Complexes carbeniques des ferroporphyrines: Preparation de complexes carbeniques comportant deux substituants electrodonneurs par alcoolyse ou thionolyse des complexes halogenocarbeniques correspondants

α-Halocarbeneporphyriniron complexes, Fe(P)(C(Cl)R) react with alcohols or thiols with substitution of the chlorine atom by OR′ or SR′ groups. This reaction has been used to obtain new carbeneporphyriniron complexes in which the carbene ligand is substituted by two electrodonating groups. The complexes Fe(P)(C(XR′)R) with XR′ = OCH₃ or OC₂H₅, R = CH₃ or (CH₃)₂CH and P = TPP (tetraphenylporphyrin) or TTP (tetratolylporphyrin) and with XR′ = SCH₂C₆H₅, R = CH₃ and P = TPP or TTP, have been isolated and fully characterized.     

Tacticity measurements on polypropylene sulphide by 13C-NMR spectroscopy

Determination of tacticity of polypropylene sulphide is studied by NMR. The measurements made by ¹³C-NMR are compared with those obtained by ¹H 100 MHz NMR on corresponding selectively deuterated polymers. The good correlation between the two sets of results shows that stereoregularity of polypropylene sulphide can be easily found with 5 per cent precision.     

A Native Ternary Complex Trapped in a Crystal Reveals the Catalytic Mechanism of a Retaining Glycosyltransferase

Glycosyltransferases (GTs) comprise a prominent family of enzymes that play critical roles in a variety of cellular processes, including cell signaling, cell development, and host–pathogen interactions. Glycosyl transfer can proceed with either inversion or retention of the anomeric configuration with respect to the reaction substrates and products. The elucidation of the catalytic mechanism of retaining GTs remains a major challenge. A native ternary complex of a GT in a productive mode for catalysis is reported, that of the retaining glucosyl‐3‐phosphoglycerate synthase GpgS from M. tuberculosis in the presence of the sugar donor UDP‐Glc, the acceptor substrate phosphoglycerate, and the divalent cation cofactor. Through a combination of structural, chemical, enzymatic, molecular dynamics, and quantum‐mechanics/molecular‐mechanics (QM/MM) calculations, the catalytic mechanism was unraveled, thereby providing a strong experimental support for a front–side substrate‐assisted S_Ni‐type reaction.     

Structural Basis of Chitin Oligosaccharide Deacetylation

Cell signaling and other biological activities of chitooligosaccharides (COSs) seem to be dependent not only on the degree of polymerization, but markedly on the specific de‐N‐acetylation pattern. Chitin de‐N‐acetylases (CDAs) catalyze the hydrolysis of the acetamido group in GlcNAc residues of chitin, chitosan, and COS. A major challenge is to understand how CDAs specifically define the distribution of GlcNAc and GlcNH₂ moieties in the oligomeric chain. We report the crystal structure of the Vibrio cholerae CDA in four relevant states of its catalytic cycle. The two enzyme complexes with chitobiose and chitotriose represent the first 3D structures of a CDA with its natural substrates in a productive mode for catalysis, thereby unraveling an induced‐fit mechanism with a significant conformational change of a loop closing the active site. We propose that the deacetylation pattern exhibited by different CDAs is governed by critical loops that shape and differentially block accessible subsites in the binding cleft of CE4 enzymes.     

Can Bluetooth succeed as a large-scale ad hoc networking technology?

We investigate issues that Bluetooth may face in evolving from a simple wire replacement to a large-scale ad hoc networking technology. We do so by examining the efficacy of Bluetooth in establishing a connected topology, which is a basic requirement of any networking technology. We demonstrate that Bluetooth experiences some fundamental algorithmic challenges in accomplishing this seemingly simple task. Specifically, deciding whether there exists at least one connected topology that satisfies the Bluetooth constraints is NP-hard. Several implementation problems also arise due to the internal structure of the Bluetooth protocol stack. All these together degrade the performance of the network, or increase the complexity of operation. Given the availability of efficient substitute technologies, Bluetooth's use may end up being limited to small ad hoc networks.     

Individual QoS versus aggregate QoS: a loss performance study

This paper explores the differences that can exist between individual and aggregate loss guarantees in an environment where guarantees are only provided at the aggregate level. The focus is on understanding which traffic parameters are responsible for inducing possible deviations and to what extent. In addition, we seek to evaluate the level of additional resources, e.g., bandwidth or buffer, required to ensure that all individual loss measures remain below their desired target. This paper's contributions are in developing analytical models that enable the evaluation of individual loss probabilities in settings where only aggregate losses are controlled, and in identifying traffic parameters that have a major influence on the differences between individual and aggregate losses. The latter allows us to further construct practical tools and guidelines for rapidly assessing if specific traffic sources can be safely multiplexed into a common service class.     

Protein structural dynamics of photoactive yellow protein in solution revealed by pump-probe X-ray solution scattering

Photoreceptor proteins play crucial roles in receiving light stimuli that give rise to the responses required for biological function. However, structural characterization of conformational transition of the photoreceptors has been elusive in their native aqueous environment, even for a prototype photoreceptor, photoactive yellow protein (PYP). We employ pump-probe X-ray solution scattering to probe the structural changes that occur during the photocycle of PYP in a wide time range from 3.16 μs to 300 ms. By the analysis of both kinetics and structures of the intermediates, the structural progression of the protein in the solution phase is vividly visualized. We identify four structurally distinct intermediates and their associated five time constants and reconstructed the molecular shapes of the four intermediates from time-independent, species-associated difference scattering curves. The reconstructed structures of the intermediates show the large conformational changes such as the protrusion of N-terminus, which is restricted in the crystalline phase due to the crystal contact and thus could not be clearly observed by X-ray crystallography. The protrusion of the N-terminus and the protein volume gradually increase with the progress of the photocycle and becomes maximal in the final intermediate, which is proposed to be the signaling state. The data not only reveal that a common kinetic mechanism is applicable to both the crystalline and the solution phases, but also provide direct evidence for how the sample environment influences structural dynamics and the reaction rates of the PYP photocycle.