Where is the Limit to which the Qualitative Theory of Chemical Reactivity can be Extended? Study of Dienophilic Activity of Cyanoethylenes

A qualitative quantum-chemical approach, based on the stabilization energy method, is applied to discuss the mechanism and some kinetic regularities observed in Diels-Alder (DA) reactions of cyanoethylenes with cyclopentadiene and phencyclone. For the former reaction series the polar effect predominates over the localizationeffect and both the regular activity sequence and the great range of the rate constants can be understood as a result of the charge transfer from diene to dienophile in a synchronous transition state (TS). The inversion of the regulaT activity sequence observed for the latter reaction series seems to be caused by localizationeffects in asynchronous TS. The relative weakening of the charge transfer is associated with the increased role of electron correlation. Possible distortions of the TS structure within a reaction series are analyzed. The shift of the TS a<long the antisymmetricmode appears to be significant and should be taken into account when considering the kinetic data. The. TS shift along the symmetric coordinate is negligible. The influence of the prereaction complex formation on DA reaction kinetics is found to be inconsequential for the reactions under consideration

Universal architecture of bacterial chemoreceptor arrays

Chemoreceptors are key components of the high-performance signal transduction system that controls bacterial chemotaxis. Chemoreceptors are typically localized in a cluster at the cell pole, where interactions among the receptors in the cluster are thought to contribute to the high sensitivity, wide dynamic range, and precise adaptation of the signaling system. Previous structural and genomic studies have produced conflicting models, however, for the arrangement of the chemoreceptors in the clusters. Using whole-cell electron cryo-tomography, here we show that chemoreceptors of different classes and in many different species representing several major bacterial phyla are all arranged into a highly conserved, 12-nm hexagonal array consistent with the proposed “trimer of dimers” organization. The various observed lengths of the receptors confirm current models for the methylation, flexible bundle, signaling, and linker sub-domains in vivo. Our results suggest that the basic mechanism and function of receptor clustering is universal among bacterial species and was thus conserved during evolution

A Minimal Model of Metabolism Based Chemotaxis

Since the pioneering work by Julius Adler in the 1960's, bacterial chemotaxis has been predominantly studied as metabolism-independent. All available simulation models of bacterial chemotaxis endorse this assumption. Recent studies have shown, however, that many metabolism-dependent chemotactic patterns occur in bacteria. We hereby present the simplest artificial protocell model capable of performing metabolism-based chemotaxis. The model serves as a proof of concept to show how even the simplest metabolism can sustain chemotactic patterns of varying sophistication. It also reproduces a set of phenomena that have recently attracted attention on bacterial chemotaxis and provides insights about alternative mechanisms that could instantiate them. We conclude that relaxing the metabolism-independent assumption provides important theoretical advances, forces us to rethink some established pre-conceptions and may help us better understand unexplored and poorly understood aspects of bacterial chemotaxis

Azospirillum Genomes Reveal Transition of Bacteria from Aquatic to Terrestrial Environments

Fossil records indicate that life appeared in marine environments ∼3.5 billion years ago (Gyr) and transitioned to terrestrial ecosystems nearly 2.5 Gyr. Sequence analysis suggests that “hydrobacteria” and “terrabacteria” might have diverged as early as 3 Gyr. Bacteria of the genus Azospirillum are associated with roots of terrestrial plants; however, virtually all their close relatives are aquatic. We obtained genome sequences of two Azospirillum species and analyzed their gene origins. While most Azospirillum house-keeping genes have orthologs in its close aquatic relatives, this lineage has obtained nearly half of its genome from terrestrial organisms. The majority of genes encoding functions critical for association with plants are among horizontally transferred genes. Our results show that transition of some aquatic bacteria to terrestrial habitats occurred much later than the suggested initial divergence of hydro- and terrabacterial clades. The birth of the genus Azospirillum approximately coincided with the emergence of vascular plants on land

Free radical reactions at high pressures (Modern aspects of physical chemistry at high pressure : the 50th commemorative volume)

In this article the author does not present the complete review of the works concerning free radical reactions at high pressures. He aims at the presentation of the principles, conclusions, and explanations which are the most inetresting from his point of view. Thus the author dwells upon two domains viz. Free radical reactions in the liquid phase and solid-phase reactions initiated by shear. These two divisions considerably differ in the amount of experimental data as well as in the level of theoretical development. The review includes in particular the following points. The rate of radical "cage" reactions may be determined by that of the rotational diffusion and consequently may decrease as the pressure increases. The chain termination in thermal polymerization of styrene seems to proceed in kinetical area because the reaction is characterized by a negative activation volume. There exist quantitative relations between the rate constant and activation volume of the radical substitution reactions involving the transfer of H atom from organic compounds to the Free radical. The existence of the relations is stipulated by different lengthening of valence bonds in the transition state. Deformation of organic substances at high pressure gives rise to new chemical and kinetical phenomena which can not be interpreted from the known points of view on high pressure effects