPCW) in a matrix of 1 mg ml(-1) diesel oil.     

Observation of superconductivity (Tc = 50 K) in a new tetragonal alkaline-earth cuprate Sr0.8Ba1.2CuO3+δ, synthesised at ambient pressure

The ambient-pressure synthesis of a new tetragonal alkaline-earth superconducting cuprate, Sr_0.8Ba_1.2CuO_3+δ, from a cupro-oxycarbonate is reported. Magnetic-susceptibility measurements show the presence of a superconducting transition 50 K in a post-annealed sample. The crystal structure, refined from time-of-flight powder neutron-diffraction data was found to have an oxygen-deficient La₂CuO₄-type tetragonal T structure (a = 3.8988(3) Å and C = 12.815(3) Å) with oxygen vacancies located within the CuO₂ planes. Ordering of these oxygen vacancies is responsible for the observation of a superlattice in both neutron- and electron-diffraction measurements. An interpretation of the electron-diffraction patterns suggests that the superlattice in Sr_0.8Ba_1.2CuO_3+δ and also in the isostructural superconductor Sr₂CuO_3+δ are of an identical nature.     

Finite element approximation of a model for phase separation of a multi-component alloy with a concentration-dependent mobility matrix

We consider a model for phase separation of a multi-componentalloy with a concentration-dependent mobility matrix and logarithmicfree energy. In particular we prove that there exists a uniquesolution for sufficiently smooth initial data. Further, we provean error bound for a fully practical piecewise linear finiteelement approximation in one and two space dimensions. Finallynumerical experiments with three components in one space dimensionare presented.     

Spin-dependent transport in molecular tunnel junctions

We present measurements of magnetic tunnel junctions made using a self-assembled-monolayer molecular barrier. Ni-octanethiol-Ni samples were fabricated in a nanopore geometry. The devices exhibit significant changes in resistance as the angle between the magnetic moments in the two electrodes is varied, demonstrating that low-energy electrons can traverse the molecular barrier while remaining spin polarized. An analysis of the voltage and temperature dependence of the data suggests that the spin-polarized transport signals can be degraded by localized states in the molecular barriers.     

A simple prediction of total body water to aid quality control in isotope dilution studies in subjects 3-87 years of age

The principal method of measuring total body water (TBW) is by isotope dilution. Also, the doubly labelled water method, which is the method of choice for measuring total energy expenditure (TEE) in free-living individuals, includes calculation of TBW as the dilution space of the tracer. TBW was measured in 261 subjects (135 males and 126 females), aged 3-87, including healthy children, children with HIV and adults with non-insulin dependent diabetes mellitus (type 2 diabetes), mild hypertension, pancreatic cancer and lung cancer, either in studies of body composition or TEE. A linear relationship was found between TBW and height in all subjects. When TBW is plotted against height cubed (Ht3, m3) the regression line can be forced through the origin. Considering only adults with 18.5>body mass index <29.9 and all children (n=220), this yielded TBW (l)=7.40 x Ht3, R2=0.95. This simple linear relationship between measured TBW and Ht3 compared favourably with other prediction methods, assuming TBW is a constant proportion (55%) of body weight and TBW predicted from height and weight (mean difference between measured and predicted TBW 0.55 l compared with -1.95 and -1.20 l, respectively). Absolute errors were greater at higher TBW, but use of a log10 transformation reduced this effect. This simple relationship of TBW with Ht3 is too crude to be used as a body composition predictor in individual subjects as it ignores, for instance, body shape. However, it can be used as a quality control tool. Here, use of a log10 transformation and residual plot can serve to identify outliers, which can be checked for gross errors in data input and if necessary samples are re-analysed.