Enzyme-catalysed nitrate reduction-themes and variations as revealed by protein film voltammetry

Protein film voltammetry has been used to define the catalytic performance of two nitrate reductases: the respiratory nitrate reductase, NarGH, from Paracoccus pantotrophus and the assimilatory nitrate reductase, NarB, from Synechococcus sp. PCC 7942. NarGH and NarB present distinct "fingerprints" of catalytic activity when viewed in this way. Potentials that provide insufficient driving force for significant rates of nitrate reduction by NarB result in appreciable rates of nitrate reduction by NarGH. However, both enzymes display complex modulations in their rate of substrate reduction when viewed across the electrochemical potential domain.     

Excitonic effects in solids described by time-dependent density-functional theory

Starting from the many-body Bethe-Salpeter equation we derive an exchange-correlation kernel f(xc) that reproduces excitonic effects in bulk materials within time-dependent density functional theory. The resulting f(xc) accounts for both self-energy corrections and the electron-hole interaction. It is static, nonlocal, and has a long-range Coulomb tail. Taking the example of bulk silicon, we show that the -alpha/q(2) divergency is crucial and can, in the case of continuum excitons, even be sufficient for reproducing the excitonic effects and yielding excellent agreement between the calculated and the experimental absorption spectrum.     

An optimal control problem for the multidimensional diffusion equation with a generalized control variable

The present paper is concerned with an optimal control problem for then-dimensional diffusion equation with a sequence of Radon measures as generalized control variables. Suppose that a desired final state is not reachable. We enlarge the set of admissible controls and provide a solution to the corresponding moment problem for the diffusion equation, so that the previously chosen desired final state is actually reachable by the action of a generalized control. Then, we minimize an objective function in this extended space, which can be characterized as consisting of infinite sequences of Radon measures which satisfy some constraints. Then, we approximate the action of the optimal sequence by that of a control, and finally develop numerical methods to estimate these nearly optimal controls. Several numerical examples are presented to illustrate these ideas.     

Speciation of organic and inorganic arsenic by HPLC-HG-ICP

This paper deals with the application of high performance liquid chromatography (HPLC), hydride generation (HG) and inductively coupled plasma atomic emission spectrometry (ICP) to determine four species of arsenic: As(III), As(V), monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA). The coupling conditions of HPLC-HG-ICP are given. Two anionic exchange columns (Nucleosil-5SB and Hamilton PRP X-100) are tested and the separation conditions optimized. Two acids (H₂SO₄ and HCl at different concentrations) are tested to obtain the hydrides. The proposed method is applied to determine four arsenic species in a synthetic matrix simulating a fish extract.     

Collective excitations in148Gd populated in thep p′-reaction on a radioactive target

Collective excitations in the two-particle nucleus¹⁴⁸Gd up to 2.9 MeV have been investigated by thepp′-reaction at 25 MeV beam energy, and angular distributions have been analyzed with standard DWBA calculations. It is found that¹⁴⁸Gd has larger octupole- than quadrupole collectivity in the energy range investigated. The 3^? strength distribution is in quantitative agreement with predicted results for the levels formed by the coupling of two valence particles to the¹⁴⁶Gd core octupole phonon.     

Optical and loss spectra of carbon nanotubes: depolarization effects and intertube interactions

We performed ab initio calculations of the anisotropic dielectric response of small-diameter single-walled carbon nanotubes in the framework of time-dependent density-functional theory. The calculated optical spectra are in very good agreement with experiment, both concerning absolute peak positions and anisotropy effects. The latter can only be described correctly when crystal local-field effects ("depolarization" effects) are fully taken into account. Moreover, interactions between the tubes can strongly modify their absorption and electron energy-loss spectra.     

Search for the effect of pressure on liquid-liquid critical phenomena

The coexistence curve of methanol + cyclohexane has been reanalyzed using an intrinsically simpler equation to study the influence of pressure on its asymmetry. The results confirm that no pressure dependence is discernible in the coexistence curve. Suggestions for future work in this direction are made.     

Scattering of antiplane shear wave by a propagating crack at the interface of two dissimilar elastic media

An analysis of the scattering of horizontally polarized shear wave by a semi-infinite crack running with uniform velocity along the interface of two dissimilar semi-infinite elastic media has been carried out. The mixed boundary value problem has been solved completely by the Wiener-Hopf technique. The effect of different values of the material parameter, the angle of incidence of incident wave and the crack propagation velocity on the stress intensity factor have been illustrated graphically.