The only active mutant of thymidylate synthase D169, a residue far from the site of methyl transfer, demonstrates the exquisite nature of enzyme specificity

Cysteine is the only variant of D169, a cofactor-binding residuein thymidylate synthase, that shows in vivo activity. The 2.4Å crystal structure of Escherichia coli thymidylate synthaseD169C in a complex with dUMP and the antifolate CB3717 showsit to be an asymmetric dimer, with only one active site covalentlybonded to dUMP. At the active site with covalently bound substrate,C169 S     

Molybdenum Electrodes in Glass Tanks

The available industrial experience is reviewed and summarized in order to define the general principles of electrode design and operation. The discussion includes the properties of molybdenum that affect its use in electrodes, the relative merits of different styles of electrodes, and electrode holders, procedures for installation and operation, and the effect of electrical and convection currents on electrode placement and tank performance.     

Characterization of the thermal and fluid flow behavior of industrial ribbon burners

This study focuses on the key parameters that enhance the stability of ribbon burners used in various industrial processes, characterizing the flame environment and flow regimes that the burner creates under changing operating conditions. The research includes the exploration of ribbon-pack configurations in order to define the effects of separation distance and port arrangement on flame stability and thermal and fluid flow. Flow visualization studies reveal that burners having a 1.5–2.0 mm division between rows appear to be the most stable. More than four rows of ports in the ribbon-pack are required to resist entrainment of ambient air as a result of impinging surface motion. The flame environment created by the ribbon burner experiences higher levels of mixing, at a lower Reynolds number in comparison to a single non-reacting jet due to combustion-induced turbulence and jet interaction. Under these high flow velocity conditions there is little entrainment of ambient air along the impingement surface and a very stable flame environment is created.     

Enantioselective degradation of organochlorine pesticides in background soils: variability in field and laboratory studies

Variability in the enantioselective degradation of chiral organochlorine pesticides (alpha-HCH, cis- and trans-chlordane (CC and TC), and o,p'-DDT) in the field and laboratory was investigated. Background soils presumably receive the same EF signature of a compound via atmospheric deposition and then degrade that compound in a way that can vary over small spatial areas. Background soils from woodland and grassland areas were sampled to compare chiral signatures and determine the spatial variability within a few square meters. The enantiomer fractions, EF = areas of the (+)/[(+)+(-)]-enantiomers, showed variability between and within ecosystems. For example, the EF of CC varied between 0.272 -and 0.558 in nine samples taken over a few square meters, and a range of 0.431-0.506 was found within depths of a few centimeters. Woodland and grassland soils were spiked with alpha-HCH, TC, CC, and o,p'-DDT, and one portion was placed in the field to monitor changes in EF under in situ conditions and the other taken to the laboratory. In general, the enantiomer degradation preferences in the laboratory paralleled those in the field, with some differences. Soil organic matter content and pH exerted a minor influence on this variability. The results of this study have implications for the use of chiral compounds to make inferences about air-soil exchange and for the mechanisms of biodegradation/ biotransformation of anthropogenic compounds in soils.     

Are emissions of black carbon from gasoline vehicles underestimated? Insights from near and on-road measurements

Measurements of black carbon (BC) with a high-sensitivity laser-induced incandescence (HS-LII) instrument and a single particle soot photometer (SP2) were conducted upwind, downwind, and while driving on a highway dominated by gasoline vehicles. The results are used with concurrent CO(2) measurements to derive fuel-based BC emission factors for real-world average fleet and heavy-duty diesel vehicles separately. The derived emission factors from both instruments are compared, and a low SP2 bias (relative to the HS-LII) is found to be caused by a BC mass mode diameter less than 75 nm, that is most prominent with the gasoline fleet but is not present in the heavy-duty diesel vehicle exhaust on the highway. Results from both the LII and the SP2 demonstrate that the BC emission factors from gasoline vehicles are at least a factor of 2 higher than previous North American measurements, and a factor of 9 higher than currently used emission inventories in Canada, derived with the MOBILE 6.2C model. Conversely, the measured BC emission factor for heavy-duty diesel vehicles is in reasonable agreement with previous measurements. The results suggest that greater attention must be paid to black carbon from gasoline engines to obtain a full understanding of the impact of black carbon on air quality and climate and to devise appropriate mitigation strategies.