Search results for "pair"
showing 10 items of 2908 documents
Direct observation of a borane-silane complex involved in frustrated Lewis-pair-mediated hydrosilylations.
2014
Perfluorarylborane Lewis acids catalyse the addition of silicon-hydrogen bonds across C=C, C=N and C=O double bonds. This 'metal-free' hydrosilylation has been proposed to occur via borane activation of the silane Si-H bond, rather than through classical Lewis acid/base adducts with the substrate. However, the key borane/silane adduct had not been observed experimentally. Here it is shown that the strongly Lewis acidic, antiaromatic 1,2,3-tris(pentafluorophenyl)-4,5,6,7-tetrafluoro-1-boraindene forms an observable, isolable adduct with triethylsilane. The equilibrium for adduct formation was studied quantitatively through variable-temperature NMR spectroscopic investigations. The interactio…
A new model for C–C bond formation processes derived from the Molecular Electron Density Theory in the study of the mechanism of [3+2] cycloaddition …
2016
Abstract The [3+2] cycloaddition (32CA) reactions of the nitrile ylide (NY) with ethylene and with dicyanoethylene (DCE) have been studied using the Molecular Electron Density Theory through DFT calculations at the MPWB1K/6-31G(d) level. The analysis of the electronic structure of NY indicates that it presents a carbenoid structure with an sp 2 lone pair at the carbon atom. While the 32CA reaction with ethylene presents a low activation energy, 6.1 kcal mol −1 , the transition state structure associated with the 32CA reaction of NY with DCE is located 7.5 kcal mol −1 below the reagents, the reaction being completely regioselective. The topological analysis of the Electron Localisation Funct…
RNase H1 and H2 are differentially regulated to eliminate RNA-DNA hybrids
2019
SUMMARYRNA-DNA hybrids are tightly regulated to ensure genome integrity. The RNase H enzymes, RNase H1 and H2, contribute to chromosomal stability through the removal of RNA-DNA hybrids. Loss of RNase H2 function is implicated in human diseases of the nervous system and cancer. To better understand RNA-DNA hybrid dynamics, we have focused on elucidating the regulation of the RNase H enzymes themselves. Using yeast as a model system, we demonstrate that RNase H1 and H2 are controlled in different manners. RNase H2 is regulated in a strict cell cycle dependent manner, both in terms of its R-loop removal, and ribonucleotide excision repair functions. RNase H1, however, can function independent…
Determination of steady-state levels of oxidative DNA base modifications in mammalian cells by means of repair endonucleases
1997
The alkaline elution technique in combination with various repair endonucleases (Fpg protein, endonuclease III, exonuclease III, T4 endonuclease V) was used to quantify steady-state (background) levels of oxidative base modifications in various types of mammalian cells. In human lymphocytes the number of base modifications sensitive to Fpg protein, which include 8-hydroxyguanine, was 0.25 +/- 0.05 per 10(6) base pairs. Even lower levels (0.07 +/- 0.02 per 10(6) bp) were observed in HeLa cells. The numbers of sites sensitive to the other repair endonucleases were below the detection limit (0.05 per 10(6) bp). In a direct comparison, the background level of Fpg-sensitive modifications determi…
Molecular Design of the Cαβ Interface Favors Specific Pairing of Introduced TCRαβ in Human T Cells
2008
Abstract A promising approach to adoptive transfer therapy of tumors is to reprogram autologous T lymphocytes by TCR gene transfer of defined Ag specificity. An obstacle, however, is the undesired pairing of introduced TCRα- and TCRβ-chains with the endogenous TCR chains. These events vary depending on the individual endogenous TCR and they not only may reduce the levels of cell surface-introduced TCR but also may generate hybrid TCR with unknown Ag specificities. We show that such hybrid heterodimers can be generated even by the pairing of human and mouse TCRα- and TCRβ-chains. To overcome this hurdle, we have identified a pair of amino acid residues in the crystal structure of a TCR that …
Supramolecular Assembly of Metal Complexes by (Aryl)I⋅⋅⋅d[PtII] Halogen Bonds
2020
The theoretical data for the half-lantern complexes [{Pt( CN^ )(μ- SN^ )}2 ] [1-3; CN^ is cyclometalated 2-Ph-benzothiazole; SN^ is 2-SH-pyridine (1), 2-SH-benzoxazole (2), 2-SH-tetrafluorobenzothiazole (3)] indicate that the Pt⋅⋅⋅Pt orbital interaction increases the nucleophilicity of the outer d z2 orbitals to provide assembly with electrophilic species. Complexes 1-3 were co-crystallized with bifunctional halogen bonding (XB) donors to give adducts (1-3)2 ⋅(1,4-diiodotetrafluorobenzene) and infinite polymeric [1⋅1,1'-diiodoperfluorodiphenyl]n . X-ray crystallography revealed that the supramolecular assembly is achieved through (Aryl)I⋅⋅⋅d z2 [PtII ] XBs between iodine σ-holes and lone pa…
Isolation of a stable, acyclic, two-coordinate silylene.
2012
The synthesis and characterization of a stable, acyclic two-coordinate silylene, Si(SAr(Me(6)))(2) [Ar(Me(6)) = C(6)H(3)-2,6(C(6)H(2)-2,4,6-Me(3))(2)], by reduction of Br(2)Si(SAr(Me(6)))(2) with a magnesium(I) reductant is described. It features a V-shaped silicon coordination with a S-Si-S angle of 90.52(2)° and an average Si-S distance of 2.158(3) A. Although it reacts readily with an alkyl halide, it does not react with hydrogen under ambient conditions, probably as a result of the ca. 4.3 eV energy difference between the frontier silicon lone pair and 3p orbitals.
Injectable in situ forming hydrogels based on natural and synthetic polymers for potential application in cartilage repair
2015
In this work we prepared two new hyaluronic acid (HA) based in situ forming hydrogels for the potential treatment of articular cartilage damages. In particular the amino derivative of HA (HA-EDA) and its graft copolymer with α-elastin (HA-EDA-g-α-elastin) were crosslinked, in mild physiological conditions via Michael-type addition, with α,β-poly(N-2-hydroxyethyl)-dl-aspartamide (PHEA) derivatized with divinylsulfone (DV). The swelling and degradation profile of the obtained hydrogels as well as the metabolic activity of incorporated bovine articular chondrocytes were investigated. Histological analysis and scanning electron microscopy (SEM) were performed to analyze the morphology of cells …
Chemical and spectroscopical evidence for an electron‐transfer mechanism in the reaction of arenesulfonyl chlorides with anions
1989
The reaction of amide and amidate anions 2 with p-toluenesulfonyl chloride (1) under different reaction conditions gives rise to the total or partial reduction of the acyl halide to p-toluenesulfinic acid (5) and acylation compounds in variable amounts depending on the crowding at the anionic center. This indicates that a Single-Electron Transfer (SET) mechanism is involved in the reactions of 1 with anions. Unpaired electron species are detected by ESR in the course of the reactions.
The first structurally characterized cationic lanthanide–alkyl complexesElectronic supplementary information (ESI) available: experimental and spectr…
2002
Reaction of rare earth metal–alkyl complexes [Ln(CH2SiMe3)3(THF)2] (Ln = Y, Lu) with B(C6X5)3 (X = H, F) in the presence of crown ethers gives crystallographically characterized ion pairs [Ln(CH2SiMe3)2(CE)(THF)n]+[B(CH2SiMe3)(C6X5)3]– (CE = [12]-crown-4, n = 1; CE = [15]-crown-5 and [18]-crown-6, n = 0).