Organophosphazenes. 26. Factors controlling the pathways observed in the reactions of ethynyl lithium reagents with hexafluorocyclotriphosphazene

In contrast to previously studied reactions of ethynyl lithium reagents, the reactions of propynyl and hexynyl lithium with N(3)P(3)F(6) lead to predominantly nongeminal isomers. A modest cis stereo selectivity was observed. The sequential addition of a lithio acetylene reagent which follows a predominately geminal pathway (lithiophenylacetylene) and an aryl lithium reagent (p-propenylphenyl lithium) which follows a predominantly nongeminal pathway were examined. The relative order of addition of the two reagents was interchanged, resulting in a change of reaction pathway demonstrating ring substituent control of the regio and stereo chemical pathways. Hydrogenation of the ethynyl unit in the phosphazene derivatives provides a facile pathway to the difficult to prepare alkylphosphazenes. The chemical shift of the organosubstituted phosphorus center undergoes a remarkably large change (46-47 ppm) on going from the ethynyl to the alkyl derivatives, which reduces the complex (31)P and (19)F NMR spectra of the ethynyl derivatives to easily interpretable first-order spectra, thus allowing for structure assignment. The (13)C NMR data shows that nongeminal regio selectivity increases with the amount of s character on the ethynyl carbon atom attached to the phosphorus center. These observations allow for an understanding of the factors controlling regio and stereo chemical control in the reactions of carbanionic nucleophiles with N(3)P(3)F(6).     

Buffers for the Physiological pH Range: Studies of 4-(N-Morpholino)butanesulfonic Acid (MOBS) from 5 to 55 °C

In this study, we report the pH values of two buffer solutions without chloride ion and eight buffer solutions with NaCl with an ionic strength I=0.16 mol?kg^?1. Electromotive force (emf) techniques have been used to get the cell potentials at 12 temperatures from 5 to 55?°C, including 37?°C. An extended form of the Bates-Guggenheim convention is used in the entire ionic strength range, 0.04 to 0.16?mol?kg^?1. The residual liquid junction potentials (??E _j ) of the buffer solutions of MOBS have been estimated from previous measurements with a flowing junction cell. These values of ??E _j have been used for correction in order to ascertain the operational pH values of four buffer solutions of MOBS at 25 and 37?°C. These solutions are recommended as pH standards for physiological application in the pH range 7.4 to 7.7.     

Liquid chromatographic determination of diminazene diaceturate (Berenil) in raw bovine milk

A simple liquid chromatographic (LC) method is presented for the determination of diminazene (DZ) in raw bovine milk. DZ is extracted from raw milk by chilled aqueous centrifugation and is isolated from milk components on a cyano solid-phase extraction column. DZ is eluted by using a methanol-ion pairing reagent. A Phenomenex LUNA CN column and an acetonitrile-buffered mobile phase with a counter ion are used for gradient LC. The LC effluent is monitored at a detection wavelength of 372 nm by using a deuterium lamp. Under the parameters described, the retention time of DZ is 8-10 min with a peak area response of 6.5 mAU/ng. The method demonstrated excellent precision over all levels tested (25-400 ppb) with an overall average recovery of 90.4 +/- 14.5%. The method is applicable to the monitoring of milk for DZ residues at the 25 ppb level with a limit of quantitation of 10 ppb.     

Synthesis, cyclic voltammetric studies, and electrogenerated chemiluminescence of a new phenylquinoline-biphenothiazine donor-acceptor molecule

We report the synthesis, electrochemistry, and luminescence of a novel ECL emitting compound containing two electron-accepting hexyl-phenylquinoline groups covalently attached to the 3,3'-positions of the electron-donating 10,10'-dimethylbiphenothiazine group. The optimized geometry as determined from semiempirical MNDO calculations shows that the two quinoline groups are twisted 82.5 degrees from the two phenothiazine rings, indicating a lack of electron delocalization among these groups. This unique geometry allows generation of localized radical cations and radical anions capable of generating ECL upon annihilation. However, the phenothiazine rings are twisted 46.5 degrees relative to each other, suggesting possible interactions between the two moieties. This is evident in the electrochemical behavior in which two closely spaced one-electron oxidations, rather than a single two-electron oxidation wave, were observed. The photophysical properties of BHQ-BPZ show strong resemblances to the parent compound, BPQ-PTZ, which contains a single phenothiazine moiety. In addition, the ECL spectrum produced via radical ion annihilation shows good agreement with the fluorescence emission of the compound.     

Mapping the Geometry of Metal Three‐Coordination Using Crystal Structure Data: Reaction Pathway for Ligand Addition to Linear HgII Species

Symmetry‐modified principal‐component analysis has been used to visualise the geometrical distortions of three‐coordinate metal centres observed in crystal structures retrieved from the Cambridge Structural Database. It is shown that compounds of Cu, Ag, Hg, Zn, and Au dominate the dataset, and exhibit distortions away from the trigonal planar archetype towards a) T‐shaped and b) Y‐shaped geometries. A small number of compounds, principally of Cu and Ag, also show distortions towards trigonal pyramidal geometries. The interconversions from Y‐shaped geometries, through the trigonal planar form to T‐shaped geometries are clearly mapped by the PC analysis. For Hg^II complexes, it is possible to interpret the transition from T‐shaped geometries to the trigonal planar form in terms of a reaction pathway for ligand addition to linear L₁? Hg^II? L₂ species.     

Novel titanocene anti-cancer drugs derived from fulvenes and titanium dichloride

Starting from 6-(p−N,N-dimethylanilinyl)fulvene (1a) or 6-(pentamethylphenyl)fulvene (1b) [1,2-di(cyclopentadienyl)-1,2-di(p−N,N-dimethylaminophenyl)ethanediyl] titanium dichloride (2a) and [1,2-di(cyclopentadienyl)-1,2-bis(pentamethylphenyl)ethanediyl] titanium dichloride (2b) and their corresponding dithiocyanato complexes (3a, 3b) were synthesized. Titanocene 2b did not show a cytotoxic effect, but when 2a was tested against pig kidney carcinoma cells (LLC-PK) or human ovarian carcinoma cells (A2780/cp70) inhibitory concentrations (IC₅₀) of 2.7 × 10⁻⁴ and 1.9 ×  10⁻⁴ M, respectively, were observed.     

Structural effects on interconversion of oxygen-substituted bisketenes and cyclobutenediones

Cyclobutenediones 5 disubstituted with HO (a), MeO (b), EtO (c), i-PrO (d), t-BuO (e), PhO (f), 4-MeOC6H4O (g), 4-O2NC6H4O (h), and 3,4-bridging OCH2CH2O (i) substituents upon laser flash photolysis gave the corresponding bisketenes 6a-i, as detected by their distinctive doublet IR absorptions between 2075 and 2106 and 2116 and 2140 cm-1. The reactivities in ring closure back to the cyclobutenediones were greatest for the group 6b-e, with the highest rate constant of 2.95 x 10(7) s-1 at 25 degrees C for 6e (RO = t-BuO) in isooctane, were less for 6a (RO = OH, k = 2.57 x 10(6) s-1 in CH3CN), while 6f-i were the least reactive, with the lowest rate constant of 3.8 x 10(4) s-1 in CH3CN for 6h (RO = 4-O2NC6H4O). The significantly reduced rate constants for 6f-i are attributed to diminution of the electron-donating ability of oxygen to the cyclobutenediones 5f-h by the ArO substituents compared to alkoxy groups and to angle strain in the bridged product cyclobutenedione 5i. The reactivities of the ArO-substituted bisketenes 6f-h in CH3CN varied by a factor of 50 and gave an excellent correlation of the observed rate constants log k with the sigma p constants of the aryl substituents. Computational studies at the B3LYP/6-31G(d) level of ring-closure barriers are consistent with the measured reactivities. Photolysis of squaric acid (5a) in solution provides a convenient preparation of deltic acid (7).     

Simulating ion channel activation mechanisms using swarms of trajectories

Atomic-level studies of protein activity represent a significant challenge as a result of the complexity of conformational changes occurring on wide-ranging timescales, often greatly exceeding that of even the longest simulations. A prime example is the elucidation of protein allosteric mechanisms, where localized perturbations transmit throughout a large macromolecule to generate a response signal. For example, the conversion of chemical to electrical signals during synaptic neurotransmission in the brain is achieved by specialized membrane proteins called pentameric ligand-gated ion channels. Here, the binding of a neurotransmitter results in a global conformational change to open an ion-conducting pore across the nerve cell membrane. X-ray crystallography has produced static structures of the open and closed states of the proton-gated GLIC pentameric ligand-gated ion channel protein, allowing for atomistic simulations that can uncover changes related to activation. We discuss a range of enhanced sampling approaches that could be used to explore activation mechanisms. In particular, we describe recent application of an atomistic string method, based on Roux's “swarms of trajectories” approach, to elucidate the sequence and interdependence of conformational changes during activation. We illustrate how this can be combined with transition analysis and Brownian dynamics to extract thermodynamic and kinetic information, leading to understanding of what controls ion channel function. © 2019 Wiley Periodicals, Inc.