A sensitive and accurate automatic coulometric detector system for enzymatically produced carbon dioxide

A system for the determination of carbon dioxide liberated in an enzyme reactor is described. The properties of the system are illustrated by the determination of urea. Enzymatically generated carbon dioxide is expelled by boiling, dried and determined coulometrically. The lowest concentration of urea which can be determined with a relative standard deviation of 5% is 1 μM. Compared with other methods based on immobilized enzymes for the determination of compounds as carbon dioxide, this sensitivity constitutes an improvement of about two orders of magnitude. Possible application to sea water is discussed.     

Interactions of Humicola insolens cutinase with an anionic surfactant studied by small-angle neutron scattering and isothermal titration calorimetry

The interaction of cutinase from Humicula insolens (HiC) and sodium dodecyl sulfate (SDS) has been investigated by small-angle neutron scattering (SANS) and isothermal titration calorimetry (ITC). The concerted interpretation of structural and thermodynamic information for identical systems proved valuable in attempts to elucidate the complex modes of protein-detergent interaction. Particularly so at the experimental temperature 22 degrees C, where the formation of SDS micelles is athermal (deltaH = 0), and the effects of protein-detergent interactions stand out clearly in the thermograms. It was found that the effect of SDS on cutinase depended strongly on the sample composition. Thus, addition of SDS corresponding to a molar ratio, n(s) = n(SDS)/n(HiC) of about 10, was associated with the formation of HiC/SDS aggregates, which include more than one protein molecule. The SANS results suggested that on the average such adducts contained two HiC, and the ITC traces showed that they form and break down slowly. At slightly higher SDS concentrations (n(s) = 10-25) these "dimers" dissociated, and the protein denatured. The denaturation showed the characteristic positive enthalpy change, but the SDS denatured state of HiC was unusually compact with a radius of gyration close to that of the native conformation. Further titration with SDS was associated with exothermic binding to the denatured protein until the saturation point at about n(s) = 90. At this point, the free monomer concentration was 2.2 mM and the binding number was approximately 40 SDS/HiC. Interestingly, this degree of SDS binding (approximately 0.5 g of SDS/g of HiC) is less than half the amount bound to typical water-soluble proteins.     

Investigations of reactions involved in flameless atomic absorption procedures : Part II. An experimental study of the rôle of hydrogen in eliminating the interference from chlorine in the determination of lead in steel

The rôle of hydrogen in eliminating the interference from chlorine in the flameless a.a.s. method for lead in steel has been experimentally studied with the Perkin-Elmer HGA 72 and Varian-Techtron CRA 63 graphite furnaces. Hydrogen is generated at high temperatures in both systems by the reaction of graphite with water left in the tube after the drying step. The amount of hydrogen formed during the ashing procedure in the Perkin-Elmer tube is about five times larger than that in the Varian tube. The removal of chlorine from graphite tubes containing dissolved steel samples has been determined as a function of temperature for (i) the CRA 63 without hydrogen added, (ii) the CRA 63 with hydrogen added, and (iii) the HGA 72 without hydrogen added. At 950 K, a significant amount of chlorine is left only in case (i); thus the amount of hydrogen formed in the HGA 72 is large enough to remove all of the chlorine through the reaction FeCl₂(g) + H₂(g) → Fe(s) + 2 HCl(g). If a sample is ashed at 950 K, losses of lead as lead chlorides only occur for the case (i) which is in accordance with theoretical prediction. The theoretically predicted optimum ashing temperature range of 900–1000 K is shown to be in good agreement with the experimental results.     

SCF and CASSCF studies of geometrical rearrangements in LiBO

The minimum energy path for the rearrangement LiOB OBLi was calculated with the SCF approximation using a double-zeta plus polarization basis set. Stationary points on the potential surface were studied with the help of the CASSCF method using different choices of active space. The results indicate that LiBO may be regarded as a polytopic type molecule. The relative energies of different geometrical configurations changed considerably when the CASSCF method was used, compared to those obtained at the SCF level, demonstrating the importance of correlation effects for this system.     

Microdetermination of caffeine in blood by gas chromatography--mass spectrometry

A micromethod for the quantitative analysis of caffeine present in small quantities (100 microliter) of whole blood is described. It is based on the gas chromatographic--mass spectrometric analysis of chloroform extracts of biological samples. The method is relatively simple, rapid, specific and sensitive; as little as 20 ng of caffeine can be measured.     

Determination of the 15N/14N ratio of ammonium and ammonia in aqueous solutions by equilibrium headspace-gas chromatography-combustion-isotope ratio mass spectrometry

A method for determination of the 15N/14N ratio of total ammoniacal nitrogen (TAN; ammonium and ammonia) in aqueous solutions was developed, primarily intended for use with soil extracts, which have a high TAN level, e.g. from recently fertilised agricultural soils. Ammonium was converted to ammonia by addition of NaOH, followed by nitrogen isotopic analysis of the headspace by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) where complete separation of TAN from the matrix was not necessary. The ammonia concentration in the gas phase was maximised by increasing the temperature and salt concentration and by decreasing the gas liquid ratio in the headspace vials. Isotopic equilibrium was reached after less than 1 h at 80 degrees C. The measured isotopic ratio was constant for solutions containing 30-200 mM NH4-N, corresponding to 950-7000 ng NH3-N detected with the IRMS. The integrated area response at m/z 28 increased linearly with the ammonium ion concentration in the interval 10-200 mM NH4-N. The fractionation factor between the liquid and gas phases was 1.0054 +/- 0.0007 within the linear range, which is in agreement with values reported in the literature, but with a higher precision. Changes in temperature, gas:liquid ratio or salt concentration did not affect the measured ratio, demonstrating the robustness of the developed method.     

Photo-stability of photo-cured organic coatings: Part III—Photo-degradation of organic coatings photo-cured with alkoxyacetophenone and hydroxy alkylacetophenone photo-initiators

Photo-degradation studies of polystyrene (as model) and coating resins were made with three different photo-fragmenting initiators: 2,2-dimethoxy-2-phenylacetophenone, 1-[4-(2-propyl)-phenyl]-2-hydroxy-2-methyl-propanone-1 and 1-benzoyl-1-hydroxy-cyclohexane. Addition of the initiators to a benzene solution of polystyrene gave a rapid decrease in molecular weight when the solutions were uv irradiated. The degradation rate was increased by the presence of oxygen. Irradiation of polystyrene films, containing the same initiators, showed similar effects. In addition, the polymer became partially crosslinked. Addition of a free radical scavenger decreased the degradation rate. Both photo-oxidation rate and yellowing rate were affected by the concentration and the reactivity of the radicals. Photo-oxidation during photo-curing of a coating occurs in a thin surface layer. This is due to a combination of free radicals formed by the uv irradiation and the oxygen present. Oxidation during the photo-curing of a urethane resin corresponds to more than 50% of the total surface oxidation measured with ESCA.     

High-performance liquid chromatographic method for the analysis of a candidate 8-aminoquinoline antileishmanial drug using oxidative electrochemical detection

An analytical method was developed for the quantitation of a candidate antileishmanial drug, 6-methoxy-8-(6-diethylaminohexylamino)-4-methylquinoline, dihydrochloride, in canine plasma. The assay utilized internal standard technique with a structural similar 8-aminoquinoline, 6-methoxy-8-(7-diethylaminoheptylamino)-4-methylquinoline, dihydrochloride, as the internal standard. The method employs a liquid-solid extraction procedure with prepackaged silica gel columns upon which the drug and internal standard are adsorbed, then selectively washed and eluted. Reversed-phase chromatography was then employed to analyze the extracted sample by means of oxidative electrochemical detection at +0.75 V. Good accuracy and precision were obtained over the range of concentration tested (10-1500 ng/ml plasma). Analyses of plasma samples from human volunteers given the drug demonstrate the method is also suitable for analysis of human plasma samples. The entire procedure is relatively simple and requires only 1 ml of plasma.     

Curve-straightening in closed Euclidean submanifolds

The flow in the negative direction of the gradient vector field associated with the functional total squared (geodesic) curvature k ² ds is the so-called curvestraightening flow. This paper will consider spaces of closed curves in closed Euclidean submanifolds. It will define these spaces of curves as submanifolds of certain Hilbert manifolds representing all curves. The main result will then be to show the existence of a particular set of functionals defined on the entire Hilbert manifold which have the following four properties: 1. The directional derivatives of these functionals may be computed by solving an initial value problem for a system of ordinary differential equations. 2. By introducing a suitable Hilbert space basis for the Sobolev spaces used, the gradients may be effectively computed (but of course not explicitly computed, except in very special cases). 3. The gradients span the space normal to the tangent space of the space of closed curves. 4. Despite the fact that these gradients in general are not given explicitly it is nevertheless possible to compute the projection onto the tangent space to the space of closed curves. In particular we do this for the gradient of k ² ds. When all details are worked out this gives us an algorithm (which we supply) for finding critical points in the space of closed curves. It is not known if the trajectories actually always converge to critical points. If the functional is modified to include a multiple of the length so the functional becomes k ²+ds then the above convergence is known for >0. The motivating application for the curvestraightening flow is the possibility of using it to find (non-trivial) closed (periodic) geodesics. Note that if =0 then a closed geodesic is a global minimum. For any , geodesics are critical but there are also other critical points, the so-called elastic curves. The paper concludes by deriving the second variation formula for k ²+ds along closed geodesics. The quadratic functional associated with the second derivative is shown to be positive definite even for non-zero along some closed geodesics in some particular manifolds of interest.     

Solvent effects on the n-->pi* electronic transition in formaldehyde: a combined coupled cluster/molecular dynamics study

We present a study of the blueshift of the n-->pi* electronic transition in formaldehyde in aqueous solution using a combined coupled cluster/molecular mechanics model including mutual polarization effects in the Hamiltonian. In addition, we report ground and excited state dipole moments. Configurations are generated from molecular dynamics simulations with two different force fields, one with and one without an explicit polarization contribution. A statistical analysis using 1200 configurations is presented. Effects of explicit polarization contributions are found to be significant. It is found that the main difference in the effects on the excitation energies arises from the fact that the two force fields result in different liquid structures, and thus a different set of configurations is generated for the coupled cluster/molecular mechanics calculations.