Different transition-state structures for the reactions of beta-lactams and analogous beta-sultams with serine beta-lactamases

Beta-sultams are the sulfonyl analogues of beta-lactams, and N-acyl beta-sultams are novel inactivators of the class C beta-lactamase of Enterobacter cloacae P99. They sulfonylate the active site serine residue to form a sulfonate ester which subsequently undergoes C-O bond fission and formation of a dehydroalanine residue by elimination of the sulfonate anion as shown by electrospray ionization mass spectroscopy. The analogous N-acyl beta-lactams are substrates for beta-lactamase and undergo enzyme-catalyzed hydrolysis presumably by the normal acylation-deacylation process. The rates of acylation of the enzyme by the beta-lactams, measured by the second-order rate constant for hydrolysis, kcat/K(m), and those of sulfonylation by the beta-sultams, measured by the second-order rate constant for inactivation, k(i), both show a similar pH dependence to that exhibited by the beta-lactamase-catalyzed hydrolysis of beta-lactam antibiotics. Electron-withdrawing groups in the aryl residue of the leaving group of N-aroyl beta-lactams increase the rate of alkaline hydrolysis and give a Bronsted beta(lg) of -0.55, indicative of a late transition state for rate-limiting formation of the tetrahedral intermediate. Interestingly, the corresponding Bronsted beta(lg) for the beta-lactamase-catalyzed hydrolysis of the same substrates is -0.06, indicative of an earlier transition state for the enzyme-catalyzed reaction. By contrast, although the Bronsted beta(lg) for the alkaline hydrolysis of N-aroyl beta-sultams is -0.73, similar to that for the beta-lactams, that for the sulfonylation of beta-lactamase by these compounds is -1.46, compatible with significant amide anion expulsion/S-N fission in the transition state. In this case, the enzyme reaction displays a later transition state compared with hydroxide-ion-catalyzed hydrolysis of the beta-sultam.     

ICP-AES determination of silver after chemical separation from uranium matrix

The separation of silver from a uranium matrix has been carried out using Cyanex-471X (triisobutylphosphine sulphide) in xylene. The effects of various parameters such as the Cyanex-471X concentration, the nitric acid molarity, the contact time and the nitrate ion concentration on the extraction of silver have been studied. The silver metal ion species extracted into the organic phase was found to be Ag(NO₃)·2S (where S is Cyanex-471X). The stripping of silver into an aqueous medium was carried out with 5% NaHSO₃, followed by its determination using ICP-AES.     

Radio-tracer methods for studying speciation in natural waters

Radio-tracer experiments have shown that antimony, mercury and zinc interact to form complexes with humic and/or fulvic acids, whose molar masses can be estimated by gel chromatography. Sb(III) and (V) humates are stable in the pH range 7–11, but are largely dissociated below pH 4; humic acid does not reduce Sb(V) to Sb(III) in solution. Mercury forms a strong complex with humic+fulvic acids. Zinc forms complexes with both humic acids and glycine, and the humic acid complex has similar elution behaviour on dextran gel to a fraction from river water equilibrated with⁶⁵Zn. At least one other form of zinc, in addition to Zn²⁺, occurs in this river.     

Force Calibration in Lateral Force Microscopy

An analysis of the contact mechanics and the forces of interaction in lateral force microscopy measurements is presented. This analysis allows for a new method of interpretation of the frictional forces, the lateral contact stiffness, and the contact shear strength. The technique was developed for the interpretation of frictional data obtained with colloidal probes, although results are presented which illustrate its ability to interpret measurements recorded with both colloidal probes and standard atomic force microscopy tips. The technique is found to compensate for the variations in the contact geometry, giving repeatable results for probes of different sizes. A critical review of other techniques which have been employed to interpret the frictional force in lateral force microscopy is also presented. Copyright 2000 Academic Press.     

The polarographic determination of aluminium in zinc-base die-casting alloys

A procedure for the direct polarographic determination of aluminium is described, with particular reference to zinc-base die-casting alloys. Interfering elements are removed by electrolysis of a solution of the alloy in a mercury-cathode cell. The solution is polarographed at a carefully controlled pH in a supporting electrolyte of magnesium chloride and sodium sulfate of appropriate concentration. The precision and accuracy of the method are shown to be satisfactory by the analysis of standard samples from three different laboratories.     

Self-gravitating radiation in anti-de sitter space

Equilibrium configurations of self-gravitating massless thermal radiation inside spherical boxes of radiusR in asymptotically anti-de Sitter space (A = -3/b ₂) are constructed numerically for a range of central densities. For each box radius considered (R/b = 0, 1/2, 1, 2, 4, ), there is a unique configuration with maximal total mass and entropy, and another (at a lower central density) with maximum asymptotic red-shifted temperature. With the box removed toR=, the maximum total mass and entropy of self-gravitating thermal radiation areM _max 0.4598b0.7964(–A)^–1/2 andS _max1.3560a ^1/4 b ^3/2 3.0910a ^1/4(–A)^–3/4, and the maximum red-shifted temperature is