Synthesis, spectral and biological studies of nitrogen–sulphur donor macrocyclic ligands and their transition metals complexes

Mn(II), Co(II), Ni(II) and Cu(II) complexes have been synthesized with 22 and 24 membered tetramide macrocyclic ligands viz; 1,9,12,20-tetraaza-2,8,13,19-tetraone-5,16-dithiacyclodocosane [L¹] and 1,9,13,21-tetraaza-2,8,14,20-tetraone-5,17-dithiacyclotetracosane [L²] and characterized by elemental analysis, molar conductance, magnetic susceptibility measurements, mass, IR, electronic EPR spectral studies and electrochemical properties. The molar conductance of all the complexes in DMSO solution is corresponding to 1:2 electrolyte. Thus these complexes may be formulated as [M(L′)]Cl₂ [where M = Mn(II), Co(II), Ni(II) and Cu(II) L′ = L¹ and L²]. On the basis of spectral studies a distorted octahedral geometry has been assigned for all the complexes. The ligands and their complexes were also screened in vitro against two pathogenic fungi (F. moniliformae and R. solani) to assess their growth inhibiting potential.     

Single-drop micro-extraction and diffuse reflectance Fourier transform infrared spectroscopic determination of chromium in biological fluids

The present paper deals with a new micro-extraction procedure for selective separation of Cr(VI) in the form of a metaloxy anionic species namely dichromate (Cr₂O₇²⁻) with N¹-hydroxy-N¹,N²-diphenylbenzamidine (HOA) in to dichloromethane and its subsequent and rapid diffuse reflectance Fourier transform infrared spectroscopic (DRS-FTIR) determination employing potassium bromide matrix. The diffuse reflectance Fourier transform infrared spectroscopy gives both qualitative and quantitative information about the dichromate. The determination of chromium is based on the analytical peak selection, among the various vibrational peaks, at 902 cm⁻¹. The micro-extraction was based on the liquid-liquid solvent extraction (LLSE) principle. The dichromate binds with the nitrogen and oxygen atoms of N¹-hydroxy-N¹,N²-diphenylbenzamidine (HOA) and forms 1:2, Cr(VI):HOA complex in 0.1 mol L⁻¹ HCl medium. The formation of above complex, in the acidic medium, is confirmed by the appearance of chocolate-brown color in the micro-extract. The speciation studies of Cr(III) and Cr(VI) is done by conversion of Cr(III) into Cr(VI) utilizing H₂O₂ as an oxidizing agent. The chemistry of pure dichromate and that of its HOA complex is discussed. The limit of detection (LoD) and the limit of quantification (LoQ) of the method are found to be 0.01 μg g⁻¹ Cr₂O₇²⁻ and 0.05 μg g⁻¹ Cr₂O₇²⁻, respectively. The standard deviation value and the relative standard value at a level of 10 μg Cr₂O₇²⁻/0.1 g KBr for n = 10 is found to be 0.26 μg Cr₂O₇²⁻ and 2.6%, respectively. The relative standard deviation (n = 8 and 6) for the determination of dichromate (Cr₂O₇²⁻) in real human biological fluid samples is observed to be in the range 3.1-7.8%.     

Volatile constituents of Origanum vulgare L., 'thymol' chemotype: variability in North India during plant ontogeny

Essential oils derived from six different phenophases, namely early vegetative stage, late vegetative stage, early flowering stage, full flowering stage (FFS), late flowering stage and seed shattering stage of Origanum vulgare L. grown in Kumaon region of Uttarakhand, India were investigated by GC and GC-MS. A total of 38 constituents, representing 97.4-99.7% of the total oil composition, were identified. Major components of oils were thymol (40.9-63.4%), p-cymene, (5.1-25.9%), γ-terpinene (1.4-20.1%), bicyclogermacrene (0.2-6.1%), terpinen-4-ol (3.5-5.9%), α-pinene (1.6-3.1%), 1-octen-3-ol (1.4-2.7%), α-terpinene (1.0-2.2%), carvacrol (<0.1-2.1%), β-caryophyllene (0.5-2.0%) and β-myrcene (1.2-1.9%). Thymol, terpinen-4-ol, 3-octanol, α-pinene, β-pinene, 1,8-cineole, α-cubebene and (E)-β-ocimene were observed to be higher during FFS. The study showed that plant stage had a significant effect on the essential oil content and composition of O. vulgare grown in the hilly tracks of Northern India.     

Intriguing H-aggregate and H-dimer formation of coumarin-481 dye in aqueous solution as evidenced from photophysical studies

Photophysical properties of coumarin-481 (C481) dye in aqueous solution show intriguing presence of multiple emitting species. Concentration and wavelength dependent fluorescence decays and time-resolved emission spectra and area-normalized emission spectra suggest the coexistence of dye monomers, dimers, and higher aggregates (mostly trimers) in the solution. Because of the efficient intramolecular charge transfer (ICT) state to twisted intramolecular charge transfer (TICT) state conversion, the dye monomers show very short fluorescence lifetime of ~0.2 ns. Fluorescence lifetimes of dimers (~4.1 ns) and higher aggregates (~1.4 ns) are relatively longer due to steric constrain toward ICT to TICT conversion. Observed results indicate that the emission spectra of the aggregates are substantially blue-shifted compared to monomers, suggesting H-aggregation of the dye in the solution. Temperature-dependent fluorescence decays in water and time-resolved fluorescence results in water-acetonitrile solvent mixtures are also in support of the dye aggregation in the solution. Though dynamic light scattering studies could not recognize the dye aggregates in the solution due to their small size and low concentration, fluorescence up-conversion measurements show a relatively higher decay tail in water than in water-acetonitrile solvent mixture, in agreement with higher dye aggregation in aqueous solution. Time-resolved fluorescence results with structurally related non-TICT dyes, especially those of coumarin-153 dye, are also in accordance with the aggregation behavior of these dyes in aqueous solution. To the best of our knowledge, this is the first report on the aggregation of coumarin dyes in aqueous solution. Present results are important because coumarin dyes are widely used as fluorescent probes in various microheterogeneous systems where water is always a solvent component, and the dye aggregation in these systems, if overlooked, can easily lead to a misinterpretation of the observed results.     

meso-meso-Linked corrole dimers with modified cores: synthesis, characterization, and properties

The synthesis and characterization of the first examples of core-modified corrole dimers linked through the meso positions are described. The dimers are obtained by a simple Ag(I)- or Fe(III)-catalyzed coupling reaction in near-quantitative yields. The corroles obtained are metalated with metal ions, such as Cu(II) and Ni(II). The electronic-absorption spectral studies reveal weak electronic interaction between the two subunits and the exciton coupling observed for the free-base corrole dimer (1717 cm(-1)) is lower than that for the corresponding protonated derivative (4081 cm(-1)). The solution-state structure derived from (1)H and two-dimensional NMR spectral studies reveals a noncoplanar arrangement of two corrole units. Geometry optimization at the B3LYP-631G level also confirms the noncoplanar arrangement of corroles with a dihedral angle of 64.8 degrees between two corrole planes. The electron paramagnetic resonance (EPR) and magnetic characterization studies on the paramagnetic copper-metalated dimer indicate that both copper ions behave as independent spins without any noticeable interaction. Results of fluorescence studies reveal a bathochromic shift of about 60 nm upon dimerization. The first hyperpolarizability (beta) measured by using the hyper Rayleigh scattering (HRS) method reveals doubling of the beta values on progressing from monomer to dimer, attributed to enhanced pi conjugation. The use of copper dimers in the photocleavage of DNA is also explored. It is shown that the bimetallic copper dimer selectively cleaves the nucleic acids without affecting the proteins, suggesting a possible application of the copper complex in the removal of nucleic acid contaminants from protein extracts through a simple photolytic pathway.     

Comparative study of volatile oil compositions of two Plectranthus species from northern India

The leaf and inflorescence essential oils of Plectranthus rugosus Wall. (syn. Rabdosia rugosa Wall.) and Plectranthus incanus L. (syn. Plectranthus mollis L.), which grow wild in Uttarakhand, India, were analysed and compared by capillary gas chromatography and gas chromatography-mass spectrometry. The analysis led to the identification of 43 constituents, forming 89.5-93.6% of the total oil compositions. Both leaf and inflorescence oil of P. rugosus were dominated by sesquiterpene hydrocarbons (71.8%, 71.7%) represented by β-caryophyllene (36.2%, 29.8%), germacrene D (25.2%, 28.2%) and α-humulene (6.6%, 8.6%) as the major constituents. Conversely, the leaf and inflorescence oil of P. incanus were dominated by monoterpenoids (74.4%, 65.8%) with piperitenone oxide (44.2%, 38.5%), piperitone (8.6%, 12.2%) and terpinolene (14.5%, 10.2%) as major constituents. Piperitenone oxide, piperitone, cis- and trans-piperitols and trans-piperitol acetate were the marker constituents in P. incanus, which were not noted in the essential oil of P. rugosus.     

Photochemical C–N bond forming reactions for the synthesis of five-membered fused N-heterocycles

Abstract

Few conversions cannot take place with ground-state reactions even with the help of a catalyst, therefore they are made to occur under photochemical conditions. The transfer of electrons took place even with the photochemical excitement of one molecule where redox reaction cannot occur at the ground state. The ground-state reactions resulted in the formation of side products. The substrates did not require any sort of chemical activation for C–N bond construction in the course of photochemical reactions. The source of energy; light has always been the interest of researchers in order to induce chemical reactions ever since the starting of scientific chemistry. The present review encloses the chemistry of photochemical transformations with a focus on their synthetic uses. The organic photochemical reactions prevent the polluting or harmful reagents thus, provides a possibility for sustainable procedures as well as green chemistry. This review article displays the formation of numerous of five-membered fused nitrogen-heterocyclic compounds.     

Highly efficient electrocatalytic hydrogen evolution promoted by O–Mo–C interfaces of ultrafine β-Mo2C nanostructures

Optimizing interfacial contacts and thus electron transfer phenomena in heterogeneous electrocatalysts is an effective approach for enhancing electrocatalytic performance. Herein, we successfully synthesized ultrafine β-Mo₂C nanoparticles confined within hollow capsules of nitrogen-doped porous carbon (β-Mo₂C@NPCC) and found that the surface layer of molybdenum atoms was further oxidized to a single Mo–O surface layer, thus producing intimate O–Mo–C interfaces. An arsenal of complementary technologies, including XPS, atomic-resolution HAADF-STEM, and XAS analysis clearly reveals the existence of O–Mo–C interfaces for these surface-engineered ultrafine nanostructures. The β-Mo₂C@NPCC electrocatalyst exhibited excellent electrocatalytic activity for the hydrogen evolution reaction (HER) in water. Theoretical studies indicate that the highly accessible ultrathin O–Mo–C interfaces serving as the active sites are crucial to the HER performance and underpinned the outstanding electrocatalytic performance of β-Mo₂C@NPCC. This proof-of-concept study opens a new avenue for the fabrication of highly efficient catalysts for HER and other applications, whilst further demonstrating the importance of exposed interfaces and interfacial contacts in efficient electrocatalysis.

Ultrafine β-Mo₂C nanostructures encapsulated in N-doped carbon capsules featuring O–Mo–C interfaces as the active sites for HER have been unveiled.     

Free Radicals as a Double-Edged Sword: The Cancer Preventive and Therapeutic Roles of Curcumin

Free radicals, generally composed of reactive oxygen species (ROS) and reactive nitrogen species (RNS), are generated in the body by various endogenous and exogenous systems. The overproduction of free radicals is known to cause several chronic diseases including cancer. However, increased production of free radicals by chemotherapeutic drugs is also associated with apoptosis in cancer cells, indicating the dual nature of free radicals. Among various natural compounds, curcumin manifests as an antioxidant in normal cells that helps in the prevention of carcinogenesis. It also acts as a prooxidant in cancer cells and is associated with inducing apoptosis. Curcumin quenches free radicals, induces antioxidant enzymes (catalase, superoxide dismutase, glutathione peroxidase), and upregulates antioxidative protein markers–Nrf2 and HO-1 that lead to the suppression of cellular oxidative stress. In cancer cells, curcumin aggressively increases ROS that results in DNA damage and subsequently cancer cell death. It also sensitizes drug-resistant cancer cells and increases the anticancer effects of chemotherapeutic drugs. Thus, curcumin shows beneficial effects in prevention, treatment and chemosensitization of cancer cells. In this review, we will discuss the dual role of free radicals as well as the chemopreventive and chemotherapeutic effects of curcumin and its analogues against cancer.     

Systematic mutational analysis of an ubiquitin ligase (MDM2)-binding peptide: computational studies

To understand the importance of amino acids that comprise the peptide PMI (p53-MDM2/MDMX inhibitor), a p53-mimicking peptide with high affinity for the ubiquitin ligase MDM2, computational alanine scanning has been carried out using various protocols. This approach is very useful for identifying regions of a peptide that can be mutated to yield peptides that bind to their targets with higher affinities. Computational alanine scanning is a very useful technique that involves mutating each amino acid of the peptide in its complex with its target (MDM2 in the current study) to alanine, running short simulations on the mutated complex and computing the difference in interaction energies between the mutant peptides and the target protein (MDM2 in the current study) relative to the interaction energy of the original (wild-type) peptide and the target protein (MDM2 in the current study). We find that running multiple short simulations yield values of computed binding affinities (enthalpies) that are similar to those obtained from a long simulation and are well correlated with the trends in the data available from experiments that used Surface Plasmon Resonance to obtain dissociation constants. The p53-mimicking peptides contain three amino acids (F19, W23 and L26) that are major determinants of the interactions between the peptides and MDM2 and form an essential motif. We find in the current study that the trends amongst the contributions to experimental binding affinities of the hydrophobic residues F19, W23 and L26 are the best reproduced in all the computational protocols examined here. This study suggests that running such short simulations may provide a rapid method to redesign peptides to obtain high-affinity variants against a target protein. We further observe that modelling an extended conformation at the C-terminus of the helical PMI peptides, in accord with the conformation of the p53-peptide complexed to MDM2, reproduces the trends seen amongst the experimental affinities of the peptides that carry the alanine mutations at their C-termini. This suggests that some of the mutant peptides possibly interconvert between helical and extended states and can bind to MDM2 in either conformation. This novel feature, not obvious from the crystallographic data, if factored into modelling protocols, may yield novel high-affinity peptides. Our findings suggest that such protocols may enable rapid investigations of at least certain types of amino acid mutations, notably from large to small amino acids.