bluuebunny/crossref_metadata_2025_split huggingface dataset:
Lasing From a Förster Transfer Fluorescent Dye Couple Dissolved in a Chiral Nematic Liquid Crystal
Authors: Chambers M. , Fox M. , Grell M. , Hill J. 
Published: December 2002

AbstractFörster exciton transfer from a ’sensitizer’ to an ’emitter’ dye allows for the decoupling of the excitation and emission processes in dye lasers. Here, we report on the lasing from a Förster transfer couple dissolved in an unconventional, mirrorless resonator; namely, a chiral nematic liquid crystal (CNLC). We found optically pumped lasing at a threshold as low as 2.1 J m–2. We also demonstrate simultaneous lasing at two widely separated wavelengths.
Non‐Covalent Chemistry on Surface‐Confined, Isolated Dendrimers
Authors: van Manen H.‐J. , Auletta T. , Dordi B. , Schönherr H. , Vancso G.J. , van Veggel F.C.J.M. , Reinhoudt D.N. 
Published: December 2002

AbstractA “bottom‐up” fabrication of nanometer‐scale dendritic structures via the growth of isolated, nanosized supramolecular structures on gold surfaces is presented. A novel dendritic wedge containing peripheral pyridines and a focal dialkylsulfide chain (9) was synthesized and its coordination chemistry in solution was tested with a second‐generation Fréchet dendron functionalized with a focal SCS PdII pincer moiety (13). Replacement of the labile acetonitrile in 13 by the pyridine groups of 9 converts dendritic wedge 9 into fourth‐generation metallodendron 9 · 134 in one step. Based on these results the spatially confined growth on a gold surface was studied. The dendritic molecules with reactive pyridine groups at the periphery (9) were first spatially isolated by inserting them into decanethiol self‐assembled monolayers (SAMs) on a gold surface. Subsequently, the peripheral pyridines were reacted via metal–ligand coordinative interactions by exposing the monolayer to a solution of dendritic wedges containing focal SCS PdII pincer moieties (13). The isolated molecules inserted into the original SAM in the first step were resolved by ex‐situ tapping‐mode atomic force microscopy (TM‐AFM). The subsequent PdII–pyridine coordination resulted in a significant increase in size of the individually resolved molecules. Reference experiments demonstrated that the increased size of the isolated dendrimers on the gold substrate was a result of specific metal–ligand interactions. The methodology presented herein allows the creation of surface‐confined architectures and potentially provides a viable complement to currently employed “top‐down” methods in nanofabrication.
Fine‐Tuning of the Chromaticity of the Emission Color of Organic–Inorganic Hybrids Co‐Doped with EuIII, TbIII, and TmIII
Authors: Carlos L.D. , Sá Ferreira R.A. , Rainho J.P. , de Zea Bermudez V. 
Published: December 2002

AbstractA fine‐tuning of the emission chromaticity from red to green is reported for organic–inorganic nanostructured hybrids co‐doped with Eu3+, Tb3+, and Tm3+. Raising the temperature from 200 to 300 K changes the color coordinates smoothly from the yellowish–green region of the chromaticity diagram towards the red. This is discussed in terms of thermally activated energy‐transfer mechanisms from Tb3+ energy states to the hybrid’s emitting levels.
Synthesis of Symmetrical 1, 5‐Bisacyloxyanthraquinone Derivatives and Their Dual Activity of Cytotoxicity and Lipid Peroxidation
Authors: Huang Hsu‐Shan , Chiu Hui‐Fen , Chiou Jeng‐Fong , Yeh Pen‐Fong , Tao Chi‐Wei , Jeng Wei‐Ren 
Published: December 2002

AbstractSymmetrical bis‐substituted anthraquinones were successfully prepared and demonstrated potent cytotoxicity against the growth of suspended murine and human tumors, i.e. rat glioma C6 cells and human hepatomaG2 cells.We report here a convenient synthetic pathway that leads to symmetrically substituted 1, 5‐bisacyloxyanthraquinone derivatives. Acylation of the hydroxyl group of 1, 5‐dihydroxyanthraquinone with the appropriate acyl chlorides in the presence of pyridine or sodium hydride, respectively, furnished this structural class of anthraquinones. The bis(butyryloxy) analog 2b, bis(2‐chlorobenzoyl) analog 2f, and bisphenylpropionyloxy analog 2n exhibit potent cytotoxicity in inhibition of human hep G2 cell growth in culture, as determined by using XTT colorimetric assay, while their antiproliferative activity is markedly enhanced and is comparable to that of the anticancer agentmitoxantrone. In addition, redox properties of the compounds for the inhibition of lipid peroxidation in model membraneswere determined.Compounds 2n also exhibited stronger antioxidant activity than ascorbic acid, (+)‐α‐tocopherol, and anthrarufin. Biological evaluation and SAR studies of these symmetrical anthraquinones have been performed and the results are discussed.
Crown Ethers as Ancillary Ligands in the Assembly of Silver(I) Aggregates Containing Embedded Acetylenediide
Authors: Wang Quan‐Ming , Mak Thomas C. W. 
Published: December 2002

AbstractFive silver(I) double salts containing embedded acetylenediide, [Ag([12]crown‐4)2][Ag10(C2)(CF3CO2)9([12]crown‐4)2(H2O)3]⋅H2O (2), [Ag2C2⋅5 AgCF3CO2⋅(benzo[15]crown‐5)⋅2 H2O]⋅0.5 H2O (3), [Ag4([18]crown‐6)4(H2O)3][Ag18(C2)3(CF3CO2)16(H2O)2.5]⋅2.5 H2O (4), [Ag2C2⋅6 AgC2F5CO2⋅2([15]crown‐5)]2 (5), and [(Ag2C2)2⋅(AgC2F5CO2)9⋅([18]crown‐6)2⋅(H2O)3.5]⋅H2O (6), have been isolated by varying the types of crown ethers and anions employed. Single‐crystal X‐ray analysis has shown that complex 2 is composed of winding anionic chains with sandwiched [Ag([12]crown‐4)2]+ ions accommodated in the concave cavities between them. In 3, silver(I) double cages each sandwiched by a couple of benzo[15]crown‐5 ligands are linked by {Ag2(CF3CO2)2} bridges to form a one‐dimensional structure. For 4, an anionic silver column is generated through fusion of two kinds of silver polyhedra (triangulated dodecahedron and bicapped trigonal antiprism), and the charge balance is provided by aqua‐ligated [Ag([18]crown‐6)]+ ions. Complex 5 is a centrosymmetric hexadecanuclear supermolecule composed of two [(η5‐[15]crown‐5)2(C2@Ag7)(μ‐C2F5CO2)5] moieties connected through a {Ag2(C2F5CO2)2} bridge. Compound 6 is a discrete supermolecule containing an asymmetric (C2)2@Ag13 cluster core capped by two [18]crown‐6 ligands in μ3‐η5 and μ4‐η6 ligation modes.
A Fluorescent Sensor for 2,3‐Bisphosphoglycerate Using a Europium Tetra‐N‐oxide Bis‐bipyridine Complex for Both Binding and Signaling Purposes
Authors: Best Michael D. , Anslyn Eric V. 
Published: December 2002

AbstractHost 1 was designed and synthesized as a fluorescent sensor for 2,3‐bisphosphoglycerate (BPG, 3). The design features a tris‐functionalized triethylbenzene core to preorganize binding groups. The three cationic moieties, a tetra‐N‐oxide bipyridine–europium complex and two ammonium groups, were included to complement the three anionic functionalities on the guest. Beyond acting as a binding site, the europium complex was used to signal binding of the guest through modification of the charge transfer emission. A 1:1 complex with BPG was determined in 50 % methanol/acetonitrile with a Ka of 6.7×105 mol−1 by monitoring the reduction of the fluorescence signal upon guest addition. In the titration of related glycolytic intermediates lacking a second phosphate (4–6) into host 1, 2:1 host to guest binding was observed. Similarly, control compound 2, which lacks the ammonium groups, binds BPG and 4–6 in a 2:1 fashion. Also, phenylphosphate 7 binds to host 1 in a 1:1 stoichiometry with a Ka over three times less than 3.
Cover Picture: Chem. Eur. J. 1/2003
Authors: Orgueira Hernán A. , Bartolozzi Alessandra , Schell Peter , Litjens Remy E. J. N. , Palmacci Emma R. , Seeberger Peter H. 
Published: December 2002

AbstractThe cover picture shows schematically a general, modular strategy for the first completely stereoselective synthesis of defined heparin oligosaccharides. Heparin is currently used as an anticoagulant to prevent the formation of thrombi. A developing thrombus is depicted as background. Di‐ and trisaccharide modules were derived from six monosaccharide building blocks in a fully selective fashion by placing a conformational constraint on uronic acid glycosyl acceptors. A new concept for stereochemical control of α‐glucosamine glycoside formation was utilized. Locking the conformation of the glucuronic acid acceptor allowed for completely selective preparation of the desired cis‐glycosides. This innovation greatly simplified the key step in the preparation of disaccharide building blocks. Combination of disaccharide modules to form trans‐uronic acid linkages proved also completely selective by virtue of C2 participating groups. Coupling reactions between disaccharide modules exhibited sequence dependence. Elaboration of disaccharide building blocks to trisaccharide modules allowed for stereoselective access to oligosaccharides as demonstrated on the example of a hexasaccharide. Final deprotection and sulfation yielded the fully synthetic heparin oligosaccharides. A detailed description of this study is reported by P. H. Seeberger et al. on p. 140 ff.
Mechanism of Dioxygen Cleavage in Tetrahydrobiopterin‐Dependent Amino Acid Hydroxylases
Authors: Bassan Arianna , Blomberg Margareta R. A. , Siegbahn Per E. M. 
Published: December 2002

AbstractThe reaction mechanism for the formation of the hydroxylating intermediate in aromatic amino acid hydroxylases (i.e., phenylalanine hydroxylase, tyrosine hydroxylase, tryptophan hydroxylase) was investigated by means of hybrid density functional theory. These enzymes use molecular oxygen to hydroxylate both the tetrahydrobiopterin cofactor and the aromatic amino acid. A mechanism is proposed in which dioxygen forms a bridging bond between the cofactor and iron. The product is an iron(II)–peroxy–pterin intermediate, and iron was found to be essential for the catalysis of this step. No stable intermediates involving a pterin radical cation and a superoxide ion O2− were found on the reaction pathway. Heterolysis of the OO bond in the iron(II)–peroxy–pterin intermediate is promoted by one of the water molecules coordinated to iron and releases hydroxypterin and the high‐valent iron oxo species FeIVO, which can carry out subsequent hydroxylation of aromatic rings. In the proposed mechanism, the formation of the bridging CO bond is rate‐limiting in the formation of FeIVO.
Guest Binding and Orientation within Open Nanoscale Hosts
Authors: Laughrey Zachary R. , Gibb Corinne L. D. , Senechal Tangi , Gibb Bruce C. 
Published: December 2002

AbstractThe synthesis of three different nanoscale molecular hosts is reported. These cavitands each possess a highly preorganized cavity with an open portal (nearly 1 nm wide), by which guests can enter and egress the cavity. Additionally, these hosts are deep‐functionalized with a crown of weakly acidic benzal CH groups which can form a variety of noncovalent interactions with guest molecules residing within the cavity. Thirty‐one guests were examined for their propensity to form complexes with the hosts. Guests that possess halogen atoms were the strongest binders, suggesting the formation of polydentate CH⋅⋅⋅XR hydrogen bonds with the deep crown of benzal hydrogens. Exchange rates between the free and bound states were noted to be dependent on the size of the guest and the solvent used to study complexation. In general, stronger binding and slower exchange were noted for complexations carried out in DMSO with highly complementary guests. The orientation of each guest within the cavity was determined using either EXSY NMR spectroscopy or 1H NMR shift data. Cumulatively these results showed that the principal factors directing orientation were interactions with the benzal groups and the type of solvent. Van't Hoff analyses of selected complexations were also carried out. As well as revealing that all complexations were entropically unfavorable, these experiments provided support for guest orientation determinations, and gave an estimation that the formation of a CH⋅⋅⋅IR hydrogen bond releases between 1 and 1.5 kcal mol−1.
Mono‐ and Binuclear Ruthenium Corroles: Synthesis, Spectroscopy, Electrochemistry, and Structural Characterization
Authors: Simkhovich Liliya , Luobeznova Inna , Goldberg Israel , Gross Zeev 
Published: December 2002

AbstractThe aim of this research was to prepare mononuclear ruthenium corroles, because of the well‐documented potency of analogous porphyrin complexes in catalysis. The syntheses of the mononuclear nitrosyl complexes [Ru(tpfc)(NO)] and [Ru(tdcc)(NO)] (tpfc=trianion of 5,10,15‐tris(pentafluorophenyl)corrole, tdcc=trianion of 5,10,15‐tris(2,6‐dichlorophenyl)corrole), and of the binuclear [{Ru(tpfc)}2] were achieved by using [{Ru(cod)Cl2}x] (cod=cyclooctadiene) as the metal source. The NMR spectra of all three complexes clearly demonstrate that they are diamagnetic; this is consistent with a triple bond between the metal ions in [{Ru(tpfc)}2] and is expected for classical {MNO}6 complexes. These features were further substantiated by the stretching frequencies of the {MNO} moieties, electrochemical measurements on all complexes, and the X‐ray crystal structures of [Ru(tpfc)(NO)] and [{Ru(tpfc)}2]. A comparison of the spectroscopic and structural characteristics of these new complexes with analogous iron corroles, as well as with iron and ruthenium porphyrins, suggests that it will be hard to obtain mononuclear ruthenium corroles without π‐accepting ligands.
Voltage‐Dependent Formation of Anion Channels by Synthetic Rigid‐Rod Push–Pull β‐Barrels
Authors: Sakai Naomi , Houdebert David , Matile Stefan 
Published: December 2002

AbstractIon channels formed by p‐octiphenyls equipped with amphiphilic, cationic tripeptide strands and either with (5) or without (6) axial dipole moment are described (preliminary communication: N. Sakai, S. Matile, J. Am. Chem. Soc. 2002, 124, 1184–1185). Fluorescence kinetics with variably polarized neutral or anionic vesicles, together with planar bilayer conductance measurements, reveal voltage dependence with weakly lyotropic anion selectivity, and deactivation by competing surface potentials of the ion channels formed by asymmetric 5. In planar bilayers, 5 forms short‐lived, poorly organized channels—similar to those produced by α‐helical natural antibiotics—capable of transforming into stable, ohmic p‐octiphenyl “β‐barrel” ion channels similar to those of the >99 % homologous but symmetric 6. Fluorescence depth quenching and circular dichroism studies confirm the effect of membrane potentials in promotion of the partitioning of 5 (but not 6) into the bilayers, identifying partitioning as the voltage‐dependent step.
Enantioselective Synthesis of Paraconic Acids
Authors: Chhor Rakeshwar B. , Nosse Bernd , Sörgel Sebastian , Böhm Claudius , Seitz Michael , Reiser Oliver 
Published: December 2002

AbstractThe development of a new method for the enantioselective synthesis of disubstituted γ‐butyrolactones is reported. Based on this strategy, the total synthesis of three paraconic acids, that is (−)‐roccellaric acid, (−)‐nephrosteranic acid and (−)‐protopraesorediosic acid, and the formal total synthesis of (−)‐methylenolactocin and (−)‐protolichesterinic acid is described, which are important because of their antibiotic and antitumor properties. Key steps of the synthesis are copper(I)‐catalyzed asymmetric cyclopropanations of furans, highly diastereoselective Sakurai allylations, Lewis acid or Lewis base catalyzed retroaldol/lactonization cascades, and ruthenium(II)‐catalyzed, intermolecular cross metathesis reactions.
Studies on Mimicry of Naturally Occurring Annonaceous Acetogenins: Non‐THF Analogues Leading to Remarkable Selective Cytotoxicity against Human Tumor Cells
Authors: Zeng Bu‐Bing , Wu Yikang , Jiang Sheng , Yu Qian , Yao Zhu‐Jun , Liu Zhong‐Hai , Li Hong‐Yan , Li Yan , Chen Xiao‐Guang , Wu Yu‐Lin 
Published: December 2002

AbstractA class of structurally simplified analogues of the naturally occurring annonaceous acetogenins were developed, amongst which some non‐THF analogues showed remarkable cytotoxicities against tumor cell lines, as well as good selectivity between human tumor cells and normal cells. The synthetic routes were significantly shortened because of the removal of the chiral centers bearing the THF rings on the natural templates. This simplification also provides access to the parallel synthesis of these mimics by a combinatorial strategy. The remaining stereogenic centers at the positions α to the ethereal links were introduced by the Chiron approach from the easily accessible chiral building blocks 6 a and/or 6 b, made in turn from L‐ascorbic acid or D‐mannitol, while the one in the butenolide segment was taken from L‐lactate. All four diastereomeric non‐THF analogues 2 a–2 d showed remarkable activity against the HCT‐8 cell line, and better differentiation was found when testing against the HT‐29 cell line. It was also discovered that both the butenolide and ethylene glycol subunits play essential roles in the cytotoxicities against tumor cell lines, while the 10‐substituted hydroxy group and the absolute configuration of methyl group at the butenolide moiety are less important for their activity.
Functional Supramolecular Devices: [M4IIL4]8+ [2×2]‐Grid‐Type Complexes as Multilevel Molecular Electronic Species
Authors: Ruben Mario , Breuning Esther , Barboiu Mihail , Gisselbrecht Jean‐Paul , Lehn Jean‐Marie 
Published: December 2002

AbstractThe [M4IIL4]8+ [2×2]‐grid‐type complexes 1–8 present a set of features of particular interest for potential applications. All complexes exhibit multiple reduction levels at low reduction potentials paired with rather high stability. The modulation of the reduction potentials is possible by introduction of appropriate substituents on the ligands. The CoII4 complexes 1–5 present a remarkable regularity in the disposition of the reduction levels, indicating the ability of the CoII sites to transmit electronic interactions between reduced ligands. In general, all investigated molecular systems 1–8 show characteristics typical for multilevel supramolecular electronic devices.
Accurate Quantum‐Chemical Prediction of Enthalpies of Formation of Small Molecules in the Gas Phase
Authors: Klopper Wim , Noga Jozef 
Published: December 2002

AbstractThe coupled‐cluster approach, including single and double excitations and perturbative corrections for triple excitations, is capable of predicting molecular electronic energies and enthalpies of formation of small molecules in the gas phase with very high accuracy (specifically, with error bars less than 5 kJ mol−1), provided that the electronic wavefunction is dominated by the Hartree–Fock configuration. This capability is illustrated by calculations on molecules containing O–H and O–F bonds, namely OH, FO, H2O, HOF, and F2O. To achieve this very high accuracy, it is imperative to account for electron‐correlation effects in a quantitative manner, either by using explicitly correlated two‐particle basis functions (R12 functions) or by extrapolating to the limit of a complete basis. Besides taking into account harmonic zero‐point vibrational energies, it is also necessary to account for anharmonic corrections to the zero‐point vibrational energies, to include the core orbitals into the coupled‐cluster calculations, and to account for spin–orbit corrections and scalar relativistic effects. These additional corrections constitute small but significant contributions in the range of 1–4 kJ mol−1 to the enthalpies of formation of the aforementioned molecules. The highly accurate coupled‐cluster results, obtained by employing R12 functions and by including various corrections, are compared with standard Kohn–Sham density‐functional calculations as well as with the Gaussian‐2 and complete‐basis‐set model chemistries.
Porphyrin‐Containing Glycodendrimers
Authors: Ballardini Roberto , Colonna Barbara , Gandolfi M. T. , Kalovidouris Stacey A. , Orzel Lukasz , Raymo Françisco M. , Stoddart J. Fraser 
Published: December 2002

AbstractTwo dendrimers, incorporating tetrasubstituted porphyrin units as their cores and, in one case, four perbenzoylated and, in the other case, twelve peracetylated β‐D‐glucopyranosyl residues at their peripheries, have been synthesized in yields of 39 and 16%, respectively. The deprotection of these dendrimers was achieved quantitatively under Zemplén conditions. The protected and deprotected dendrimers were characterized by liquid secondary‐ion or matrix‐assisted laser desorption ionization time‐of‐flight mass spectrometry and by a combination of one‐ and two‐dimensional 1H and 13C NMR spectroscopy. All the glycodendrimers were also characterized by absorption and emission spectroscopy, and lifetime measurements. The most relevant result is that both protected and deprotected dendrimers show two fluorescence lifetime values that are different from the porphyrin model compounds. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)
Stereoselective Hydrolysis of Quaternary Quinuclidinium Benzoates Catalyzed by Butyrylcholinesterase
Authors: Primoz̆ič Ines , Hrenar Tomica , Tomić Srđanka , Meić Zlatko 
Published: December 2002

AbstractFour chiral, quaternary, N‐methyl and N‐benzyl derivatives of (R)‐ and (S)‐quinuclidin‐3‐yl benzoates were synthesized and studied as substrates of horse serum butyrylcholinesterase (BChE). The kcat for the substrates decreased in the order (R)‐N‐methyl > (R)‐N‐benzyl (2.3‐fold slower) >> (S)‐N‐methyl (70.5‐fold slower reaction), while for the (S)‐N‐benzyl ester inhibition of the enzyme was observed. The kinetics of inhibition (Ka = 3.3 μM) indicated that binding to the catalytic site of BChE occurred. From the ratio of the kcat/KM values of both enantiomers an enantiomeric excess of 95% was calculated for N‐methyl derivatives. Thus, BChE is suitable as a biocatalyst for the resolution of racemic quaternary quinuclidinium esters. In order to explain the experimental data, combined quantum chemical (HF/3−21G*) and semiempirical (PM3) calculations within the ONIOM scheme of the stable species in the acylation step were performed. Geometry optimizations were carried out for all benzoate esters for an assumed active site model of BChE. It was confirmed that hydrolysis is affected to an appreciable extent by a proper geometrical orientation of substrates at the choline subsite. The energies of the optimized systems were in good agreement with the experimental data. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)
A DFT Study on the Interaction of Li+ and Na+ with Alkyl‐Substituted Ethenes
Authors: Froudakis George E. , Stratakis Manolis 
Published: December 2002

AbstractThe interaction of Li+ and Na+ with several alkyl‐substituted ethenes was calculated by the DFT method. Two trends were found. The first is that, for the majority of alkenes, the cations do not bind on top of the double bond, but close to it, either towards the more‐ or the less‐substituted side of the alkene. This type of interaction is found even with highly symmetrical alkenes such as tetramethylethene. The second trend is that there is a relatively strong interaction between both cations and the alkyl chains of the double bond at the homoallylic position. These calculations were correlated to regioselectivity results in the dye‐sensitized photooxygenation of alkenes within alkali‐metal exchanged zeolite Y. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)