Experimental and Computational Study of the Thermal Decomposition of 3‐Methyl‐3‐buten‐1‐ol in m‐Xylene Solution

An experimental study of the thermal decomposition of a β‐hydroxy alkene, 3‐methyl‐3‐buten‐1‐ol, in m‐xylene solution, has been carried out at five different temperatures in the range of 513.15–563.15 K. The temperature dependence of the rate constants for the decomposition of this compound in the corresponding Arrhenius equation is given by ln k (s^?1) = (25.65 ± 1.52) ? (17,944 ± 814) (kJ·mol^?1)·T^?1. A computational study has been carried out at the M05–2X/6–31+G(d,p) level of theory to calculate the rate constants and the activation parameters by the classical transition state theory. There is a good agreement between the experimental and calculated rate constants and activation Gibbs energies. The bonding characteristics of reactant, transition state, and products have been investigated by the natural bond orbital analysis, which provides the natural atomic charges and the Wiberg bond indices. Based on the results obtained, the mechanism proposed is a one‐step process proceeding through a six‐membered cyclic transition state, being a concerted and slightly asynchronous process. The results have been compared with those obtained previously by us (Struct Chem 2013, 24, 1811–1816) for the thermal decomposition of 3‐buten‐1‐ol, in m‐xylene solution. We can conclude that in the compound studied in this work, 3‐methyl‐3‐buten‐1‐ol, the effect of substitution at position 3 by a weakly activating CH₃ group is the stabilization of the transition state formed in the reaction and therefore a small increase in the rate of thermal decomposition.     

Structural and Kinetic Aspects of Blood Plasma Protein Adsorption on the Surface of Hydrophobic Polymers

Abstract

Due to the wide use of polymers in medicine, researchers are required to solve a very important problem–to understand the interaction between materials of nonphysiological origin and the surrounding biological liquids, and tissues, particularly blood.     

Ritter Reactions between Alcohols and Acetonitrile Mediated by the Conducting Polymer Poly-(3,4-ethylenedioxy Thiophene) (PEDOT)

Herein, we report new reactivity of the conducting polymer, poly-(3,4-ethylenedioxy thiophene) (PEDOT), where PEDOT mediates a Ritter reaction between alcohols and acetonitrile. The yields were variable and in most cases competitive with results obtained using sulfuric acid. Attempts at a stoichiometric reaction between benzonitrile and diphenylmethanol are also reported herein. Finally, described here are preliminary mechanistic studies that suggest PEDOT is behaving as an alcohol-selective or specific Lewis acid.

Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications® for full experimental and spectral details.     

CaII Binding Regulates and Dominates the Reactivity of a Transition‐Metal‐Ion‐Dependent Diesterase from Mycobacterium tuberculosis

The diesterase Rv0805 from Mycobacterium tuberculosis is a dinuclear metallohydrolase that plays an important role in signal transduction by controlling the intracellular levels of cyclic nucleotides. As Rv0805 is essential for mycobacterial growth it is a promising new target for the development of chemotherapeutics to treat tuberculosis. The in vivo metal‐ion composition of Rv0805 is subject to debate. Here, we demonstrate that the active site accommodates two divalent transition metal ions with binding affinities ranging from approximately 50 nm for Mn^II to about 600 nm for Zn^II. In contrast, the enzyme GpdQ from Enterobacter aerogenes, despite having a coordination sphere identical to that of Rv0805, binds only one metal ion in the absence of substrate, thus demonstrating the significance of the outer sphere to modulate metal‐ion binding and enzymatic reactivity. Ca^II also binds tightly to Rv0805 (K_d≈40 nm ), but kinetic, calorimetric, and spectroscopic data indicate that two Ca^II ions bind at a site different from the dinuclear transition‐metal‐ion binding site. Ca^II acts as an activator of the enzymatic activity but is able to promote the hydrolysis of substrates even in the absence of transition‐metal ions, thus providing an effective strategy for the regulation of the enzymatic activity.     

A simple method for extracting both active oily and water soluble extract (WSE) from Nigella sativa (L.) seeds using a single solvent system

The current study provides a way of extraction for both active NSO and WSE from Nigella sativa seeds using 98% methanol. About 1?kg of ground seeds was macerated by 1:2.5 w/v (g/mL) for 72?hours. After rotary evaporation and 7 days of continuous drying and chilling at 50 and 4?°C, NSO and WSE were obtained at the same instant. Solubility tests of 24 solvents and 11 thin layer chromatographic analyses while 2, 2-diphenyl-1-picrylhydrazyl free radical scavenging assay of NSO (73.66) , WSE (33.32) and NSO?+?WSE (78.22) against ascorbic acid (IC50?=?4.28?mg/mL) was performed. WSE was found to be highly soluble in water and 5% NaOH exhibiting the same Rf value of 0.95 for EtOH:DMSO (9:1) against the honey. WSE has revealed more than twofold higher anti-oxidant activity than others. Formulation of WSE with Tualang honey may provide better targeted hydrophilic drug delivery systems.     

Magnetic circular dichroism study of a dicobalt(II) methionine aminopeptidase/fumagillin complex and dicobalt II-II and II-III model complexes

The dicobalt form of the metallohydrolase methionine aminopeptidase from Escherichia coli (CoCo EcMetAP) has an active site with one 5-coordinate Co (II) and a more weakly bound 6-coordinate Co (II). These metal ions are bridged by two carboxylate amino acid side chains and water or hydroxide, potentially enabling magnetic exchange coupling between the metals. We used variable-temperature, variable-field magnetic circular dichroism to determine whether such coupling occurs. CoCo EcMetAP's MCD spectrum shows distinct d-d transitions at 495 and 567 nm caused by 6- and 5-coordinate Co (II), respectively. The magnetization curves for 5- and 6-coordinate Co (II) are very different, indicating that their electronic ground states vary considerably, ruling out any coupling. When the fungal metabolite fumagillin binds to the CoCoEcMetAP, the qualitative MCD spectrum is unchanged; however, VTVH MCD data show that 5- and 6-coordinate Co (II) ions have similarly shaped magnetization curves, indicating that the Co (II) ions now share the same electronic ground state. Fitting the VTVH MCD data to a model in which dimer wave functions are calculated using a spin Hamiltonian with zero-field splitting showed the Co (II) ions to be weakly ferromagnetically coupled, with J = 2.9 cm (-1). Ferromagnetic coupling is unusual for dinuclear Co (II); therefore, to support the CoCoEcMetAP/fumagillin complex results, we also analyzed VTVH MCD data from a matched pair of dinuclear cobalt complexes, 1 and 2. Complex 1 shares the carboxylate and hydroxide-bridged dicobalt(II) structural motif with the active site of CoCo EcMetAP. Complex 2 contains a nearly isostructural Co (II) ion, but the Co (III) is diamagnetic, so any magnetic coupling is switched off, while the spectral features of the Co (II) ion remain. Magnetization data for 1, fitted to the dimer model, showed that the Co (II) ions were weakly ferromagnetically coupled, with J = 1.7 cm (-1). Magnetization data for Co (II) ions in 2, however, reflect loss of magnetic exchange coupling.     

Dicobalt II-II, II-III, and III-III complexes as spectroscopic models for dicobalt enzyme active sites

A matched set of dinuclear cobalt complexes with II-II, II-III, and III-III oxidation states have been prepared and structurally characterized. In [(bpbp)Co2(O2P(OPh)2)2]n+ ( n = 1, 2, or 3; bpbp(-) = 2,6-bis(( N,N'-bis-(2-picolyl)amino)-methyl)-4-tertbutylphenolato), the nonbonded Co...Co separations are within the range 3.5906(17) to 3.7081(11) angstroms, and the metal ions are triply bridged by the phenolate oxygen atom of the heptadentate dinucleating ligand and by two diphenylphosphate groups. The overall structures and geometries of the complexes are very similar, with minor variations in metal-ligand bond distances consistent with oxidation state assignments. The CoIICoIII compound is a valence-trapped Robin-Day class II complex. Solid state 31P NMR spectra of the diamagnetic CoIIICoIII (3) and paramagnetic CoIICoIII (2) and CoIICoII (1) complexes show that 31P isotropic shifts broaden and move downfield by about 3000 ppm for each increment in oxidation state. Cyclic voltammetry corroborates the existence of the CoIICoII, CoIICoIII, and CoIIICoIII species in solution. The redox changes are not reversible in the applied scanning timescales, indicating that chemical changes are associated with oxidation and reduction of the cobalt centers. An investigation of the spectroscopic properties of this series has been carried out for its potential usefulness in analyses of the related spectroscopic properties of the dicobalt metallohydrolases. Principally, magnetic circular dichroism (MCD) has been used to determine the strength of the magnetic exchange coupling in the CoIICoII complex by analysis of the variable-temperature variable-field (VTVH) intensity behavior of the MCD signal. The series is ideal for the spectroscopic determination of magnetic coupling since it can occur only in the CoIICoII complex. The CoIICoIII complex contains a nearly isostructural CoII ion, but since CoIII is diamagnetic, the magnetic coupling is switched off, while the spectral features of the CoII ion remain. Analysis of the MCD data from the CoIICoIII complex has been undertaken in the theoretical context of a 4T1g ground-state of the CoII ion, initially in an octahedral ligand field that is split by both geometric distortion and zero-field splitting to form an isolated doublet ground state. The MCD data for the CoIICoII pair in the [(bpbp)Co2(O2P(OPh)2)2]+ complex were fitted to a model based on weak antiferromagnetic coupling with J = -1.6 cm (-1). The interpretation is confirmed by solid state magnetic susceptibility measurements.