Structural studies of penicillin acylase

Penicillin acylases are used in the pharmaceutical industry for the preparation of antibiotics. The 3-D structure of Penicillin Gacylase from Escherichia coli has been solved. Here, we present structural data that pertain to the unanswered questions that arose from the original strucutre. Specificity for the amide portion of substrate was probed by the structure determination of a range of complexes with substitutions around the phenylacetyl ring of the ligand. Altered substrate specificity mutations derived from an in vivo positive selection process have also been studied, revealing the structural consequences of mutation at position B71. Protein processing has been analyzed by the construction of site-directed mutants, which affect this reaction with two distinct phenotypes. Mutations that allow processing but yield inactive protein provide the structure of an ES complex with a true substrate, with implications for the enzymatic mechanism and stereospecificity of the reaction. Mutations that preclude processing have allowed the structure of the precursor, which includes the 54a mino acid linker region normally removed from between the A and B chains, to be visualized.     

A resolution of Stewartson’s quarter-infinite plate problem

We revisit a problem originally considered by Stewartson in 1961: the incompressible, high-Reynolds-number flow past a quarter-infinite plate, with a leading edge that is perpendicular to, and a side edge that is parallel to, an undisturbed oncoming freestream. Particular emphasis is placed on the key region close to the side edge, where the flow is (superficially) three-dimensional, although the use of similarity variables reduces the dimensionality of the problem down to two. As noted by Stewartson, this problem has several intriguing features; it includes singularities and is also of a mixed parabolic type, with edge conditions influencing the solution in both directions across the flow domain. These features serve to greatly complicate the (numerical) solution process (the problem is of course also highly non-linear), and computation was clearly infeasible in 1961. In the present paper, a detailed computational study is presented, answering many of the questions that arose from the 1961 study. We present detailed numerical results together with asymptotic analyses of the key locations in the flow.     

Keggin type heteropolyacids-catalyzed synthesis of quinoxaline derivatives in water

Keggin type heteropolyacids was found to be an efficient and reusable catalyst for the synthesis of biologically active quinoxaline derivatives from the condensation of 1,2-diamine with 1,2-dicarbonyl compounds in excellent yields in water.This method provides a new and efficient protocol in terms of small quantity of catalyst,a wide scope of substrates,and simple work-up procedure.     

A convenient synthesis of methyl 2-hydroxy-4-oxo-4-(substituted phenyl) butanoates from malic acid

A synthesis of methyl 2-hydroxy-4-oxo-4-(substituted phenyl)butanoates has been accomplished using Friedel-Crafts acylation of substituted benzenes with 2-acetoxybutanoyl chloride.The key feature of this strategy is that various 4-arylbutanoates can be obtained conveniently,which are important intermediates for the synthesis of biologically active compounds including ACE inhibitors.     

PEG-SO_3H as catalyst for the Beckmann rearrangement and dehydration of oximes

Under mild conditions, conversion of a variety of ketoximes and aldoximes to their corresponding amides and nitriles proceeded in the presence of PEG-SO3H with high yields.     

Fluid flow in curved ducts

The advent of standard algorithms for the numerical solution of partial differential equations has given researchers a new tool for fluid flow calculations. In this paper, single-phase flow in curved ducts is numerically simulated by imposing a spatially varying centrifugal force on a fluid flowing in a straight tube. The resulting set of partial differential equations is solved using the HARWELL-FLOW3D computer program. Comparison with other numerical and experimental results shows that this simplified formulation gives accurate results. The model neglects certain geometric terms of the order d/D, the duct-to-coil diameter ratio. The effect of these terms is investigated by considering the flow in a 90° bend for large d/D. It is shown that while there may be significant error in the prediction of the local variables for large d/D, the circumference-averaged quantities are well predicted.     

Methylation of arsenic by anaerobic microbial consortia isolated from lake sediment

Anaerobic enrichment cultures, isolated from arsenic–contaminated lake sediment in the Canadian sub–arctic and grown in five selective media, methylated arsenate/arsenite to produce mono?, di? and tri–methyl arsenicals. The extent of methylation and methylarsenic species produced varied with the type of enrichment. Iron–reducing, manganese–reducing, sulfate–reducing and broad–spectrum anaerobic heterotrophic mixed cultures all produced methylarsenicals. Sulfate–reducing cultures produced higher concentrations of methylarsenicals (especially trimethyl species) than iron- or maganese–reducers. There is evidence that several of the methylarsenicals, which were hydride–reactive at pH 6, were methylarsenic(III) thiols. The organoarsenicals produced by enrichment cultures were the same as those detected in the porewater of the lake sediments used to initiate the enrichment cultures. Overall, this study demonstrates that microbes from anaerobic lake sediments can methylate (and demethylate) arsenic, a capability shared by manganese?, iron?, and sulfate–reducing microbial consortia.     

Variabilities of measured and simulated soil structures

The macro-structure of tilled soil varies significantly between replicate samples collected from the field. This is illustrated with data from a grey swelling clay from Victoria, Australia. Soil structure was quantified statistically from studies of linear transects on sections cut through impregnated blocks of undisturbed soil as described previously.Simulated soil structures were generated using the mean parameters measured from actual soil structures. The simulated and actual soil structures exhibited similar variabilities. Not all of the statistical parameters are independent, and some covariances of these are examined.The variabilities of some derived structural quantities and dissimilarity coefficients between replicate soil structures are also examined as functions of sample size. Minimum sample sizes are set which are necessary to distinguish between soil structure of various degrees of similarity.     

Decreased molybdenum emission in a carbon arc caused by zirconium carbide formation on the sample electrode

The presence of about 10% zirconium (as oxide) in alumina samples obtained by the ignition of aluminum 8-quinolinolate (precipitated as a trace-element collector) greatly dereased the intensity of molybdenum emission when the alumina was analysed by means of a cathode-layer carbon arc. Retention of the molybdenum on the sample electrode was confirmed with radioactive molybdenum-99. A prominent crystalline residue on the cathode, identified as ZrC by x-ray powder diffraction, was probably responsible for the decreased emission. Thus, zirconium crucibles cannot be used for the sodium carbonate fusion of samples in a spectrochemical procedure for determining traces of molybdenum in soils and plants.