Size-selective synthesis and stabilization of gold organosol in C(n)TAC: enhanced molecular fluorescence from gold-bound fluorophores

Gold nanoparticles of variable sizes have been synthesized in toluene employing two-phase (water-toluene) extraction of AuCl4- followed by its reduction with sodium borohydride in the presence of a series of cationic surfactants of a homologous series having the general formula C(n)TAC. The solubility features of the gold particles in the organic solvent have been accounted qualitatively by calculating the van der Waals interaction potential between the particles. The effect of thermal energy and medium dielectric constant on the stability of metal particles has been studied by measuring the surface plasmon resonance. The stabilization of surfactant-mediated gold particles as hydrosol or organosol has been elucidated by considering the double-layer interaction as a function of the dielectric constant of the solvent medium. The influence of the counterion of the phase transfer reagent and stabilizing ligand on the photochemical stability of the gold colloids has been investigated. The fluorescence probe 1-methylaminopyrene (MAP) was considered for the surface functionalization of the gold particles, and it has been found that there is an enhancement of molecular fluorescence from the gold-probe assembly.     

A convenient palladium catalyzed synthesis of symmetric biaryls, biheterocycles and biaryl chiral diamides

A series of symmetrical diamido biaryls has been synthesized in good yield by direct homocoupling of iodoarylbenzamides by palladium-catalysis. No cross product has been isolated from the reaction mixture of two different iodoarylbenzamides under similar conditions. However, only in the case of 2-iodo-N-phenylbenzamide, the intramolecularly coupled product phenanthridone has been isolated as a minor product along with the major intermolecularly coupled product. Biphenyl chiral diamides have been synthesized by this coupling method. This coupling reaction also works well with the reductive dimerization of functionalized heterocyclic compounds. Thus 6,6′-dipivaloylamino-3,3′-bipyridine and 6,6′-dimethyl-2,2′-bipyridine have been efficiently synthesized. In two cases, the X-ray crystal structures have been solved to establish the structures of symmetrical and chiral diamido biaryls and their supramolecular networks.     

Mechanistic Studies on the Oxidation of Nitrite by a μ‐Oxodiiron(III,III) Complex in Aqueous Acidic Media

Examined in this study is the kinetics of a net 2e⁻ transfer between [Fe₂(μ‐O)(phen)₄(H₂O)₂]⁴⁺ ( 1 ) and its hydrolytic derivatives [Fe₂(μ‐O)(phen)₄(H₂O)(OH)]³⁺ ( 2 ) and [Fe₂(μ‐O)(phen)₄(OH)₂]²⁺ ( 3 ) with in aqueous media and in presence of excess 1,10‐phenanthroline (phen). The reaction is quantitative with a 1 : 1 stoichiometry between the oxidant and reductant to produce ferroin ([Fe(phen)₃]²⁺) and . The order of reactivity of the oxidant species is 1 > 2 > 3 , in agreement with the progressive cationic charge reduction. The reactions appear to be inner‐sphere where the initial one‐electron proton‐coupled redox (1e⁻, 1H⁺; electroprotic) seems to be rate‐determining.     

Transition-Metal-Free Heterocyclic Carbon-Boron Bond Formation

Heteroaryl boronic acids and esters are extremely important and valuable intermediates because of their wide application in the synthesis of marketed drugs and bioactive compounds. Over the last couple of decades, the construction of highly important heteroaryl carbon-boron bonds has created huge attention. The transition-metal-free protocols are more green, less sensitive to air and moisture, and also economically advantageous over the transition-metal-based protocols. The transition-metal-free C−H borylation of heteroarenes and C−X (X=halogen) borylation of heteroaryl halides represents an excellent approach for their synthesis. Also, various cyclization and alkyne activation protocols have been recently established for their synthesis. The goal of this review article is to summarize the existing literature and the current state of the art for transition-metal-free synthesis of heteroaryl boronic acid and esters.     

Biyclo[2.2.1]heptane as cyclopentane precursor. Part 4 Stereocontrolled syhthesis of a potential intermediate to chromophycane dolastane and clavularane

The regio– and stereoselective synthesis of the keto-esters 20 and 22 are described, the latter being a potential intermediate to several diterpenes. The key steps involve the Diels-Alder cycloaddition between the benzocycloheptenone 5 and cyclopentadiene followed by a regioselective functionalisation of the adduct 6. A remarkable reversal of regioselectivity was observed during oxymercuration of the unsymmetric double bond in 6 and its reduced product 11 leading to 8 and 12 which were subsequently transformed to 20 and 22 respectively.     

Excited-state intramolecular proton transfer in 2-(2'-arylsulfonamidophenyl)benzimidazole derivatives: the effect of donor and acceptor substituents

A series of water-soluble 2-(2'-arylsulfonamidophenyl)benzimidazole derivatives containing electron-donating and accepting groups attached to various positions of the fluorophore pi-system has been synthesized and characterized in aqueous solution at 0.1 M ionic strength. The measured pK(a)'s for deprotonation of the sulfonamide group of monosubstituted derivatives range between 6.75 and 9.33 and follow closely Hammett's free energy relationship. In neutral aqueous buffer, all compounds undergo efficient excited-state intramolecular proton transfer (ESIPT) to yield a strongly Stokes-shifted fluorescence emission from the phototautomer. Upon deprotonation of the sulfonamide nitrogen at high pH, ESIPT is interrupted to yield a new, blue-shifted emission band. The peak absorption and emission energies were strongly influenced by the nature of the substituents and their attachment positions on the fluorophore pi-system. The fluorescence quantum yield of the ESIPT tautomers revealed a significant correlation with the observed Stokes shifts. The study provides valuable information regarding substituent effects on the photophysical properties of this class of ESIPT fluorophores in an aqueous environment and may offer guidelines for designing emission ratiometric pH or metal-cation sensors for biological applications.     

Isomeric complexes of ruthenium(II) with neutral heterocyclic schiff base ligands. High resolution proton resonance spectra of trans-cis isomeric pairs of RuX2L2 (L = 2-arylpyridinecarboxaldimine, X = Cl, Br) and comparison of their physical properties

The reaction of 2-arylpyridinecarboxaldimine [RH₄C₆NC(H)Py, L (1)] with hydrated RuX₃ (X = Cl, Br) in boiling C₂H₅OH affords dark crystals of RuX₂L₂. Two geometrical isomers of the compound have been isolated and characterized by analytical and spectroscopic data. The trans isomer of RuCl₂L₂ shows a single sharp band for ν(Ru---Cl), whereas two bands are observed for the corresponding cis isomer. The highresolution ¹H NMR spectra of the isolated complexes are reported and completely assigned. All the complexes have multiple t₂→π^*(L) transitions in the visible region. Each of the complexes display a quasi-reversible oxidative response due to an Ru^III/Ru^II couple in the range 0.25–0.40 V vs S.C.E. at a platinum working electrode. The formal potentials of this couple obey the Hammett relationship. The ligand-based irreversible oxidations are also briefly noted.     

Leveraging an enzyme/artificial substrate system to enhance cellular persulfides and mitigate neuroinflammation

Persulfides and polysulfides, collectively known as the sulfane sulfur pool along with hydrogen sulfide (H₂S), play a central role in cellular physiology and disease. Exogenously enhancing these species in cells is an emerging therapeutic paradigm for mitigating oxidative stress and inflammation that are associated with several diseases. In this study, we present a unique approach of using the cell''s own enzyme machinery coupled with an array of artificial substrates to enhance the cellular sulfane sulfur pool. We report the synthesis and validation of artificial/unnatural substrates specific for 3-mercaptopyruvate sulfurtransferase (3-MST), an important enzyme that contributes to sulfur trafficking in cells. We demonstrate that these artificial substrates generate persulfides in vitro as well as mediate sulfur transfer to low molecular weight thiols and to cysteine-containing proteins. A nearly 100-fold difference in the rates of H₂S production for the various substrates is observed supporting the tunability of persulfide generation by the 3-MST enzyme/artificial substrate system. Next, we show that the substrate 1a permeates cells and is selectively turned over by 3-MST to generate 3-MST-persulfide, which protects against reactive oxygen species-induced lethality. Lastly, in a mouse model, 1a is found to significantly mitigate neuroinflammation in the brain tissue. Together, the approach that we have developed allows for the on-demand generation of persulfides in vitro and in vivo using a range of shelf-stable, artificial substrates of 3-MST, while opening up possibilities of harnessing these molecules for therapeutic applications.

A persulfide/hydrogen sulfide generation strategy through artificial substrates for 3-mercaptopyruvate sulfurtransferase (3-MST) is reported, which enhances cellular persulfides, attenuates reactive oxygen species (ROS), and alleviates inflammation.     

Enantiomeric synthesis of the key synthon of the oviposition-deterring pheromone of Rhagoletis cerasi L

A convenient and enantioselective synthesis of ethyl (8RS, 15R)-8-acetoxy-15-hydroxyhexadecanoate15 via a chemoenzymatic approach has been described. The salient features of the synthesis were operational simplicity, use of easily accessible materials, and excellent enantiocontrol via lipase catalyzed acetylation of 2-alkanol.     

Applicability and effectiveness of classifications models for achieving the twin objectives of growth and outreach of microfinance institutions

Measuring performance of microfinance institutions (MFIs) is challenging as MFIs must achieve the twin objectives of outreach and sustainability. We propose a new measure to capture the performance of MFIs by placing their twin achievements in a 2 × 2 grid of a classification matrix. To make a dichotomous classification, MFIs that meet both their twin objectives are classified as ‘1’ and MFIs who could not meet their dual objectives simultaneously are designated as ‘0’. Six classifiers are applied to analyze the operating and financial characteristics of MFIs that can offer a predictive modeling solution in achieving their objectives and the results of the classifiers are comprehended using technique for order preference by similarity to ideal solution to identify an appropriate classifier based on ranking of measures of performance. Out of six classifiers applied in the study, kernel lab-support vector machines achieved highest accuracy and lowest classification error rate that discriminates the best achievement of the MFIs’ twin objective. MFIs can use both these steps to identify whether they are on the right path to attaining their multiple objectives from their operating characteristics.