Kinetics of thermal degradation of poly(-caprolactone)

The thermal degradation/modification dynamics of poly(-caprolactone) (PCL) was investigated in a thermogravimetric analyzer under non-isothermal and isothermal conditions. The time evolution of the molecular weight distribution during degradation was studied using gel permeation chromatography. Experimental molecular weight evolution and weight loss profile were modeled using continuous distribution kinetics. The degradation exhibited distinctly different behavior under non-isothermal and isothermal heating. Under non-isothermal heating, the mass of the polymer remained constant at initial stages with rapid degradation at longer times. The Friedman and Chang methods of analysis showed a 3-fold change (from 18 to 55–62 kcal mol⁻¹) in the activation energy from low temperatures to high temperatures during degradation. This suggested the governing mechanism changes during degradation and was explained using two parallel mechanisms (random chain scission and specific chain end scission) without invoking the sequential reaction mechanisms. Under isothermal heating, the polymer degraded by pure unzipping of specific products from the chain end.     

Synthesis,characterization, and anti-cancer activity of chalcone derivatives as-potent anaplastic lymphoma kinase inhibitors

Structural Chemistry - Chalcone derivatives (7a–k) have been synthesized and characterized by 1H-NMR, 13C-NMR, mass, and elemental analysis. The synthesized compounds 7a, 7d, and 7g have been...     

Contrasting Response of Two Dipolar Fluorescence Probes in a Leucine‐Based Organogel and Its Implications

The microenvironments of a leucine‐based organogel are probed by monitoring the fluorescence behavior of coumarin 153 (C153) and 4‐aminophthalimide (AP). The steady‐state data reveals distinctly different locations of the two molecules in the gel. Whereas AP resides close to the hydroxyl moieties of the gelator and engages in hydrogen‐bonding interactions, C153 is found in bulk‐toluene‐like regions. In contrast to C153, AP exhibits excitation‐wavelength‐dependent emission, indicating that the environments of the hydrogen‐bonded AP molecules are not all identical. A two‐component fluorescence decay of AP in gel, unlike C153, supports this model. A time‐resolved fluorescence anisotropy study of the rotational motion of the molecules also reveals the strong association of only AP with the gelator. That AP influences the critical gelation concentration implies its direct involvement in the gel‐formation process. The results highlight the importance of guest–gelator interactions in gels containing guest molecules.     

Comparative evaluation of antibacterial activity of silver nanoparticles synthesized using Rhizophora apiculata and glucose

The focus of the study is to compare the antibacterial efficacy of silver nanoparticles (AgNPs) fabricated by exploiting biological (a mangrove plant, Rhizophora apiculata) and chemical means (Glucose). The synthesized nanoparticles were characterised using UV-visible absorption spectrophotometry (UV-vis), Fourier transform Infra-red Spectroscopy (FTIR) and Transmission electron microscopy (TEM). Biologically synthesized silver nanoparticles (BAgNPs) were observed at 423 nm with particle sizes of 19-42 nm. The chemically synthesized silver nanoparticles (CAgNPs) showed a maximum peak at 422 nm with particle sizes of 13-19 nm. An obvious superiority of the antibacterial potency of BAgNPs compared to the CAgNPs as denoted by the zone of inhibition (ZoI) was noted when the nanoparticles were treated against seven different Microbial Type Culture Collection (MTCC) strains. The current study therefore elucidates that the synthesized AgNPs were efficient against the bacterial strains tested.     

A perturbation-based super-CI approach for the orbital optimization of a CASSCF wave function

A perturbation theory-based algorithm for the iterative orbital update in complete active space self-consistent-field (CASSCF) calculations is presented. Following Angeli et al. (J. Chem. Phys. 2002, 117, 10525), the first-order contribution of singly excited configurations to the CASSCF wave function is evaluated using the Dyall Hamiltonian for the determination of a zeroth-order Hamiltonian. These authors employ an iterative diagonalization of the first-order density matrix including the first-order correction arising from single excitations, whereas the present approach uses the single-excitation amplitudes directly for the construction of the exponential of an anti-Hermitian matrix resulting in a unitary matrix which can be used for the orbital update. At convergence, the single-excitation amplitudes vanish as a consequence of the generalized Brillouin's theorem. It is shown that this approach in combination with direct inversion of the iterative subspace (DIIS) leads to very rapid convergence of the CASSCF iteration procedure. © 2019 Wiley Periodicals, Inc.     

Purification and biochemical characterization of two major thermophilic xylanase isoforms (T70 and T90) from xerophytic Opuntia vulgaris plant spp.

Thermostable xylanase isoforms T₇₀ and T₉₀ were purified and characterized from the xerophytic Opuntia vulgaris plant species. The enzyme was purified to homogeneity employing three consecutive steps. The purified T₇₀ and T₉₀ isoforms yielded a final specific activity 134.0 and 150.8 U mg^?1 protein, respectively. The molecular mass of these isoforms was determined to be 27 kDa. The optimum pH for the T₇₀ and T₉₀ xylanase isoforms was 5.0 and the temperature for optimal activity was 70 and 90 °C, respectively. The Km value of T₇₀ and T₉₀ enzyme isoforms was 3.49, 2.1 mg ml^?1, respectively when oat spelt xylan was used as a substrate. The T₇₀ had a Vmax of 10.4 μmol min^?1 mg^?1, and T₉₀ had a Vmax of 8.9 μmol min^?1 mg^?1, respectively. In the presence of 10 mM Co²⁺, and Mn²⁺ the activity of T₇₀ and T₉₀ isoforms increased, where as 90 % inhibition was noted with of the use 10 mM Hg²⁺, Cd²⁺, Cu²⁺, Zn²⁺ while partial inhibition was observed in the presence of Fe³⁺, Ni²⁺, Ca²⁺and Mg²⁺. The T₇₀ and T₉₀ isoforms retained nearly 50 % activity in the presence of 2.0 M urea, while use of 40 mM SDS lowered the activity nearly 38–41 %. The substrate specificity of both T₇₀ and T₉₀ isoforms showed maximum activity for oat spelt xylan. Western blot, immunodiffusion, and in vitro inhibition assays confirmed reactivity of the T₉₀ isoform with polyclonal anti-T₉₀ antibody raised in rabbit, as well as cross-reactivity of the antibody with the T₇₀ xylanase isoform.     

Lipase specificity for the hydrolysis of poly (vinyl acetate)

The effect of lipases on the side chain hydrolysis of poly (vinyl acetate) (PVAc) was investigated in toluene by various lipases, Hog-pancreas (HP), Candida Rugosa (CR), Lipolase-100T (LL), and Novozyme 435 (NV) at 60 °C. Gas chromatographs and GC–MS spectra showed the presence of different ester side chains. The different size side chain esters in PVAc get hydrolyzed at different specific rates by the various lipases. Longer side chains are hydrolyzed in the order HP>NV>LL>CR whereas the short chains are hydrolyzed in the reverse order. A continuous distribution kinetics model was proposed that accounts for lipase deactivation and determines the rate coefficients of hydrolysis of various chain lengths. Lipase deactivation and hydrolysis rate coefficients were determined for each case. The proposed model predicts the experimental data satisfactorily.     

Mechanism of (Salen)manganese(III)‐Catalyzed Oxidation of Aryl Phenyl Sulfides with Sodium Hypochlorite

The oxidation of 4‐substituted phenyl phenyl sulfides was carried out with several oxo(salen)manganese(V) complexes in MeCN/H₂O 9 : 1. The kinetic data show that the reaction is first‐order each in the oxidant and sulfide. Electron‐attracting substituents in the sulfides and electron‐releasing substituents in salen of the oxo(salen)manganese(V) complexes reduce the rate of oxidation. A Hammett analysis of the rate constants for the oxidation of 4‐substituted phenyl phenyl sulfides gives a negative ρ value (ρ=?2.16) indicating an electron‐deficient transition state. The log k₂ values observed in the oxidation of each 4‐substituted phenyl phenyl sulfide by substituted oxo(salen)manganese(V) complexes also correlate with Hammett σ constants, giving a positive ρ value. The substituent‐, acid‐, and solvent‐effect studies indicate direct O‐atom transfer from the oxidant to the substrate in the rate‐determining step.