Stereodynamics of Small 1,2-Dialkyldiaziridines

Diaziridines are very interesting representatives of organic compounds containing stereogenic nitrogen atoms. In particular, 1,2-dialkyldiaziridines show extraordinarily high stereointegrity. The lone electron pairs of the nitrogen atoms are in trans configuration, avoiding a four-electron repulsive interaction. Furthermore, the trans configuration of the substituents at the nitrogen atoms is energetically favored because of reduced steric interactions. Therefore only two stereoisomers (enantiomers) are observed. At elevated temperatures the enantiomers are interconverting because of the limited stereointegrity of the chirotopic nitrogen atoms. The enantiomerization rate constants and the activation parameters of interconversion are of great interest. Here, we investigated the stereodynamics of a set of small 1,2-dialkyldiaziridines bearing short substituents (Me, Et, iPr, tBu), using enantioselective dynamic gas chromatography (DGC). Separation of enantiomers of all compounds, including the highly volatile 1,2-dimethyldiaziridine, was achieved using heptakis(2,3-di-O-ethyl-6-O-tert-butyldimethylsilyl)-β-cyclodextrin in 50% PS086 (w/w) as chiral stationary phase in fused silica capillaries with a length of up to 50 m. Measurements at variable temperatures were performed and reaction rate constants were determined using the unified equation of chromatography implemented in the software DCXplorer. The activation barriers at room temperature for 1-(tert-butyl)-2-ethyldiaziridine, ΔG^╪_298K = 123.8 kJ mol^–1 (ΔH^╪ = 115.5 ± 2.9 kJ mol^–1, ΔS^╪ = –28 ± 1 J mol^–1 K^–1), and 1-ethyl-2-isopropyldiaziridine, ΔG^╪_298K = 124.2 kJ mol^–1 (ΔH^╪ = 113.1 ± 2.4 kJ mol^–1, ΔS^╪ = –37 ± 2 J mol^–1 K^–1), were determined, representing some of the highest values observed for nitrogen inversion in diaziridines. Chirality 00:000–000, 2013. © 2013 Wiley Periodicals, Inc.     

The stereodynamics of 1,2‐dipropyldiaziridines

N‐alkylated trans‐diaziridines are an intriguing class of compounds with two stereogenic nitrogen atoms which easily interconvert. In the course of our investigations of the nature of the interconversion process via nitrogen inversion or electrocyclic ring opening ring closure, we synthesized and characterized the three constitutionally isomeric diaziridines 1,2‐di‐n‐propyldiaziridine 1 , 1‐isopropyl‐2‐n‐propyldiaziridine 2 , and 1,2‐diisopropyldiaziridine 3 to study the influence of the substituents on the interconversion barriers. Enantiomer separation was achieved by enantioselective gas chromatography on the chiral stationary phase Chirasil‐β‐Dex with high separation factors α (1‐isopropyl‐2‐n‐propyldiaziridine: 1.18; 1, 2‐diisopropyldiaziridine: 1.24; 100°C 50 kPa He) for the isopropyl substituted diaziridines. These compounds showed pronounced plateau formation between 100 and 150°C, and peak coalescence at elevated temperatures. The enantiomerization barriers ΔG? and activation parameters ΔH? and ΔS? were determined by enantioselective dynamic gas chromatography (DGC) and direct evaluation of the elution profiles using the unified equation implemented in the software DCXplorer. Interestingly, 1‐isopropyl‐2‐n‐propyldiaziridine and 1,2‐diisopropyldiaziridine exhibit similar high interconversion barriers ΔG? (100°C) of 128.3 ± 0.4 kJ mol^?1 and 129.8 ± 0.4 kJ mol^?1, respectively, which indicates that two sterically demanding substituents do not substantially increase the barrier as expected for a distinct nitrogen inversion process. Chirality, 2010. © 2009 Wiley‐Liss, Inc.     

Study of the interaction between sodium salts of (2E)‐3‐(4'‐halophenyl)prop‐2‐enoyl sulfachloropyrazine and bovine serum albumin by fluorescence spectroscopy

Three sodium salts of (2E)‐3‐(4'‐halophenyl)prop‐2‐enoyl sulfachloropyrazine (CCSCP) were synthesized and their structures were determined by ¹H and ¹³C NMR, LC‐MS and IR. The binding properties between CCSCPs and bovine serum albumin (BSA) were studied using fluorescence spectroscopy in combination with UV–vis absorbance spectroscopy. The results indicate that the fluorescence quenching mechanisms between BSA and CCSCPs were static quenching at low concentrations of CCSCPs or combined quenching (static and dynamic) at higher CCSCP concentrations of 298, 303 and 308 K. The binding constants, binding sites and corresponding thermodynamic parameters (ΔH, ΔS, ΔG) were calculated at different temperatures. All ΔG values were negative, which revealed that the binding processes were spontaneous. Although all CCSCPs had negative ΔH and positive ΔS, the contributions of ΔH and ΔS to ΔG values were different. When the 4'‐substituent was fluorine or chlorine, van der Waals interactions and hydrogen bonds were the main interaction forces. However, when the halogen was bromine, ionic interaction and proton transfer controlled the overall energetics. The binding distances between CCSCPs and BSA were determined using the Förster non‐radiation energy transfer theory and the effects of CCSCPs on the conformation of BSA were analyzed by synchronous fluorescence spectroscopy. Copyright © 2012 John Wiley & Sons, Ltd.     

Novel Oxaliplatin Derivatives with 1‐(Substituted Benzyl)azetidine‐3,3‐dicarboxylate Anions. Synthesis,Cytotoxicity, and Interaction with DNA

A series of oxaliplatin derivatives with (1R,2R)‐N¹‐alkyl‐1,2‐cyclohexane‐1,2‐diamine (alkyl=Bu or ⁱPr) as carrier ligands and 1‐(methoxy‐ or methyl‐substituted benzyl)azetidine‐3,3‐dicarboxylate anions as leaving groups were synthesized and spectrally characterized. Generally, Complexes 10 – 15 with an ⁱPr substituent at N(1) showed higher activities in vitro than carboplatin against MCF‐7 human breast carcinoma and A549 human non‐small‐cell lung cell lines, although they were less potent than oxaliplatin. The typical complex 14 exhibited cytotoxicity superior to that of carboplatin and comparable to that of oxaliplatin against two selected tumor cell lines. Additionally, agarose gel electrophoresis was applied to investigate the DNA‐cleavage ability of complex 14 , which demonstrated that it has a different mode of DNA distortion from that of oxaliplatin.     

Enantiomeric Recognition of Racemic 4‐Aryl‐1,4‐dihydropyridine Derivatives via Chiralpak AD‐H Stationary Phases

The chromatographic chiral resolution of two new series of racemic 4‐substituted‐1,4‐dihydropyridine derivatives was studied on a commercial Chiralpak AD‐H column. Analytes without 5,5‐dimethyl substituents ( 1–15 ) are more efficiently resolved than analytes with 5,5‐dimethyl groups ( 16–30 ). The AD‐H column discriminated between enantiomers through both hydrogen bonding attractions and π–π interactions. This interpretation is in accord with plots of the logarithm of separation factors, log(α), versus σ (Hammett–Swain substituent parameter) and σ⁺ (Brown substituent constant) plots. By elucidating the effects of the remote substituents on these chiral separations, it was shown that the influence of π–π interaction forces increase when steric bulk effects act to decrease the hydrogen bonding attractive forces on the AD‐H column. Chirality 24:854–859, 2012. © 2012 Wiley Periodicals, Inc.     

A study on the interaction between 3‐spiro‐piperidones and bovine serum albumin using spectroscopic approaches

The interaction between 3‐spiro‐2′‐pyrrolidine‐3′‐spiro‐3″‐piperidine‐2,3″‐dione (PPD) and bovine serum albumin (BSA) in aqueous solution was studied using fluorescence and UV–vis spectroscopy. Fluorescence emission data revealed that BSA (1.00 × 10^‐5 mol/L) fluorescence was statically quenched by PPD at various concentrations, which implies that a PPD–BSA complex was formed. The binding constant (K_A), the number of binding sites (n) and the specific binding site of the PPD with BSA were determined. Energy‐transfer efficiency parameters were determined and the mechanism of the interaction discussed. The thermodynamic parameters, ΔG, ΔH and ΔS, were obtained according to van't Hoff's equation, showing the involvement of hydrophobic forces in these interactions. The effect of PPD acting on the BSA conformation was detected by synchronous fluorescence. Copyright © 2012 John Wiley & Sons, Ltd.     

Scavenging of superoxide anion radical and hydroxyl radical by novel thiazolyl‐thiazolidine‐2,4‐dione compounds

The antioxidant behavior of a series of new synthesized substituted thiazolyl‐thiazolidine‐2,4‐dione compounds (TZDs) was examined using chemiluminescence and electron paramagnetic resonance spin trapping techniques. 5,5‐Dimethyl‐1‐pyrroline‐N‐oxide (DMPO) was used as the spin trap. The reactivity of TZDs with superoxide anion radical (O) and hydroxyl radical (HO^?) was evaluated using potassium superoxide/18‐crown‐6 ether dissolved in dimethylsulfoxide, and the Fenton‐like reaction (Fe²⁺ + H₂O₂), respectively. The results showed that TZDs efficiently inhibited light emission from the O generating system at a concentration of 0.05–1 mmol L^?1 (5–94% reductions were found at 1 mmol L^?1 concentration). The TZD compounds showed inhibition of HO^?‐dependent DMPO–OH spin adduct formation from DMPO (the amplitude decrease ranged from 8 to 82% at 1 mmol L^?1 concentration). The findings showed that examined TZDs had effective activities as radical scavengers. Copyright © 2009 John Wiley & Sons, Ltd.     

The γ‐aminobutyric acid shunt contributes to closing the tricarboxylic acid cycle in Synechocystis sp. PCC 6803

A traditional 2‐oxoglutarate dehydrogenase complex is missing in the cyanobacterial tricarboxylic acid cycle. To determine pathways that convert 2‐oxoglutarate into succinate in the cyanobacterium Synechocystis sp. PCC 6803, a series of mutant strains, Δsll1981, Δslr0370, Δslr1022 and combinations thereof, deficient in 2‐oxoglutarate decarboxylase (Sll1981), succinate semialdehyde dehydrogenase (Slr0370), and/or in γ‐aminobutyrate metabolism (Slr1022) were constructed. Like in Pseudomonas aeruginosa, N‐acetylornithine aminotransferase, encoded by slr1022, was shown to also function as γ‐aminobutyrate aminotransferase, catalysing γ‐aminobutyrate conversion to succinic semialdehyde. As succinic semialdehyde dehydrogenase converts succinic semialdehyde to succinate, an intact γ‐aminobutyrate shunt is present in Synechocystis. The Δsll1981 strain, lacking 2‐oxoglutarate decarboxylase, exhibited a succinate level that was 60% of that in wild type. However, the succinate level in the Δslr1022 and Δslr0370 strains and the Δsll1981/Δslr1022 and Δsll1981/Δslr0370 double mutants was reduced to 20–40% of that in wild type, suggesting that the γ‐aminobutyrate shunt has a larger impact on metabolite flux to succinate than the pathway via 2‐oxoglutarate decarboxylase. ¹³C‐stable isotope analysis indicated that the γ‐aminobutyrate shunt catalysed conversion of glutamate to succinate. Independent of the 2‐oxoglutarate decarboxylase bypass, the γ‐aminobutyrate shunt is a major contributor to flux from 2‐oxoglutarate and glutamate to succinate in Synechocystis sp. PCC 6803.     

Efficient blue organic light‐emitting diodes using N2,N2,N11,N11,5,6,7,8‐octaphenyltriphenylene‐2,11‐diamine derivatives

In this study, we have synthesized phenyl‐substituted triphenylene derivatives, using the Diels–Alder reaction and the Buchwald–Hartwig reaction. To investigate electroluminescence properties of these materials, multilayer organic light‐emitting diode (OLED) devices were fabricated with a structure of indium–tin–oxide (ITO) (180 nm)/4,4′‐bis(N‐(1‐naphthyl)‐N‐phenylamino)biphenyl (NPB) (50 nm)/blue‐emitting materials (1–3) (30 nm)/bathophenanthroline (Bphen) (35 nm)/lithium quinolate (Liq) (2 nm)/Al (100 nm). A device using N²,N²,N¹¹,N¹¹,5,6,7‐heptaphenyltriphenylene‐2,11‐diamine (2) exhibited efficient blue emission with luminous, power, and external quantum efficiencies of 0.92 cd/A, 0.67 lm/W, and 1.17% at 20 mA/cm², respectively. The Commission International de L'Éclairage coordinates of this device were (x = 0.15, y = 0.09) at 6.0 V. Copyright © 2015 John Wiley & Sons, Ltd.     

Molecular design and optimization of hepatic cancer SLP76‐derived PLCγ1 SH3‐binding peptide with the systematic N‐substitution of peptide PXXP motif

The phospholipase Cγ1 (PLCγ1) is essential for T‐cell signaling and activation in hepatic cancer immune response, which has a regulatory Src homology 3 (SH3) domain that can specifically recognize and interact with the PXXP‐containing decapeptide segment (¹⁸⁵QP P VP P QRPM¹⁹⁴, termed as SLP76^185–194 peptide) of adaptor protein SLP76 following T‐cell receptor ligation. The isolated peptide can only bind to the PLCγ1 SH3 domain with a moderate affinity due to lack of protein context support. Instead of the traditional natural residue mutagenesis that is limited by low structural diversity and shifted target specificity, we herein attempt to improve the peptide affinity by replacing the two key proline residues Pro187 and Pro190 of SLP76^185–194 PXXP motif with nonnatural N‐substituted amino acids, as the proline is the only endogenous N‐substituted amino acid. The replacement would increase peptide flexibility but can restore peptide activity by establishing additional interactions with the domain. Structural analysis reveals that the domain pocket can be divided into a large amphipathic region and a small negatively charged region; they accommodate hydrophobic, aromatic, polar, and moderate‐sized N‐substituted amino acid types. A systematic replacement combination profile between the peptide residues Pro187 and Pro190 is created by structural modeling, dynamics simulation, and energetics analysis, from which six improved and two reduced N‐substituted peptides as well as native SLP76^185–194 peptide are identified and tested for their binding affinity to the recombinant protein of the human PLCγ1 SH3 domain using fluorescence‐based assays. Two N‐substituted peptides, SLP76^185–194(N‐Leu187/N‐Gln190) and SLP76^185–194(N‐Thr187/N‐Gln190), are designed to have high potency (K_d = 0.67 ± 0.18 and 1.7 ± 0.3 μM, respectively), with affinity improvement by, respectively, 8.5‐fold and 3.4‐fold relative to native peptide (K_d = 5.7 ± 1.2 μM).     

Asymmetric Addition of Triethylaluminium to Aromatic Aldehydes Catalyzed by Titanium‐(5,5’‐Biquinoline‐6,6’‐Diol) Complexes

A novel convenient procedure for the resolution of 5,5’‐biquinoline‐6,6’‐diol (BIQOL) was achieved by separating the corresponding diastereomeric mixture of (S)‐(+)‐camphorsulfonates on a semiprepared XDB‐C8 column followed by hydrolysis. The efficient asymmetric addition of triethylaluminium to aromatic aldehydes catalyzed by Ti‐(+)/(–)BIQOL complexes under mild conditions is described. The reactions led to the formation of 1‐arylpropan‐1‐ol in up to 87.5% ee. Chirality 26:268‐271, 2014. © 2014 Wiley Periodicals, Inc.     

Application of NMR spectroscopy for assignment of the absolute configuration of 8‐tert‐butyl‐2‐hydroxy‐7‐methoxy‐8‐methyl‐9‐oxa‐6‐azaspiro[4.5]dec‐6‐en‐10‐one

In order to assign the absolute configurations of 8‐tert‐butyl‐2‐hydroxy‐7‐methoxy‐8‐methyl‐9‐oxa‐6‐azaspiro[4.5]dec‐6‐en‐10‐one ( 2a , 2b ), their esters ( 5a , 5b , 5c , 5d ) with (R)‐ or (S)‐2‐methoxyphenylacetic acid ( 4a , 4b ) have been synthesized. The absolute configurations of these compounds have been determined on the basis of NOESY correlations between the protons of the tert‐butyl group and the cyclopentane fragment of the molecules. The crucial part of this analysis was assignment of the absolute configuration at C‐5. Additionally, by calculation of the chemical shift anisotropy, δ^RS, for the relevant protons, it was also possible to confirm the absolute configurations at the C‐2 centres of compounds 2a , 2b and 5a , 5b , 5c , 5d . Chirality, 25:422–426, 2013.© 2013 Wiley Periodicals, Inc.     

Simultaneous determination of thermodynamic and kinetic parameters of aminopolycarbonate complexes of cobalt(II) and nickel(II) based on isothermal titration calorimetry data

The influence of the different side chain residues on the thermodynamic and kinetic parameters for complexation reactions of the Co²⁺ and Ni²⁺ ions has been investigated by using the isothermal titration calorimetry (ITC) technique supported by potentiometric titration data. The study was concerned with the 2 common tripodal aminocarboxylate ligands, namely, nitrilotriacetic acid and N‐(2‐hydroxyethyl) iminodiacetic acid. Calorimetric measurements (ITC) were run in the 2‐(N‐morpholino)ethanesulfonic acid hydrate (2‐(N‐morpholino) ethanesulfonic acid), piperazine‐N ,N ′‐bis(2‐ethanesulfonic acid), and dimethylarsenic acid buffers (0.1 mol L⁻¹, pH 6) at 298.15 K. The quantification of the metal‐buffer interactions and their incorporation into the ITC data analysis enabled to obtain the pH‐independent and buffer‐independent thermodynamic parameters (K , ΔG , ΔH , and ΔS ) for the reactions under study. Furthermore, the kinITC method was applied to obtain kinetic information on complexation reactions from the ITC data. Correlations, based on kinetic and thermodynamic data, between the kinetics of formation of Co²⁺ and Ni²⁺ complexes and their thermodynamic stabilities are discussed.     

Base Induced Condensation of Malononitrile with Erlenmeyer Azlactones: An Unexpected Synthesis of Multi‐Substituted Δ2‐Pyrrolines and Their Cytotoxicity

An efficient, metal free approach to synthesize multi‐substituted Δ²‐pyrroline derivatives by mild base catalyzed cyclocondensation of malononitrile with Erlenmeyer azlactones via 1,2 addition was developed. The modularity of this reaction was used to assemble a range of poly‐substituted pyrrolines. Further, synthesized products were screened for cytotoxic properties on different cancer cell lines such as A549 (Human lung adenocarcinoma cells), HeLa (Human cervical adenocarcinoma cells), Jurkat (Human chronic myeloid leukemia cells) and K562 (Human leukemic T cell Lymphoblast cells). Among the synthesized library of compounds, 6f and 6q displayed potent cytotoxic activity.     

CARBON SOURCE DEPENDENCE OF THE RATIO OF δ‐AMINOLEVULINIC ACID BIOSYNTHESIS VIA THE C5 AND SHEMIN PATHWAYS IN EUGLENA GRACILIS (EUGLENOPHYCEAE)1

The ratio of two biosynthetic pathways was estimated, the C5 and Shemin pathways, to δ‐aminolevulinic acid (ALA, a biosynthetic intermediate of tetrapyrrole) from the ¹³C‐enrichment ratios (¹³C‐ER) at the carbon atoms of chl a (after conversion to methyl pheophorbide a) biosynthesized by Euglena gracilis G. A. Klebs when l ‐[3‐¹³C]alanine was used as a carbon source. On the basis of these estimations, we confirmed that ALA was efficiently biosynthesized via both the C5 and Shemin pathways in the plastids of E. gracilis, and we determined that the ratio of ALA biosynthesis via the Shemin pathway was increased in the ratio of 14%–67%, compared with that in our previous d ‐[1‐¹³C]glucose feeding experiment ( Iida et al. 2002 ). This carbon source dependence of the contributions of the two biosynthetic pathways might be related to activation of gluconeogenesis by the amino acid substrate. The methoxy carbon of the methoxycarbonyl group at C‐13² of chl a was labeled with the ¹³C‐carbon of l ‐[methyl‐¹³C]methionine derived from l ‐[3‐¹³C]alanine via [2‐¹³C]acetyl coenzyme A (CoA), through the atypical tricarboxylic acid (TCA) cycle, gluconeogenesis, and l‐ [3‐¹³C]serine. The phytyl moiety of chl a was also labeled on C‐P2, C‐P3¹, C‐P4, C‐P6, C‐P7¹, C‐P8, C‐P10, C‐P11¹, C‐P12, C‐P14, C‐P15¹, and C‐P16 from ¹³C‐isoprene (2‐[1,2‐methyl,3‐¹³C₃]methyl‐1,3‐butadiene) generated from l ‐[3‐¹³C]alanine via [2‐¹³C]acetyl CoA.     

A novel cadmium(II) complex of bipyridine derivative: synthesis,X-ray crystal structure,DNA-binding and antibacterial activities

Abstract

A mononuclear cadmium(II) complex of formula [Cd(5,5′-dmbipy)₂(OAc)₂]·2H₂O (5,5′-dmbipy = 5,5′-dimethyl-2,2′-bipyridine and OAc?=?acetato ligand) has been synthesized and characterized by FT-IR, UV–Vis, ¹H-NMR, elemental analysis and single-crystal X-ray structure analysis. The molecular structure of the complex shows a distorted tetragonal antiprism CdN₄O₄ coordination geometry around the cadmium atom, resulting in coordination by four nitrogen atoms from two 5,5′-dmbipy ligands and four oxygen atoms from two acetate anions. The interaction of this complex to FS-DNA (fish sperm DNA) has also been studied by electronic absorption, fluorescence and gel electrophoresis techniques. Binding constant (K_b), Stern–Volmer constant (K_sv), number of binding sites (n) and bimolecular quenching rate constant (k_q) have been calculated from these spectroscopic data. These results have revealed that the metal complex can bind effectively to FS-DNA via groove binding. The calculated thermodynamic parameters (ΔH°, ΔS° and ΔG°) show that hydrogen bonding and van der Waals forces have an important function in the Cd(II) complex–DNA interaction. The antibacterial effects of the synthesized cadmium complex have also been examined in vitro against standard bacterial strains: one Gram-positive (Staphylococcus aureus, ATCC 25923) and one Gram-negative (Escherichia coli, ATCC 25922) bacteria, using disk diffusion and macro-dilution broth methods. The obtained results show that the Cd(II) complex exhibits a marked antibacterial activity which is significantly better than those observed for its free ligand and metal salt for both Gram-positive and Gram-negative bacteria. However, this metal complex is a more potent antibacterial agent against the Gram-positive than that of the Gram-negative bacteria.

Communicated by Ramaswamy H. Sarma     

Structural characterization of 2,6‐dichloro‐p‐hydroquinone 1,2‐dioxygenase (PcpA) from Sphingobium chlorophenolicum,a new type of aromatic ring‐cleavage enzyme

PcpA (2,6‐dichloro‐p‐hydroquinone 1,2‐dioxygenase) from Sphingobium chlorophenolicum, a non‐haem Fe(II) dioxygenase capable of cleaving the aromatic ring of p‐hydroquinone and its substituted variants, is a member of the recently discovered p‐hydroquinone 1,2‐dioxygenases. Here we report the 2.6 Å structure of PcpA, which consists of four βαβββ motifs, a hallmark of the vicinal oxygen chelate superfamily. The secondary co‐ordination sphere of the Fe(II) centre forms an extensive hydrogen‐bonding network with three solvent exposed residues, linking the catalytic Fe(II) to solvent. A tight hydrophobic pocket provides p‐hydroquinones access to the Fe(II) centre. The p‐hydroxyl group is essential for the substrate‐binding, thus phenols and catechols, lacking a p‐hydroxyl group, do not bind to PcpA. Site‐directed mutagenesis and kinetic analysis confirm the critical catalytic role played by the highly conserved His10, Thr13, His226 and Arg259. Based on these results, we propose a general reaction mechanism for p‐hydroquinone 1,2‐dioxygenases.     

Six New Polyhydroxysteroidal Glycosides,Anthenosides S1 – S6, from the Starfish Anthenea sibogae

Six new polyhydroxysteroidal glycosides, anthenosides S1  –  S6 ( 1  –  6 ), along with a mixture of two previously known related glycosides, 7 and 8 , were isolated from the methanolic extract of the starfish Anthenea sibogae. The structures of 1  –  6 were established by NMR and HR‐ESI‐MS techniques as well as by chemical transformations. All new compounds have a 5α‐cholest‐8(14)‐ene‐3α,6β,7β,16α‐tetrahydroxysteroidal nucleus and differ from majority of starfish glycosides in positions of carbohydrate moieties at C(7) and C(16) ( 1  –  4 , 6 ) or only at C(16) ( 5 ). The 4‐O‐methyl‐β‐d ‐glucopyranose residue ( 2 ) and Δ²⁴‐cholestane side chain ( 3 ) have not been found earlier in the starfish steroidal glycosides. The mixture of 7 and 8 slightly inhibited the proliferation of human breast cancer T‐47D cells and decreased the colony size in the colony formation assay.     

Spectroscopic investigation of water‐soluble alloyed QDs with bovine serum albumin

We present here a systematic investigation on the interaction between a water‐soluble alloyed semiconductor quantum dot and bovine serum albumin using various spectroscopic techniques i.e. fluorescence quenching, resonance light scattering and synchronous fluorescence spectroscopy. The analysis of fluorescence spectrum and fluorescence intensity indicates that the intrinsic fluorescence of bovine serum albumin (BSA) gets quenched by both static and dynamic quenching mechanism. The Stern‐Volmer quenching constants, energy transfer efficiency parameters, binding parameters and corresponding thermodynamic parameters (ΔH⁰, ΔS⁰ and ΔG⁰) have been evaluated by using van 't Hoff equation at different temperatures. A positive entropy change with a positive enthalpy change was observed suggesting that the binding process was an entropy‐driven, endothermic process associated with the hydrophobic effect. The intermolecular distance (r) between donor (BSA) and acceptor (CdSeS/ZnS quantum dots) was estimated according to Förster's theory of non‐radiative energy transfer. The synchronous fluorescence spectra revealed a blue shift in the emission maxima of tryptophan which is indicative of increasing hydrophobicity. Negative ΔG⁰ values implied that the binding process was spontaneous. It was found that hydrophobic forces played a role in the quenching process. Copyright © 2016 John Wiley & Sons, Ltd.     

Antibacterial Δ1‐3‐Ketosteroids from the South China Sea Gorgonian Coral Subergorgia rubra

Three new Δ¹‐3‐ketosteroids characterized with a 9‐OH, subergosterones A–C ( 1 – 3 ), together with five known analogs 4 – 8 , were obtained from the gorgonian coral Subergorgia rubra collected from the South China Sea. The structures of 1 – 3 , including their absolute configurations, were determined by comprehensive spectroscopic methods and electronic circular dichroism (ECD) experiments. Compounds 2 and 3 exhibited inhibitory antibacterial activities against Bacillus cereus with MIC values of 1.56 μM .