Role of steric effects in protein-directed enediyne cycloaromatization of neocarzinostatin

The antibiotic neocarzinostatin comprises a carrier protein with a well-defined cavity for accommodating an active enediyne chromophore. The protein has two disulfides, one (Cys(37)-Cys(47)) lies on the cavity bottom and the other (Cys(88)-Cys(93)) in a constrained short loop. When the chromophore is not bound to the protein, a thiol-induced cycloaromatization of the enediyne into a tetrahydroindacene derivative is responsible for the potent antitumor activity. When it is protein-bound, the protein diverts the cycloaromatization pathway to form a distinct hydroxyisochromene-type product. How the protein directs the enediyne chemistry is an interesting puzzle, and various suggestions have been proposed in the past. We screened more than fifty thiols and manipulated conditions to locate reaction features and search for factors that could influence the protein directing strength. Thiol- and oxygen-concentration-dependence studies suggested that disulfides, which maintain the steric rigidity of the protein, could play a key role in diverting the cycloaromatization pathway. For direct proofs, we made mutations at each of the two disulfides by replacing sulfur atoms with oxygen. Circular dichroism and two-dimensional NMR spectroscopy studies suggested that the mutations changed neither the protein conformation nor the ligand interactions. Analyses of the thiol-induced cycloaromatization revealed that rupture of Cys(37)-Cys(47) made the protein almost completely lose its chemical directing ability, whereas rupture of Cys(88)-Cys(93) had only a minor influence. The results demonstrated that the steric rigidity of the binding cavity, but not necessary the whole protein, played an important role in the protein-directed mechanism.     

Layered hydrogel of poly(γ-glutamic acid), sodium alginate, and chitosan: fluorescence observation of structure and cytocompatibility

In this study, a novel layered hydrogel composing of poly (γ-glutamic acid) (PGA), chitosan (CS), and alginate (AL) were prepared. Furthermore, PGA, CS, and AL were labeled with different fluorescent dyes. The bilayer structure of hydrogel was then revealed using these fluorescent labeled polymers. To mimic the stability of these hydrogels in physiological fluids, the dissolution of PGA and the release of Ca2+ from these hydrogels in normal saline were also monitored. The results showed that by adding CS to the hydrogel, the dissolution of PGA was decreased by 67%, and the release of Ca2+ was reduced by 40%. In addition, the hydrogel exhibited no cytotoxicity for L929 fibroblasts.     

Synthesis and Application of Epoxidized Natural Rubber

Abstract

North Vietnamese NR latexes were successfully epoxidized using peracetic acid at moderate temperature and pH range. The epoxide contents of pure epoxidized natural rubber (ENR) are from 5 to 70 mol%. The ENR products were characterized and determined by spectral and thermal analysis besides the chemical titration. Conditions of longlasting or excessive temperature, or high acidic pH led to side ring opened products, proved easily by IR, ¹H-NMR, and DSC analysis. The ENR were vulcanized using a semiefficient system. The epoxidation increased the adhesion between rubber and tire cord and metal. This effect becomes stronger beyond 25 mol% and tends to be limited at over 60 mol%. The ENRs were used to formulate special-purpose adhesives. The shear strength of the adhesive ranges from 32 to 45 kg/cm² for bonding rubber to nylon and rubber to metal, respectively.     

Diketopyrrolopyrrole?Thiophene‐Based Acceptor–Donor–Acceptor Conjugated Materials for High‐Performance Field‐Effect Transistors

We report the synthesis, morphology, and field‐effect‐transistor (FET) characteristics of new acceptor–donor–acceptor conjugated materials that consist of diketopyrrolopyrrole (DPP) acceptor groups and one of four different thiophene moieties, that is, dithiophene (2T), thieno[3,2‐b]‐thiophene (TT), dithieno[3,2‐b:2′,3′‐d]‐thiophene (DTT), and 5,5′′′‐di‐(2‐ethylhexyl)‐[2,3′;5′,2′′;4′′,2′′′]quaterthiophene (4T). The optical band gaps of the as‐prepared materials are smaller than 1.7 eV, which is attributed to the strong intramolecular charge transfer and the backbone coplanarity of the thiophene moieties. The order of both crystallinity and FET mobility (×10^?2–×10^?4 cm² V^?1 s^?1) is TT2DPP > 4T2DPP > 2T2DPP >DTT2DP, which differ in the structure of the π‐conjugated cores and core symmetry. Well‐ordered intermolecular chain packing was confirmed by the GIXD and AFM results. In particular, the FET hole mobility of TT2DPP was further improved to 0.1 cm² V^?1 s^?1, which was attributed to the well‐interconnected structure through solution‐shearing. These experimental results suggest the potential applications of the new DPP? thiophene? DPP conjugated materials for organic electronic devices.     

Zirconium complexes containing a diarylamido diphosphine ligand: Structural preferences controlled by ligand electronics and sterics

This work describes the preparation of [PNP]ZrX₃ ([PNP]⁻ = [N(o-C₆H₄PⁱPr₂)₂]⁻; X = Cl, Me, CH₂SiMe₃) whose structural preference is found to be a function of the electronic and steric nature of the monodentate ligand X⁻. The reaction of ZrCl₄(THF)₂ with [PNP]Li in toluene at room temperature generates [PNP]ZrCl₃ as a red solid in 60% yield. Alkylation of [PNP]ZrCl₃ with three equivalents of Grignard reagents in diethyl ether at −35 °C produces cleanly [PNP]ZrR₃ (R = Me, CH₂SiMe₃) as yellow crystalline materials. An X-ray diffraction study of [PNP]ZrCl₃ showed it to be a chloride-bridged binuclear species {[PNP]ZrCl₂(μ−Cl)}₂ in which both zirconium atoms are 7-coordinate whereas that of [PNP]ZrMe₃ revealed a mononuclear, 6-coordinate core structure. Interestingly, with the incorporation of more sterically demanding alkyls, [PNP]Zr(CH₂SiMe₃)₃ is a 5-coordinate compound wherein the amido phosphine ligand is κ²-N,P bound to zirconium. The solution structures of these molecules were also assessed by variable-temperature NMR spectroscopy.     

Catalysis by Using TiO2 Nanoparticles and Nanotubes

TiO2 has attracted considerable attention due to its stability, non-toxicity, low cost, and great potential for use as a photocatalyst in environmental applications. Since strong metal-support interaction (SMSI) of titania-supported noble metals was first reported in 1978, titania supported catalyst has been intensively studied in heterogeneous catalysis. However, the effective catalytic activity was restricted due to the low surface area of TiO2. Recently, TiO2-based nanotubes were extensively investigated because of their potentials in many areas such as highly efficient photocatalysis and hydrogen sensor.In the present study, formation of titanium oxide (TiO2) nanotubes was carried out by hydrothermal method, with TiO2 nanoparticle-powders immersed in concentrated NaOH solution in an autoclave at 110 ℃. Preparation of nano-size Pt on TiO2-nanoparticles or TiO2-nanotubes was performed by photochemical deposition method with UV irradiation on an aqueous solution containing TiO2 and hexachloroplatinic acid or tetrachloroauric acid. The TEM micrographs show that TiO2-nanotubes exhibit ～300 nm in length with an inner diameter of ～ 6 nm and the wall thickness of ～ 2 nm, and homogeneous nanosize Pt particles (～ 2 nm) were well-dispersed on both nanoparticle- and nanotube- titania supports. It also shows the nanotube morphology was retained up2o n Pt-immobilization. Nitrogen adsorption isotherm at 77K resulted a high surface area (～ 200m/g) of TiO2-nanotubes, which is about 40 times greater than that of "mother" TiO2 nanoparticles (～5 m/g). All the spectroscopic results exhibited that the nanotube structure was not significantly affected by the immobilized Pt particles. Ti K-edge XANES spectra of TiO2 nanotube and Pt/TiO2-nanotube represent that most titanium are in a tetrahedral coordination with few retained in the octahedral structure.In the in-situ FT-IR experiments, an IR cell was evacuated to a pressure of 10-5 torr at room temperature as soon as the catalyst-pellet, Pt/TiO2 or Pt/TiO2-nanotube, was placed inside the cell.Then, 60 torr of hydrogen was introduced into the cell and subsequently the temperature was programmed to increase from room temperature to 300℃ at a constant heating rate of 5℃/min.For Pt/TiO2, an IR peak at 2083 em-1 started to appear at 200℃ with a maximum intensity at 250℃ and then decreasing as temperature increased. The 2083 em-1 IR peak corresponds to the linearly adsorption of CO on the well-dispersed Pt sites. Simultaneously, the IR bands of gaseous methane at 3016 em-1 started to appear at 225℃ and the peak intensity increased with temperature. The results reveal that Pt/TiO2 can adsorb gaseous CO2 and further catalyzes the reduction of CO2 by H2 through the intermediate CO, which further produces gaseous methane. While for the Pt/TiO2-nanotube catalyst, methane was produced at relatively low temperature, 100℃, and it catalyzed the direct conversion of CO2 to CH4. The absence of intermediate CO-adsorption signals durinng the temperature programmed process indicates that the prepared TiO2 nanotube-supported nanosize Pt possesses a potent capability for CO2 adsorption and highly catalytic activity in the hydrogenation of CO2, and was superior to the conventional Pt/TiO2 catalyst. The catalytic activity of Pt/TiO2-nanotube was indeed significantly enhanced by the high surface area of TiO2-nanotubes.Details will be discussed.     

H-Bonded metallomesogens: preparation, characterization, and mesomorphic studies of (3-hydroxypropylimino)propan-1,2-diols

Two series of new Schiff bases 2 (n=8, 12, 16) derived from (3-hydroxypropyl imino)propan-1, 2-diol with a hydroxyl group at C₁₉/C₂₀-position and their palladium complexes 1 were prepared and their mesomorphic properties investigated by DSC, POM, and XRD. The presence of both hydroxyl groups was found to be crucial in forming the liquid crystalline behavior. All compounds 2a exhibited smectic A or and C phases, in contrast, all compounds 2b formed hexagonal columnar phases. The formation of mesophases in both compounds 1-2 was probably induced by inter-molecular H-bonds. Single crystallographic data in mesogenic compound 2a (n=8) indicated that a dimeric structure with a better linear or rod-like molecular shape was formed by an inter-molecular H-bond (O4-O1′, ∼1.854 Å). Another inter-molecular H-bond (∼1.903 Å) between two dimeric structures was also observed. It crystallizes in a monoclinic space group P2(1)/c. On the other hand, all palladium complexes 1 formed enantiotropic smectic A phases. Single crystallographic data in mesogenic compound 1a (n=8) indicated that the geometry at Pd²⁺ center was coordinated as slightly twisted square planar. It crystallizes in a monoclinic space group P2(1)/n. An inter-molecular H-bond (∼1.799 Å) between neighboring molecules were observed, which might have facilitated the formation of mesophases. Variable-temperature powder XRD experiments confirmed their mesophase structures.     

Synthesis of N‐Arylated 1,2‐Dihydroheteroaromatics Through the Three‐Component Reaction of Arynes with N‐Heteroaromatics and Terminal Alkynes or Ketones

The reaction of benzynes with N‐heteroaromatics including quinolines, isoquinolines, and pyridines and various terminal alkynes or ketones with an α‐hydrogen in the presence of KF and 18‐crown‐6 in THF at room temperature for 8 h gave various N‐arylated 1,2‐dihydroheteroaromatics in good to moderate yields. Some of these product structures are found in various naturally occurring and biologically active heterocyclic compounds. The reaction involves an unusual multiple construction of new C? C, C? N, and C? H bonds and the cleavage of a C? H bond in one pot. It is likely that the three‐component coupling proceeds through the nucleophilic addition of quinoline to benzyne, which generates a zwitterionic species. The latter then attracts a proton from terminal alkyne (or ketone) to generate an N‐arylated quinolinium cation and an acetylide anion. Further reaction of these two ions provides the final substituted 1,2‐dihydroquinolines. In the reaction, the terminal alkyne acts first as a proton donor and then as a nucleophile. The application of a three‐component coupling reaction product, 1,2‐dihydro‐2‐pyridinyl alkyne in a stereospecific [4+2] Diels–Alder cycloaddition reaction with N‐phenyl maleimide to give an isoquinuclidine derivative, an important core present in various natural products, is demonstrated.     

Phytoquinoids and Secoprezizaane‐Type Sesquiterpenes from Illicium arborescens

A phytochemical investigation of the MeOH extract of Illicium arborescens yielded the two new phytoquinoid epimers, 2,3‐didehydro‐5‐O‐methyl‐11‐epiillifunone E ( 1 ) and 2,3‐didehydro‐5‐O‐methylillifunone E ( 2 ), as well as five new sesquiterpene lactones (8,9‐secoprezizaane‐type sesquiterpenes). Two of them, i.e., 3 and 4 , were minwanensin‐type sesquiterpenes, the other two, i.e., 5 and 6 , had the anisatin‐type (or floridanolide type) skeleton, and the fifth, i.e., 7 , was a dunnianin‐type sesquiterpene. Their structures were established by analyses of 1D‐ and 2D‐NMR, HR‐MS, and chemical evidence. The in vitro cytotoxic activity of compounds 1 – 7 was tested against four human tumor cell lines, including HeLa (cervical epitheloid), WiDr (colon), Daoy (medulloblastoma), and Hep2 (liver carcinoma) human‐tumor cells.     

Bipyridine‐Modulated Palladium‐Catalyzed Oxidative Heck‐Type Reactions of Arylboronic Acids with Olefins

We have demonstrated that 4,4′‐dimethyl 2,2′‐bipyridine as ligand for Pd(II) catalysts was very efficient for oxidative Heck‐type coupling reaction of arylboronic acids with olefins in DMA or CH₃CN under atm air at 80 °C. The presence of chelated bipyridine ligand isindispensable to achieve high reaction yields and to suppress the formation of biphenyl as homocoupled byproduct.