The initial step of silicate versus aluminosilicate formation in zeolite synthesis: a reaction mechanism in water with a tetrapropylammonium template

The initial step for silicate and aluminosilicate condensation is studied in water in the presence of a realistic tetrapropylammonium template under basic conditions. The model corresponds to the synthesis conditions of ZSM5. The free energy profile for the dimer formation ((OH)(3)Si-O-Si-(OH)(2)O(-) or [(OH)(3)Al-O-Si-(OH)(3)](-)) is calculated with ab initio molecular dynamics and thermodynamic integration. The Si-O-Si dimer formation occurs in a two-step manner with an overall free energy barrier of 75 kJ mol(-1). The first step is associated with the Si-O bond formation and results in an intermediate with a five-coordinated Si, and the second one concerns the removal of the water molecule. The template is displaced away from the Si centres upon dimer formation, and a shell of water molecules is inserted between the silicate and the template. The main effect of the template is to slow down the backward hydrolysis reaction with respect to the condensation one. The Al-O-Si dimer formation first requires the formation of a metastable precursor state by proton transfer from Si(OH)(4) to Al(OH)(4)(-) mediated by a solvent molecule. It then proceeds through a single step with an overall barrier of 70 kJ mol(-1). The model with water molecules explicitly included is then compared to a simple calculation using an implicit continuum model for the solvent. The results underline the importance of an explicit and dynamical treatment of the water solvent, which plays a key role in assisting the reaction.     

Selectivity, directionality, and promiscuity in peptide processing from a Bacillus sp. Al Hakam cyclodehydratase

The thiazole/oxazole-modified microcins (TOMMs) represent a burgeoning class of ribosomal natural products decorated with thiazoles and (methyl)oxazoles originating from cysteines, serines, and threonines. The ribosomal nature of TOMMs allows for the generation of derivative products from mutations in the amino acid sequence of the precursor peptide, which ultimately manifest in differing structures and, sometimes, biological functions. Employing a TOMM system for the purpose of creating new structures and functions via combinatorial biosynthesis requires processing machinery that can tolerate highly variable substrates. In this study, TOMM enzymatic promiscuity was assessed using a currently uncharacterized cluster in Bacillus sp. Al Hakam. As determined by Fourier transform tandem mass spectrometry (FT-MS/MS), azole rings were formed in both a regio- and chemoselective fashion. Cognate and noncognate precursor peptides were modified in an overall C- to N-terminal directionality, which to date is unique among characterized ribosomal natural products. Studies focused on the inherent promiscuity of the biosynthetic machinery elucidated a modest bias for glycine at the preceding (-1) position and a remarkable flexibility in the following (+1) position, even allowing for the incorporation of charged amino acids and bisheterocyclization. Two unnatural substrates were utilized as the conclusive test of substrate flexibility, of which both were processed in a predictable fashion. A greater understanding of substrate processing and enzymatic tolerance toward unnatural substrates will prove beneficial when designing combinatorial libraries to screen for artificial TOMMs that exhibit desired activities.     

Enhanced adsorption of methylene blue onto graphene oxide-doped XFe<Subscript>2</Subscript>O<Subscript>4</Subscript> (X = Co,Mn, Ni) nanocomposites: kinetic,isothermal, thermodynamic and recyclability studies

The nanocomposites XFe₂O₄/GO with various metal sites (X = Co, Mn, and Ni) were successfully synthesized via the polymerized complex method. The XFe₂O₄/GO family was characterized using X-Ray diffraction analysis, scanning electron microscopy (SEM), and a vibrating sample magnetometer. We also investigated the effect of three fundamental parameters (initial concentration, dosage, and pH) on the removal of methylene blue using the response surface methodology. A high F value, very low P value (< 0.00001), a non-significant lack of fit, and the determination coefficient (R ² > 0.95) demonstrated a strong correlation between experimental and predicted values of the responses. The predicted optimal conditions for maximum removal efficiency were easily determined to adhere to the following trend for actual test experiments: MnFe₂O₄/GO (60.1%) < CoFe₂O₄/GO (80.3%) < NiFe₂O₄/GO (87.7%). Moreover, the adsorption behavior was well-described by the Langmuir isotherm and a pseudo-second-order kinetic model. The maximum capacity for adsorption of methylene blue onto XFe₂O₄/GO was found from 42.2 to 80.6 mg/g. Moreover, the XFe₂O₄/GO could be regenerated for several cycles without a considerable decrease in removal yield, suggesting that this highly promising XFe₂O₄/GO could be applied as an efficient and novel adsorbent.     

When can the Ogston-Morris-Rodbard-Chrambach model be applied to gel electrophoresis?

The Ogston-Morris-Rodbard-Chrambach theory of gel electrophoresis is consistent with predictions from the volume averaging method with respect to the equivalence of the accessible volume fraction to the ratios of gel mobility to free solution mobility and the gel diffusion coefficient to free solution diffusion coefficient for the limiting case of small molecule electrophoresis with low electrical fields, low gel concentrations, and nonconductive gel fibers. When these conditions are not valid, more extensive calculations are required to determine the mobility and diffusion coefficient ratios as functions of the geometry and electrical field within the gel. The volume averaging theory shows that it is important to account for the electrical conductivity properties of the fibers that make up a gel electrophoresis medium, and this aspect is consistent with early theories of transport phenomena in gel electrophoresis.     

A procedure for designing linear functional observers

We present low-order functional observer existence conditions; the proposed existence conditions always lead to a design of functional observers of order less than or equal to that of a Luenberger observer. The proposed low-order observer always exists if a Luenberger observer exists.     

Direct oxidative C(sp3)─H/C(sp2)─H coupling reaction using recyclable Sr-doped LaCoO3 perovskite catalyst

A strontium-doped lanthanum cobaltite perovskite, La_0.6Sr_0.4CoO₃, was prepared and utilized as a recyclable heterogeneous catalyst for the direct oxidative C(sp³)─H/C(sp²)─H coupling reaction between cyclic ethers and alkenes or coumarins to achieve corresponding α-functionalized ethers. The α-functionalization of cyclic thioethers or amides with alkenes or coumarins was also achieved via this protocol. The La_0.6Sr_0.4CoO₃ catalyst exhibited better performance than a variety of homogeneous and heterogeneous catalysts. Utilizing a recyclable catalyst would offer a greener option for the direct oxidative C(sp³)─H/C(sp²)─H coupling reaction. To our best knowledge, the C(sp³)─H/C(sp²)─H coupling between olefins and ethers to generate α-functionalized ethers using a heterogeneous catalyst has not been previously reported, and the α-functionalization of cyclic thioethers or amides with alkenes or coumarins is new.     

Sorption Recovery of Lanthanum,Iron, Aluminum,and Calcium Ions from Phosphoric Acid

Russian Journal of Applied Chemistry - Sorption recovery of lanthanum, iron(III), aluminum, and calcium ions from phosphoric acid solutions with MTC 1600 macroporous sulfonic cation exchanger under...     

A model study on the mechanism and kinetics for the dissociation of water anion

Quantum chemical computations using both density functional theory (B3LYP functional) and wavefunction (MP2 and CCSD(T)) methods, with the 6-311++G(3df,2p) and aug-cc-pVnZ (n = D,T,Q) basis sets, in conjunction with a polarizable continuum model (PCM) method for treating structures in solution, were carried out to look again at a series of small negatively charged water species [(H₂O)_n]^•–. For each size n of [(H₂O)_n]^•– in aqueous solution with n = 2, 3, and 4, two distinct structural motifs can be identified: a classical water radical anion formed by hydrogen bonds and a molecular pincer in which the excess electron is directly interacting with H atoms. In aqueous solution, both motifs have comparable energy content and likely coexist and compete for the ground state. Some water anion isomers can dissociate when interaction with a water molecule, [(H₂O)_n]^•– + H₂O → H^•(H₂O)_m + OH^–(H₂O)_n–m, through successive hydrogen transfers with moderate energy barriers. This reaction can also be regarded as a water-splitting process in which the H transfers involved take place mainly within a water trimer, whereas other water molecules tend to stabilize transition structures through microsolvation rather than direct participation. Calculated absolute rate constants for the reversed reaction H^•(H₂O)₂ + OH^–(H₂O)₂ → [(H₂O)₄]^•־ + H₂O with both H and D isotopes agree well with the experimentally evaluated counterpart and lend a kinetic support for the involvement of a tetramer unit.