Kinetics and mechanism of the acid-catalysed hydrolysis of the carbonatotetraimidazolecobalt(III) ion

Summary The kinetics of the acid-catalysed hydrolysis of the [(imidazole)₄Co(CO₃)]⁺ ion was found to follow the rate law -dln[complex]/dt = k ₁ K[H⁺](1 + K[H ⁺]) in the 25–45 °C range, [H⁺] 0.05–1.0 m range and I = 1.0m. The reaction sequence consists of a rapid protonation equilibrium followed by the one-end dissociation of the coordinated carbonato ligand (rate-determining step) and subsequent fast release of the monodentate carbonato ligand. The rate parameter values, k ₁ and ITK, at 25 °C are 6.48 × 10⁻³s⁻¹ and 0.31m ⁻¹, respectively, and activation parameters for k ₁ are ΔH ₁ ^≠ = 86.1 ± 1.2kJ mol⁻¹ and ΔS ₁ ^≠ = 2.1 ± 6.3 J mol⁻¹K⁻¹. The hydrolysis rate increases with increase in ionic strength. The different ways of dealing with the data fit are presented and discussed. The kinetic results are compared with those for the similar cobalt(III) complexes.     

Membrane mediated regulation in free peptides of HIV-1 gp41: minimal modulation of the hemifusion phase

Membrane-mediated structural modulation in two short fragments of the human HIV-1 envelope protein gp41 is demonstrated. Derived from the C-terminal membrane proximal external (MPE) and N-terminal fusion peptide proximal (FPP) regions, these peptides are widely separated in the primary sequence but form tertiary contacts during the intermediate (hemifusion) phase of HIV infection. The structural perturbations observed at the membrane interface offer evidence of rudimentary regulatory mechanisms operating in the free peptides which may be relevant in the biological system. No such regulatory phenomena were observed for the individual peptides in a membrane environment or between the peptides in aqueous solutions. Structure determination is made using a combination of circular and linear dichroism spectroscopy (supported by calorimetric measurements) and molecular dynamics simulations. Specifically, we show that these peptides interact locally without the conformational support of helical heptad repeat regions in native gp41 and that the modulation is not mutual with the FPP peptide operating as a primary regulator of the MPE-FPP interactions in the hemifusion phase.     

Crystal structure of tris(2-pyridylthio-2-pyridinium)pentaisothiocyanato-dioxouranate(VI)

Summary The crystal structure of [DPSH] ₃ ⁺ [UO₂[NCS)₅]^3– (DPSH = 2 pyridylthio-2-pyridinium) has been determined by standard methods using diffractometer data. Crystals are monoclinic, space groupP2₁/a, witha=33.16 (3),b=16.68(2),c=7.85(1) Å, and=97.3(1); D_c=1.65 g · cm^–3 for Z=4. The structure has been refined to R=5.8% by least squares methods. Five thiocyanate groups are equatorially bonded to the linear uranyl group. The mean of the five U-N, N-C, and C-S bond distances are 2.45, 1.17, and 1.59 Å respectively. The protonated DPS molecules have the N,N-inside conformations.     

The conformations of two copper(I) complexes of 1H‐benzimidazole‐2(3H)‐thione and thiosaccharinate

(Acetonitrile‐1κN)[μ‐1H‐benzimidazole‐2(3H)‐thione‐1:2κ²S:S][1H‐benzimidazole‐2(3H)‐thione‐2κS]bis(μ‐1,1‐dioxo‐1λ⁶,2‐benzothiazole‐3‐thiolato)‐1:2κ²S³:N;1:2κ²S³:S³‐dicopper(I)(Cu—Cu), [Cu₂(C₇H₄NO₂S₂)₂(C₇H₆N₂S)₂(CH₃CN)] or [Cu₂(tsac)₂(Sbim)₂(CH₃CN)] [tsac is thiosaccharinate and Sbim is 1H‐benzimidazole‐2(3H)‐thione], (I), is a new copper(I) compound that consists of a triply bridged dinuclear Cu—Cu unit. In the complex molecule, two tsac anions and one neutral Sbim ligand bind the metals. One anion bridges via the endocyclic N and exocyclic S atoms (μ‐S:N). The other anion and one of the mercaptobenzimidazole molecules bridge the metals through their exocyclic S atoms (μ‐S:S). The second Sbim ligand coordinates in a monodentate fashion (κS) to one Cu atom, while an acetonitrile molecule coordinates to the other Cu atom. The Cu^I—Cu^I distance [2.6286 (6) Å] can be considered a strong `cuprophilic' interaction. In the case of [μ‐1H‐benzimidazole‐2(3H)‐thione‐1:2κ²S:S]bis[1H‐benzimidazole‐2(3H)‐thione]‐1κS;2κS‐bis(μ‐1,1‐dioxo‐1λ⁶,2‐benzothiazole‐3‐thiolato)‐1:2κ²S³:N;1:2κ²S³:S³‐dicopper(I)(Cu—Cu), [Cu₂(C₇H₄NO₂S₂)₂(C₇H₆N₂S)₃] or [Cu₂(tsac)₂(Sbim)₃], (II), the acetonitrile molecule is substituted by an additional Sbim ligand, which binds one Cu atom via the exocylic S atom. In this case, the Cu^I—Cu^I distance is 2.6068 (11) Å.     

New coumarin-urea based receptor for anions: a selective off–on fluorescence response to fluoride

A novel fluorosensor for anions, 1,11-bis(4-methylcoumarin-7-yl)-6-methyl-1,3,6,9,11-pentaazaundeca-2,10-dione (L), which contains two coumarin–urea units spaced by a flexible diethylenetriamine fragment, was easily prepared in 65% yield by a one-pot two-step procedure. The binding ability of L toward several anions (G) was evaluated by ¹H NMR, UV–vis, and fluorescence titration. It was found that L forms stable [LG] and [LG₂] species in both DMSO and CH₃CN solvents. Furthermore, the fluorescence emission of L in the visible range (400 nm) is affected by the presence of anions; it is quenched by acetate, chloride, and pyrophosphate while it is enhanced by fluoride. Thus, this novel fluorosensor provides a selective off–on response to the presence of fluoride in solution.     

Influence of the Mn(II) and CH4 on the evolution of light induced radicals in chloroplasts

The photochemical activity of the leaves and chloroplasts of several species of plants developed in gaseous hydrocarbon environments (C_nH_2n+2) was investigated by the EPR method. The influence of different concentrations of paramagnetic ions [Mn(II), Cu(II) and VO(II), respectively] on the plants grown in dietary media containing these ions was also followed by EPR spectroscopy. The contents of paramagnetic ions, lower than 0.1%, in the nutritive solutions, are the most suitable in the biosynthesis of chlorophyll-like pigments containing paramagnetic ions in their porphyrine ring. The changes in the form, structure and photosensibility of the EPR signals corresponding to the free radicals, as well as the properties of the pheophytins containing paramagnetic ions, such as: Mn(II), Cu(II) and VO(II), were related to the photochemical synthesis in gaseous hydrocarbon environments.     

Relative binding orientations of adenosine A1 receptor ligands — A test case for Distributed Multipole Analysis in medicinal chemistry

Summary The electrostatic properties of adenosine-based agonists and xanthine-based antagonists for the adenosine A₁ receptor were used to assess various proposals for their relative orientation in the unknown binding site. The electrostatic properties were calculated from distributed multipole representations of SCF wavefunctions. A range of methods of assessing the electrostatic similarity of the ligands were used in the comparison. One of the methods, comparing the sign of the potential around the two molecules, gave inconclusive results. The other approaches, however, provided a mutually complementary and consistent picture of the electrostatic similarity and dissimilarity of the molecules in the three proposed relative orientations. This was significantly different from the results obtained previously with MOPAC AM1 point charges. In the standard model overlay, where the aromatic nitrogen atoms of both agonists and antagonists are in the same position relative to the binding site, the electrostatic potentials are so dissimilar that binding to the same receptor site is highly unlikely. Overlaying the N⁶-region of adenosine with that near C8 of theophylline (the N⁶-C8 model) produces the greatest similarity in electrostatic properties for these ligands. However, N⁶-cyclopentyladenosine (CPA) and 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) show greater electrostatic similarity when the aromatic rings are superimposed according to the flipped model, in which the xanthine ring is rotated around its horizontal axis. This difference is mainly attributed to the change in conformation of N⁶-substituted adenosines and could result in a different orientation for theophylline and DPCPX within the receptor binding site. However, it is more likely that DPCPX also binds according to the N⁶-C8 model, as this model gives the best steric overlay and would be favoured by the lipophilic forces, provided that the binding site residues could accommodate the different electrostatic properties in the N⁶/N7-region. Finally, we have shown that Distributed Multipole Analysis (DMA) offers a new, feasible tool for the medicinal chemist, because it provides the use of reliable electrostatic models to determine plausible relative binding orientations.Abbreviations DMA distributed multipole analysis - SCF self-consistent field - CPA N⁶-cyclopentyladenosine - DPCPX 1,3-dipropyl-8-cyclopentylxanthine - R-PIA R-1-phenyl-2-propyladenosine - S-PIA S-1-phenyl-2-propyladenosine     

Mechanism of aerobic decomposition of Angeli's salt (sodium trioxodinitrate) at physiological pH

The recent determination that Angeli's salt may have clinical application as a nitrogen oxide donor for treatment of cardiovascular diseases such as heart failure has led to renewed interest in the mechanism and products of thermal decomposition of Angeli's salt under physiological conditions. In this report, several mechanisms are evaluated experimentally and by quantum mechanical calculations to determine whether HNO is in fact released from Angeli's salt in neutral, aerobic solution. The mechanism of product autoxidation is also considered.     

Polymeric Ti(IV) glycolate: A heterogeneous catalyst for solvent-free sulfoxidation by t-butyl hydroperoxide

The oxidation of sulfides to sulfoxides has been conveniently performed under heterogeneous and solvent-free conditions employing polymeric Ti(IV) glycolate as reusable catalyst and t-butyl hydroperoxide as oxygen donor. The catalyst can be very efficiently recovered and reused several times without any significant change in reactivity and selectivity.