Ketene-ketenimine rearrangements in the gas phase and in polar media. 1,3-Migration intermediates and sequential transition states

[reaction: see text] Calculations of the activation barrier for the 1,3-shifts of substituents X in alpha-imidoylketenes 1 (HN=C(X)-CH=C=O), which interconverts them with alpha-oxoketenimines 3 (HN=C=CH-C(X)=O) via a four-membered cyclic transition state TS2 have been performed at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G* level. Substituents with accessible lone pairs have the lowest activation barriers for the 1,3-shift (halogens, OR, NR2). The corresponding activation barriers for the alpha-oxoketene-alpha-oxoketene rearrangement of 4 via TS5 are generally lower by 1-30 kJ/mol. A polar medium (acetonitrile, epsilon = 36.64) was simulated using the polarizable continuum (PCM) solvation model. The effect of the solvent field is a reduction of the activation barrier by an average of 12 kJ/mol. In the cases of 1,3-shifts of amino and dimethylamino groups, the stabilization of the transition state TS2 in a solvent field is so large that it becomes an intermediate, Int2, flanked by transition states (TS2' and TS2') that are due primarily to internal rotation of the amine functions, and secondarily to the 1,3-bonding interaction. In the case of the alpha-oxoketene-alpha-oxoketene rearrangement of 4, there is a corresponding intermediate Int5 for the 1,3-amine shift already in the gas phase.     

Single metal four-electron reduction by U(ii) and masked “U(ii)” compounds

The redox chemistry of uranium is dominated by single electron transfer reactions while single metal four-electron transfers remain unknown in f-element chemistry. Here we show that the oxo bridged diuranium(iii) complex [K(2.2.2-cryptand)]₂[{((Me₃Si)₂N)₃U}₂(μ-O)], 1, effects the two-electron reduction of diphenylacetylene and the four-electron reduction of azobenzene through a masked U(ii) intermediate affording a stable metallacyclopropene complex of uranium(iv), [K(2.2.2-cryptand)][U(η²-C₂Ph₂){N(SiMe₃)₂}₃], 3, and a bis(imido)uranium(vi) complex [K(2.2.2-cryptand)][U(NPh)₂{N(SiMe₃)₂}₃], 4, respectively. The same reactivity is observed for the previously reported U(ii) complex [K(2.2.2-cryptand)][U{N(SiMe₃)₂}₃], 2. Computational studies indicate that the four-electron reduction of azobenzene occurs at a single U(ii) centre via two consecutive two-electron transfers and involves the formation of a U(iv) hydrazide intermediate. The isolation of the cis-hydrazide intermediate [K(2.2.2-cryptand)][U(N₂Ph₂){N(SiMe₃)₂}₃], 5, corroborated the mechanism proposed for the formation of the U(vi) bis(imido) complex. The reduction of azobenzene by U(ii) provided the first example of a “clear-cut” single metal four-electron transfer in f-element chemistry.

Both a masked and the actual complex [U(ii){N(SiMe₃)₂}₃]⁺ effect the reduction of azobenzene to yield a U(vi) bis-imido species providing the first example of a “clear-cut” metal centred four-electron reduction in f-element chemistry.     

Synthesis of the C3-C19 segment of phorboxazole B

[reaction: see text]. Three segment-coupling Prins approaches to the C3-C19 segment of phorboxazole B have been developed. One successful strategy utilized a novel TMSBr-mediated cyclization that proceeded with complete axial selectivity. Displacement of bromide with cesium acetate provided the C13 hydroxyl stereocenter of 22. Additionally, treatment of alpha-acetoxy ether 20 with TFA enabled a more concise synthesis of the C3-C19 target 13 by allowing direct access to the equatorial alcohol.     

DNA polymerase-mediated DNA synthesis on a TNA template

TNA, or threose nucleic acid, is capable of Watson-Crick base pairing with DNA, RNA, and TNA; coupled with its chemical simplicity, this suggests that TNA is a possible progenitor of RNA. As an initial step toward developing the molecular tools necessary to investigate the functional capabilities of TNA by in vitro selection, we have screened a variety of DNA polymerases for activity on a TNA template. We report that despite having a repeating unit that is one atom shorter than that of DNA, several polymerases showed surprisingly good ability to copy limited stretches of TNA.     

1,1,3,3-Tetramethylguanidine-gallane, (Me2N)2CN(H).GaH3: an unusually strongly bound gallane adduct

The novel adduct 1,1,3,3-tetramethylguanidine-gallane, (Me2N)2CN(H).GaH3, has been prepared by the reaction of [(Me2N)2CNH2]+Cl- with LiGaH4 in Et2O solution. Its spectroscopic properties indicate a monomeric species with an unusually strong coordinate link between the imido function and GaH3, an inference confirmed by the crystal structure at 150 K which also reveals significant secondary interactions through non-classical N-H...H-Ga bridges. Despite the intrinsic strength of the Ga-N bond, however, vaporisation at ca. 310 K occurs with partial dissociation, and decomposition via more than one pathway proceeds at temperatures >330-350 K to give a variety of products, including the free base, Me2NH, H2, and a novel gallium-nitrogen compound composed of a Ga4N4 cubane-like core bridged on three edges by -N{C(NMe2)2}GaH2- units.     

Selectively detecting attomolar concentrations of proteins using gold lined nanopores in a nanopore blockade sensor

Disease diagnosis at earlier stages requires the development of ultrasensitive biosensors for detecting low-abundance biomarkers in complex biological fluids within a reasonable time frame. Here, we demonstrate the development of an ultrasensitive nanopore blockade biosensor that can rapidly diagnose a model protein biomarker, prostate-specific antigen (PSA) with high selectivity. The solid-state nanopores have gold located only along the length of the nanopore whilst the rest of the membrane is silicon nitride. The orthogonal use of materials allows nanopore arrays with a different surface chemistry inside the nanopore relative to the rest of the membrane to be fabricated. The importance of this differential surface chemistry is it can improve the detection limit of nanopore blockade sensors in quantitative analysis. Based on such functionalized nanopore devices, nanopore blockade sensors lower the limit of detection by an order of magnitude and enable ultrasensitive detection of PSA as low as 80 aM. The findings from this study open new opportunities for nanopore sensors in further developments including optical detection and ultralow detection limit biosensing at complex biological fluids.

Selective detection of attomolar proteins was achieved using gold lined nanopores in a nanopore blockade sensor.     

Structures of neat and hydrated 1-octanol from computer simulations

Molecular dynamics computer simulations of 1-octanol and its mixtures with water have been performed. The liquid is composed of regions enriched in either hydrocarbons or hydroxyl groups. In neat octanol, the hydroxyl groups form clusters of long, thin chains. Upon the addition of water, the clusters become longer and more spherical, forming a structure that can be described as consisting of "overlapping elongated inverse micelles". The structures of the mixtures obtained at different hydration levels are consistent with those of experimental diffraction studies of water/octanol mixtures and previous computer simulations of neat and water-saturated octanol. The saturation point of the model has been calculated using the cavity-bias particle insertion method. The solubility of water in octanol is slightly too low compared to experimental results, and suggestions for possible improvements to the force field are made.     

Fluorinated heterocyclic compounds. An effective strategy for the synthesis of fluorinated Z-oximes of 3-perfluoroalkyl-6-phenyl-2h-1,2,4-triazin-5-ones via a ring-enlargement reaction of 3-benzoyl-5-perfluoroalkyl-1,2,4-oxadiazoles and hydrazine

The reaction of 3-benzoyl-5-perfluoroalkyl-1,2,4-oxadiazoles with hydrazine has been investigated, evidencing the possibility of competitive reaction paths. Nucleophilic addition of the hydrazine to the electrophilic C(5) of the 1,2,4-oxadiazole ring, followed by ring opening and ring closure with enlargement, leads with high yield and in very mild experimental conditions to the formation of Z-oximes of 3-perfluoroalkyl-6-phenyl-2H-1,2,4-triazin-5-ones (11a-c) as major products of the reaction. In turn, the hydrazine can attack the electrophilic carbonyl carbon giving 4-perfluoroacylamino-5-phenyl-2H-1,2,3-triazoles (13a-c) through the well-known Boulton-Katritzky rearrangement of the intermediate hydrazones.     

Methanol Suppression of Trichloroethylene Degradation byMethylosinus trichosporium (OB3b) and Methane-Oxidizing Mixed Cultures

The effect of methanol on trichloroethylene (TCE) degradation by mixed and pure methylotrophic cultures was examined in batch culture experiments. Methanol was found to relieve growth inhibition ofMethylosinus trichosporium (OB3b) at high (14 mg/L) TCE concentrations. Degradation of TCE was determined by both radiolabeling and gas chromatography techniques. When cultures were grown on methanol over 10 to 14 d with 0.3 mg/L TCE, OB3b degraded 16.89 ±0.82% (mean± SD) of the TCE, and a mixed culture (DT type II) degraded 4.55±0.11%. Mixed culture (JS type I) degraded 4.34±0.06% of the TCE. When grown on methane with 0.3 mg/L TCE, 32.93±2.01% of the TCE was degraded by OB3b, whereas the JS culture degraded 24.3 ±1.38% of the TCE, and the DT culture degraded 34.3 ±2.97% of the TCE. The addition of methanol to cultures grown on methane reduced TCE degradation to 16.21 ±1.17% for OB3b and to 5.08±0.56% for JS. Although methanol reduces the toxicity of TCE to the cultures, biodegradation of TCE cannot be sustained in methanol-grown cultures. Since high TCE concentrations appear to inhibit methane uptake and growth, we suggest the primary toxicity of TCE is directed towards the methane monooxygenase.     

LC-mass spectrometry analysis of N- and C-terminal boundary sequences of polypeptide fragments by limited proteolysis

Limited proteolysis is an important and widely used method for analyzing the tertiary structure and determining the domain boundaries of proteins. Here we describe a novel method for determining the N- and C-terminal boundary amino acid sequences of products derived from limited proteolysis using semi-specific and/or non-specific enzymes, with mass spectrometry as the only analytical tool. The core of this method is founded on the recognition that cleavage of proteins with non-specific proteases is not random, but patterned. Based on this recognition, we have the ability to determine the sequence of each proteolytic fragment by extracting a common association between data sets containing multiple potential sequences derived from two or more different mass spectral molecular weight measurements. Proteolytic product sequences derived from specific and non-specific enzymes can be accurately determined without resorting to the conventional time-consuming and laborious methods of SDS-PAGE and N-terminal sequencing analysis. Because of the sensitivity of mass spectrometry, multiple transient proteolysis intermediates can also be identified and analyzed by this method, which allows the ability to monitor the progression of proteolysis and thereby gain insight into protein structures.