Large protein macromolecules in enzymatic catalysis have been shown to exert a specific electric field that reduces the reorganization energy upon barrier crossing and thus reduces the reaction free energy barrier. In this work we suggest that the charge density in the active site of an enzyme investigated using formalisms embodied by the quantum theory of atoms in molecules (QTAIM) provides a sensitive and quantum field-reactant state charge density-reaction barrier correlation. Hence, QTAIM can be used for the analysis of electric field in enzyme active sites, and further investigations and exploitations of the found correlations may prove useful in enzyme design where preorganization is optimized.
tration in the weld joint. In some cases, such hydrogen localisation is even intensi ed after weld heat treat-A brief overview is given of the role of hydrogen ment has been carried out.trapping in hydrogen management of steel At this point, it is worthwhile to discuss the hydrowelding. The concept of hydrogen trapping is introduced in conjunction with efforts in reducing gen pro le across a high strength steel weld hydrogen cracking in steel welding. Appropriate joint, reported by Musiyachenko and Kasatkin, 2 selection of hydrogen traps offers the potential to as shown in Fig. 1. A weldment, having an initial control the content and distribution of hydrogen in nominal hydrogen content of 2•5 mL/100 g metal steel weldments. The effectiveness of hydrogen (1 mL H 2 /100 g metal=0•89 wt-ppm hydrogen in traps through selective alloying in reducing the steel ), was post-weld heat treated at 700°C, for stress concentration of diffusible hydrogen during relief and hardness reduction. As expected, the nomwelding thermal cycles is evaluated, particularly in inal hydrogen content of the weldment decreased to its use as a substitute for costly weld heat 0•5 mL H 2 /100 g metal. In the meantime, hydrogen treatment practices. The relationship of weld metal accumulated at the fusion line of the weldment with microstructure and hydrogen contents to the effective use of hydrogen trapping elements is a concentration not lower than the value before the discussed. The fundamental aspects which control treatment. The localisation of hydrogen was thought hydrogen content as well as hydrogen distribution to occur due to stress assisted diVusion as well as in steel are discussed, such as the statistical dislocation assisted transport of hydrogen. It was not thermodynamics and the transport of hydrogen in clear why an unusually high post-weld heat treatment steel containing traps. As tools for investigating temperature was applied in that study. Such a temperpotential hydrogen traps, several experimental ature is more commonly used for the stress relief methods are discussed in relation to their necessary for prevention of temper embrittlement, applicability to characterise the physical nature of which is caused by embrittling species other than hydrogen traps. These methods include the hydrogen. However, a weld joint would have a higher hydrogen permeation technique and hydrogen thermal desorption analysis. Recent efforts to restraint stress if normal post-weld heat temperatures prevent hydrogen cracking in steel weldments were applied to the weld joint, which would consethrough numerical computations require quently produce a larger amplitude of hydrogen consideration of hydrogen trapping in the weld accumulation. microstructure, and hence knowledge of applicable Considering the above perspectives, attempts to data of hydrogen trapping parameters is necessary improve welding consumables have evolved towards for welding engineers to improve hydrogen the reduction of weld heat treatments. Welding promanagement in steel welding. IMR/380 ...
The exact positions of critical points in the charge density in enzyme active sites reflects electrostatic preorganization.
ABSTRACT:Much of modern chemistry is concerned with the properties and dynamics of chemical bonds. Although they have been described variously, the most familiar representation is that of a link connecting two atoms. However, no one has yet developed a scheme by which to partition a molecule into bond volumes with well-defined properties. As a consequence, the chemical bond is left as nothing more than a heuristic devise. Here, we show molecules can be partitioned into bond-bundles-volumes that share many of the properties associated with the conceptual bond. This partitioning follows naturally through an extension of Baders topological theory of molecular structure. Surprisingly, it also bounds regions of space containing nonbonding or lone-pair electrons and leads to bond orders consistent with those expected from theories of directed valance.
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