Control over Kinetic and Thermodynamically Driven Pathways of Crystallization to Yield Cofacial and Slipped-Stack Dimers in Single Crystals

Control over the molecular packing in the solid state is of utmost importance in regulating the bulk optical properties of organic semiconductors. The electronic coupling between the molecules makes it possible to improve the properties of the bulk materials. This work reports an example of control over the selective formation of polymorphic single crystals of donor–acceptor-type small-molecule compound 25TR by 1) kinetic or 2) thermodynamic course of crystallisation to yield slipped stack (S) and cofacial (C) dimers in the single crystals. The distinct optical characteristics of the C-dimer and S-dimer are summarised. Both forms show significant excitonic interactions in the solid state, and the S-dimeric form has strong yellowish orange fluorescence, whereas the C-dimeric form is non-fluorescent in the crystalline state. DFT calculations and differential scanning calorimetric experiments revealed that the C-dimer polymorph is the thermodynamically stable form with a free energy offset of 0.43 eV in comparison with the S-dimer. Interestingly, the thermodynamically driven non-fluorescent single crystal was found to be convertible to its fluorescent form irreversibly by thermal trigger. The charge-carrier-transport characteristics of these two polymorphs were computed by using the Marcus–Hush formalism. The computations of the charge-carrier-transport behaviour revealed that the S-dimer ( 25TR_(R) ) is ambipolar, whereas the C-dimer ( 25TR_(Y) ) is predominantly n-type.     

Bio-Templated Chiral Zeolitic Imidazolate Framework for Enantioselective Chemoresistive Sensing

Chiral metal–organic frameworks (MOFs) have gained rising attention as ordered nanoporous materials for enantiomer separations, chiral catalysis, and sensing. Among those, chiral MOFs are generally obtained through complex synthetic routes by using a limited choice of reactive chiral organic precursors as the primary linkers or auxiliary ligands. Here, we report a template-controlled synthesis of chiral MOFs from achiral precursors grown on chiral nematic cellulose-derived nanostructured bio-templates. We demonstrate that chiral MOFs, specifically, zeolitic imidazolate framework (ZIF), unc -[Zn(2-MeIm)₂, 2-MeIm=2-methylimidazole], can be grown from regular precursors within nanoporous organized chiral nematic nanocelluloses via directed assembly on twisted bundles of cellulose nanocrystals. The template-grown chiral ZIF possesses tetragonal crystal structure with chiral space group of P4₁, which is different from traditional cubic crystal structure of I-43 m for freely grown conventional ZIF-8. The uniaxially compressed dimensions of the unit cell of templated ZIF and crystalline dimensions are signatures of this structure. We observe that the templated chiral ZIF can facilitate the enantiotropic sensing. It shows enantioselective recognition and chiral sensing abilities with a low limit of detection of 39 μM and the corresponding limit of chiral detection of 300 μM for representative chiral amino acid, D- and L- alanine.     

Synthesis and Antitumor Activity of Heterocycles Related to Carbendazim

Three types of compounds were synthesized from carbendazim ( 1 ), a benzimidazole derivative (Scheme 1 ). They included a group of esters at N1 prepared by treating carbendazim with isocyanates bearing ester groups ( 2a , 2b , 2c ), carboxyalkyl‐1,2,3,4‐tetrahydro‐s‐triazino[1,2‐a]benzimidazole‐2,4‐dione esters ( 3a and 3b , 3d and 3c derived from 3a . The antitumor potencies of the N1 esters were in the range of 7 to 40 μM, which compares favorably with carbendazim, but their water solubilities were low. The s‐triazine derivatives showed activity against pancreatic tumor cells, and one of them ( 3b ) was active in mice, but they were not effective against other tumor types. Treatment of carbendazim with 3‐bromopropionyl chloride produced 1‐methoxycarbonyl‐4‐oxo‐1,2,3,4‐tetrahydropyrimido[1,2‐a]benzimidazole ( 4 ), which gave 1‐(3‐aminopropionyl)benzimidazole 2‐methylcarbamates, substituted on the amino nitrogen ( 5a , 5b , and 5d ), when treated with amines. These products showed some antitumor activity in cell cultures, and an ethoxy derivative ( 5c ), obtained by treating 1‐methoxycarbonyl‐4‐oxo‐1,2,3,4‐tetrahydropyrimido[1,2‐a]benzimidazole with sodium ethoxide, was active in the 67–150 μM range. Some of the new compounds had good water solubility. Carbendazim kills tumor cells by inhibiting tubulin; however, s‐triazine 3b , which differs from it in size and functional groups, does not act by this mechanism.     

Thermal stresses in rectangular plates

Summary A method of determining the thermal stresses in a flat rectangular isotropic plate of constant thickness with arbitrary temperature distribution in the plane of the plate and with no variation in temperature through the thickness is presented. The thermal stress have been obtained in terms of Fourier series and integrals that satisfy the differential equation and the boundary conditions. Several examples have been presented to show the application of the method.Nomenclature x, y rectangular coordinates - _x, _y direct stresses - _xy shear stress - ø Airy's stress function - E Young's modulus of elasticity - coefficient of thermal expansion - T temperature - ² Laplace operator: - ⁴ biharmonic operator - 2a length of the plate - 2b width of the plate - a/b aspect ratio - a _mr, b_ms, c_nr, d_ns Fourier coefficients defined in equation (6) - _m=m/a m=1, 2, 3, ... _n=n/2a n=1, 3, 5, ... - _r=r/b r=1, 2, 3, ... _s=s/2b s=1, 3, 5, ... - A _m, B_m, C_n, D_n, E_r, F_r, G_s, H_s Fourier coefficients - K _rand L _s Fourier coefficients defined in equation (20) - direct stress at infinity - T ₁(x, y) temperature distribution symmetrical in x and y - T ₂(x, y) temperature distribution symmetrical in x and antisymmetrical in y - T ₃(x, y) temperature distribution antisymmetrical in x and symmetrical in y - T ₄(x, y) temperature distribution antisymmetrical in x and y     

Food safety measurement issues: way forward

Ensuring food safety (FS) is a persistent concern frequently faced by many countries. Safeguarding the quality of food that is fit for human consumption is the primary responsibility of the governmental regulatory agencies. For most part, agro-industries and food processors assume voluntary leadership for producing safe food. However, in the event of FS breach, the regulatory responsibility kicks into identify and rectify the situation. Notwithstanding whether it is the regulator or the industry that institutes the remedial action (e.g. improved hygiene and refined agricultural and manufacturing practices), the role of laboratory measurements is central in safeguarding the integrity of a functioning FS system. There are many analytical tools available to implement this task, such as validated analytical methods, natural matrix reference materials, field tested monitoring systems (proactive assessment) and effective surveillance systems (constant vigilance to prevent repeat safety violations). Way forward: existing FS tools are insufficient and should be strengthened with innovative approaches. Examples are: assembling swift intervention logistics to face FS breaches; rapid response systems including communication; robust metrology based measurement systems located at strategic locations in the country; and inter-disciplinary human resource to match the need for capacity development. These issues are discussed.     

Relationships between molecular structure and thermomechanical properties of bio‐based thermosetting polymers

Molecular dynamics simulations are used to study highly cross‐linked epoxy networks comprised of furanyl epoxy monomer, 2,5‐bis[(2‐oxiranylmethoxy)methyl]‐furan (BOF), that is cross‐linked by two furanyl amine hardeners, 5,5'‐methylenedifurfurylamine (DFDA) and 5,5'‐ethylidenedifurfirylamine (CH₃‐DFDA). Important properties of these fully furan‐based systems, including room temperature density, glass transition temperature, and Young's modulus are found to agree with previous experimental results. We also compare the simulated and experimental values of four fully furan‐based thermosetting materials to those using the conventional resin diglycidyl ether of bisphenol A (DGEBA) cured with the two furanyl hardeners. Our simulation results predict a slight decrease in density and Young's modulus, but no impact on the glass transition temperature, upon adding the methyl group in DFDA. Detailed analyses of the MD trajectories reveal the underlying mechanisms responsible for the observed structure/property relations, which center on the lack of collinear covalent bonds in the BOF molecular structure. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 285–292     

Pathways of Enzymatic Phosphotransfer Reactions

Abstract

Mevalonate 5-pyrophosphate (MVAPP) decarboxylase catalyzes the decarboxylation of MVAPP to isopentenyl pyro-phosphate, an ATP-dependent process in which 3-phospho-MVAPP is a transient intermediate that undergoes concomitant decarboxylation and elimination of phosphate. Reaction of (S_p)-adenosine 5′-0-3-thio [3-17_{O 2},180] triphosphate in place of ATP produces (R)-[17O,18O] thiophosphate in place of phosphate. Therefore, the phosphotransfer step producing 3-phospho-MVAPP proceeds with inversion of configuration at P. Gentamicin nucleotidyl-transferase catalyzes the reaction of ATP with the C-2″ hydroxyl group of aminoglycoside antibiotics to produce AMP-2″-aminoglycosides, thereby inactivating the drugs. Enzymatic reaction of (S_p)-2′-deoxyadenosine 5′-O[α-17O] triphosphate with tobramycin produces (R_p)-[α-17O]dAMP-2″-tobramycin. Therefore, transfer of the 2′-deoxyadenosine 5′-phosphoryl group proceeds with inversion of configuration. Since both reactions are uncomplicated bisubstrate processes and both proceed with inversion at P, it is likely that both proceed by mechanisms involving direct, single-step phosphotransfer from the phospho-donor substrate to the acceptor, rather than by double-displacement mechanisms involving covalent, phosphoenzyme-intermediates.     

ZIF-Induced d-Band Modification in a Bimetallic Nanocatalyst: Achieving Over 44 % Efficiency in the Ambient Nitrogen Reduction Reaction

The electrochemical nitrogen reduction reaction (NRR) offers a sustainable solution towards ammonia production but suffers poor reaction performance owing to preferential catalyst–H formation and the consequential hydrogen evolution reaction (HER). Now, the Pt/Au electrocatalyst d-band structure is electronically modified using zeolitic imidazole framework (ZIF) to achieve a Faradaic efficiency (FE) of >44 % with high ammonia yield rate of >161 μg mg_cat⁻¹ h⁻¹ under ambient conditions. The strategy lowers electrocatalyst d-band position to weaken H adsorption and concurrently creates electron-deficient sites to kinetically drive NRR by promoting catalyst–N₂ interaction. The ZIF coating on the electrocatalyst doubles as a hydrophobic layer to suppress HER, further improving FE by >44-fold compared to without ZIF (ca. 1 %). The Pt/Au-N_ZIF interaction is key to enable strong N₂ adsorption over H atom.