Negishi coupling: an easy progress for C–C bond construction in total synthesis

Negishi cross-coupling reactions were used extensively in total synthesis. The expansion of substrate scope, the development of mild reaction conditions, the advancement of the methods to improve the stereo- and regio-selectivity of carbon–carbon bond formation, the maturity of a large number of sequential processes, and the development of non-toxic reactions signify the importance of Negishi coupling. The following review illustrates a strategic role of this reaction in constructing carbon–carbon bonds in the recent total synthesis of natural products.     

Electromagnetic fields with 217 Hz and 0.2 mT as hazardous factors for tubulin structure and assembly (in vitro study)

Nowadays exposure to extremely low frequency electromagnetic fields (ELF-EMFs) is an unavoidable fact in human societies. In spite of destructive effects of ELF-EMF such as impression on short-term memory and cognitive performances, neuroprotective and neurogeneration effects of ELF-EMF have been reported. On the other hand, microtubule (MT) proteins, the dynamic cytoskeleton proteins that are mostly located in axons, are able to produce local EMFs and possess key role in memory formation and consciousness. Therefore, MTs have strong potential to be affected by exogenous ELF-EMFs. In the present study, decline of MT polymerization was shown by transmission electron microscopy images and turbidimetry method after exposure to ELF-EMFs with 50, 100, and 217 Hz frequencies at 0.2 mT intensity. Circular dichroism and 8-anilino-1-naphthalenesulfonate (ANS) fluorometry were used to demonstrate the MT secondary and third structural changes. Decrement of MT polymerization through disruption of tubulin native structure, and increasing the β-sheets and random coils were observed in all exposure conditions. Therefore, ELF-EMFs with 50, 100, and specially 217 Hz frequencies at 0.2 mT could lead to disruption of MT functions in the neural cells.     

Aluminum(III)-doped ZnO@Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanocomposite as a magnetic sorbent for preconcentration of cadmium(II)

An Al-doped ZnO@Fe₃O₄ nanocomposite was synthesized and used as a magnetic sorbent for solid-phase extraction of Cd(II) prior to its determination by flame atomic absorption spectrometry (FAAS). The size and morphology of the nano-sorbent were characterized via X-ray diffraction analysis, scanning electron microscopy and FTIR. Following its desorption with acetic acid, cadmium was quantified by FAAS. Factors affecting the extraction of the Cd(II) were optimized. Under optimized experimental conditions, the calibration graph is linear in the 0.6 to 60 ng mL^?1 concentration range. The limit of detection is 0.17 ng mL^?1 and the pre-concentration factor is 50. The inter- and intra-day relative standard deviations for six replicate determinations at a Cd(II) level of 40 ng mL^?1 are 3.8% and 2.5%, respectively. The method was successfully applied to the trace determination of Cd(II) in spiked water samples. The accuracy of the method was confirmed by analyzing the certified reference material NIST SRM 1643e.

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Graphical abstract Schematic of the synthesis of an Al-doped ZnO@Fe₃O₄ nanocomposite and its application as a magnetic sorbent for solid-phase extraction of Cd(II) prior to its determination by flame atomic absorption spectrometry (FAAS).

     

Luciferin‐Regenerating Enzyme Crystal Structure Is Solved but its Function Is Still Unclear

Contribution of luciferin‐regenerating enzyme (LRE) for in vitro recycling of D‐luciferin has been reported. According to crystal structure of LRE, it is a beta‐propeller protein which is a type of all β‐protein architecture. In this overview, reinvestigation of the luciferase‐based LRE assays and its function is reported. Until now, sequence of LRE genes from four different species of firefly has been reported. In spite of previous reports, T‐LRE (from Lampyris turkestanicus) was cloned and expressed in Escherichia coli as well as Pichia pastoris in a nonsoluble form as inclusion body. According to recent investigations, bioluminescent signal of soluble T‐LRE–luciferase‐coupled assay increased and then reached an equilibrium state in the presence of D‐cysteine. In addition, the results revealed that both D‐ and L‐cysteine in the absence of T‐LRE caused a significant increase in bioluminescence intensity of luciferase over a long time. Based on activity measurements and spectroscopic results, D‐cysteine increased the activity of luciferase due to its redox potential and induction of conformational changes in structure and kinetics properties. In conclusion, in spite of previous reports on the effect of LRE (at least T‐LRE) on luciferase activity, most of the increase in luciferase activity is caused by direct effect of D‐cysteine on structure and activity of firefly luciferase. Moreover, bioinformatics analysis cannot support the presence of LRE in peroxisome of photocytes in firefly lanterns.     

Study of Kryptofix5 Complexation with La<Superscript>3+</Superscript> Cation in Several Individual and Binary Nonaqueous Solvents Using Conductometric Method

Complexatio of the La³⁺ cation with 1,13-bis(8-quinolyl)-1,4,7,10,13-pentaoxatridecane(Kryptofix5) was studied in pure solvents acetonitrile (AN), methanol (MeOH), nitrobenzene (NB), tetrahydrofuran (THF), methyl acetate (MeOAC) and in various binary solvent mixtures of AN–MeOH, AN–NB, AN–THF, and AN–MeOAC systems at different temperatures using the conductometric method. The stoichiometry of the complex was found to be 1 : 1 (ML). In all cases, the variation of the log kf with composition of the solvent was non-linear. This behavior is probably due to a change in the structure of these binary mixed solvents as the composition of the medium is varied. The stability order of the complex in pure nonaqueous solvents at 25°C increases in the order: AN > THF > MeOAC > MeOH > NB. The values of thermodynamic data (ΔH _c °,ΔS _c °) formation of (Kryptofix5.La)³⁺ complex are definitely solvent dependent.     

The effect of cooling rate on size,quality and morphology of KTiOPO4 (KTP) crystals grown by different nucleation techniques

KTP crystals have been grown by two nucleation techniques namely spontaneous nucleation in flux medium and nucleation on Pt rod using K₆P₄O₁₃ flux.10 °C/h, 7 °C/h, 0.8 °C/h, 0.4 °C/h and 0.2 °C/h cooling rates were applied for spontaneous nucleation and crystals up to 15 × 7 × 4 mm³ in size were grown. 1 °C/h and 0.8 °C/h cooling rates were also used for nucleation on Pt rod and crystals up to 8 × 6 × 3 mm³ in size were grown. The effect of cooling rate on size, morphology and optical quality of grown crystals by both techniques were studied. For nucleation on Pt rod upper and lower rotation rates limits and an optimum rotation rate were distinguished for each cooling rate. Quality of the grown crystals by both techniques was characterized by optical transmission analysis.     

Thermodynamic study of complex formation between Kryptofix-5 and Sn2+ in several individual and binary non-aqueous solvents using a conductometric method

The complex formation between 1,13-bis(8-quinolyl)-1,4,7,10,13-pentaoxatridecane (Kryptofix-5) and Sn²⁺ ions was studied in pure acetonitrile (AN), dimethylformamide (DMF), 1,4-dioxane (DOX), and methanol (MeOH) and in acetonitrile-1,4-dioxane (AN-DOX), acetonitrile-dichloromethane (AN-DCM), acetonitrile-methanol (AN-MeOH), and acetonitrile-dimethylformamide (AN-DMF) binary mixed solvent solutions at different temperatures using conductometric method. 1: 1 [ML] complex is formed between the metal cation and ligand in most solvent systems but in the cases of AN-MeOH (MeOH = 90 mol %) binary mixture and in pure MeOH a 2: 1 [M₂L] complex was observed, that is the stoichiometry of complexes may be changed by the nature of the medium. The stability order of the (Kryptofix-5·Sn)²⁺ complex in the studied binary mixed solvent solutions at 25°C was found to be AN-DOX > AN-DCM > AN-MeOH > AN-DMF and in the case of pure solvents at 25°C the sequence was the following: AN > DMF > DOX. A non-linear behavior was observed for changes of logK _f of (Kryptofix-5·Sn)²⁺ complex versus the composition of the binary mixed solvents, which was explained in terms of solvent-solvent intractions and also by the preferential solvation of the f species involved in the complexation reaction. The values of standard enthalpy changes (ΔH _c ^○ ) for complexation reactions were obtained from the slope of the Van’t Hoff plots and the changes in standard entropy (ΔS _c ^○ ) were calculated from the relationship ΔG _{c, 298.15} ^○ = ΔH _c ^○ ? 298.15ΔS _c ^○ . The results show that in most cases, the (Kryptofix-5·Sn)²⁺ complex is both enthalpy and entropy stabilized.     

Nickel(II) complexes of polyhydroxybenzaldehyde N4-thiosemicarbazones: synthesis, structural characterization and antimicrobial activities

Nickel(II) complexes with 2,3-dihydroxybenzaldehyde N4-substituted thiosemicarbazone ligands (H₃L¹–H₃L⁴) have been synthesized and characterized with the aim of evaluating the effect of N4 substitution in the thiosemicarbazone moiety on their coordination behavior and biological activities. Two series of nickel(II) complexes with the general formulae [Ni(H₃L)(H₂L)]ClO₄ and [Ni₂(HL)₂] were characterized by analytical and spectral techniques. The molecular structure of one of the complexes, namely, [Ni(H₃L⁴)(H₂L⁴)]ClO₄ was established by single crystal X-ray diffraction studies. The crystal structure of this complex revealed that two H₃L⁴ ligands are coordinated to nickel(II) in different modes; one as a neutral tridentate ONS ligand and the other is as a monoanionic tridentate (ONS^?) ligand. The antimicrobial activities of the compounds were tested against 25 bacterial strains via the disc diffusion method, and their minimum inhibitory concentration (MIC) and minimum microbicidal concentration were evaluated using microdilution methods. With a few exceptions, most of the compounds exhibited low-to-moderate inhibitory activities against the tested bacterial strains. However, the complexes [Ni₂(HL³)₂] (7) and [Ni₂(HL⁴)₂] (8) indicated higher inhibitory activity against Salmonella enterica ATCC 9068 (MIC values 15.7 and <15.7 μg/ml, respectively), compared with gentamicin as the positive control (MIC 25 μg/ml). Complex (7) also inhibited Streptococcus pneumoniae more efficiently (MIC 31.2 μg/ml), compared with gentamicin (MIC > 50 μg/ml). The toxicities of the compounds were tested on brine shrimp (Artemia salina), where no meaningful toxicity level was noted for both the free ligands and the complexes. The cytotoxicities of the compounds on cell viability were determined on MCF7, PC3, A375, and H413 cancer cells in terms of IC₅₀; complexes [Ni(H₃L³)(H₂L³)]ClO₄ (3), [Ni₂(HL³)₂] (7) and [Ni₂(HL⁴)₂] (8) exhibited significant cytotoxicity on the tested cell lines.     

Singlet–triplet energy differences in divalent five membered cyclic conjugated Arduengo-type carbenes XC<Subscript>2</Subscript>HN<Subscript>2</Subscript>M (M = C,Si, Ge,Sn, and Pb; X = F,Cl, Br,and I)

Singlet-triplet energy differences in Arduengo-type carbenes XC₂HN₂C compared and contrasted with their sila, germa, stana and plumba analogues; at B3LYP/6-311++G** level of theory. Free Gibbs energy differences between triplet (t) and singlet (s) states (ΔG(t–s)) change in the following order: plumbylenes > stannylenes > germylenes > silylenes > carbenes. The singlet states in XC₂HN₂C are generally more stable when the electron withdrawing groups such as–F was used at β-position. However, the singlet states in XC₂N₂HM (M = Si, Ge, Sn, and Pb) are generally more stable when the withdrawing groups such as–F was placed. The puckering energy is investigated for each the singlet and triplet states. The DFT calculations found the linear correlation to size of the group 14 divalent element (M), the ∠N–M–N angle, and the Δ(LUMO–HOMO) of XC₂HN₂M.