bluuebunny/crossref_metadata_2025_split huggingface dataset:
A Novel Extended Covalent Tripod for Assembling Nine‐Coordinate Lanthanide(III) Podates: A Delicate Balance between Flexibility and Rigidity
Authors: Koeller Sylvain , Bernardinelli Gérald , Bocquet Bernard , Piguet Claude 
Published: February 2003

AbstractThe introduction of long semirigid spacers between the capping carbon atom of the tripod and the unsymmetrical tridentate binding units provides the novel, extended covalent podand tris‐{2‐[2‐(6‐diethylcarbamoylpyridin‐2‐yl)‐1‐ethyl‐1H‐benzoimidazol‐5‐yl‐methoxy]ethyl}methane (L15). Reaction of L15 with lanthanide(III) in acetonitrile produces stable podates [Ln(L15)]3+ (Ln=La–Lu) in which three tridentate binding units are facially organized. These wrap around the nine‐coordinate pseudo‐tricapped trigonal‐prismatic metal ions. The crystal structure of [La(L15)](ClO4)3 (18, LaC67H82N12O18Cl3, trigonal, R3c, Z=6) reveals the formation of a C3‐symmetrical triple‐helical podate. Two slightly different arrangements of the flexible ethylenoxy parts of the spacer are observed in the solid state in agreement with the formation of two conformational isomers (M:m) in a 4:1 ratio. A qualitative analysis of the aromatic diamagnetic anisotropies affecting the NMR signals of [Ln(L15)]3+ (Ln=La, Y, Lu) in solution, combined with the quantitative determination of electron‐induced relaxation in the paramagnetic complex [Nd(L15)]3+, demonstrate that the solid state structure is maintained in solution. This leads to a mixture of two triple‐helical conformers of similar stabilities and that do not interconvert on the NMR timescale between 243 and 343 K. Particular attention has been given to the structural programming of extended covalent tripods for facially organizing unsymmetrical tridentate binding units around LnIII. Photophysical measurements show that L15 efficiently protects the metallic coordination spheres and sensitizes EuIII and TbIII upon UV irradiation.
Experimental and Theoretical Charge Density Study of the Neurotransmitter Taurine
Authors: Hibbs David E. , Austin‐Woods Charles J. , Platts James A. , Overgaard Jacob , Turner Peter 
Published: February 2003

AbstractThe experimental electron density distribution in taurine, 2‐aminoethane sulfonic acid,1, has been determined from high‐resolution X‐ray diffraction data collected at a temperature of 100 K. Taurine crystallizes as a zwitterion in the monoclinic space groupP21/c.Topological analysis of the experimental electron density and a comparison with high‐level theoretical gas‐phase calculations show that the crystal environment has a significant influence on the electronic configuration of the sulfonate moiety in1, which in the crystal is more delocalized than in the gas phase. This crystal effect is mainly due to hydrogen bonding.
Synthesis of α‐(2→5)Neu5Gc Oligomers
Authors: Ren Chien‐Tai , Chen Chien‐Sheng , Yu Yi‐Ping , Tsai Yow‐Fu , Lin Ping‐Yu , Chen Yu‐Ju , Zou Wei , Wu Shih‐Hsiung 
Published: February 2003

AbstractA facile synthesis of the sialic acid oligomers α‐(2→5)Neu5Gc (1) is presented. Monosaccharides 2–4 with suitable functionality were used as the building blocks. After selective removal of the paired carboxyl and amine protecting groups, the fully protected oligomers were assembled through consecutive coupling of the building blocks by well established peptide coupling techniques. By this approach, fully protected oligomers as large as an octasaccharide were synthesized. Deprotection of these fully protected oligomers was conducted in two steps (LiCl in refluxing pyridine and 0.1 n NaOH) to afford the desired products in high yield. Enzymatic degradation of the octamer with neuraminidase, monitored by capillary electrophoresis (CE), was also accomplished. The stepwise exo‐cleavage adducts were all well separated and identified in the CE spectrum. The strategy described here for solution‐phase synthesis also provides the basis for future solid‐phase synthesis of poly‐α‐(2→5)Neu5Gc.
Cover Picture: Structural Regulation of a Peptide‐Conjugated Graft Copolymer: A Simple Model for Amyloid Formation (Chem. Eur. J. 5/2003)
Authors: Koga Tomoyuki , Taguchi Kazuhiro , Kobuke Yoshiaki , Kinoshita Takatoshi , Higuchi Masahiro 
Published: February 2003

AbstractThe cover picture shows the self‐organization process of the poly(γ‐methyl‐L‐glutamate)‐conjugated graft copolymer in aqueous media. The self‐assembling property and high‐order structure of the peptide derivative can be easily controlled by manipulating the environmental conditions (pH, solution composition, and salt concentration etc.). Only under the condition of low pH or that of low salt concentration did the electrostatic repulsive force based on the protonated amino groups permit the controlled growth of the peptide assembly and the tight packing of hydrophobic peptide chains, resulting in induction of highly‐ordered nanofibril formation with α‐to‐β structural transition. For details see the article by M. Higuchi et al. on p. 1146 ff.
Solid‐Phase Synthesis of the Cyclic Lipononadepsipeptide [N‐Mst(Ser1), D‐Ser4, L‐Thr6, L‐Asp8, L‐Thr9]Syringotoxin
Authors: Bayó Nuria , Jiménez Jose C. , Rivas Luis , Nicolás Ernesto , Albericio Fernando 
Published: February 2003

AbstractAn optimized solid‐phase strategy for the preparation of the cyclic lipononadepsipeptide [N‐Mst(L‐Ser1), D‐Ser4, L‐Thr6, L‐Asp8, L‐Thr9]syringotoxin is reported. The strategy is based on the use of a mild orthogonal protection scheme and the incorporation of the nonproteinogenic amino acid (Z)‐Dhb into the peptide chain as the dipeptide Fmoc‐Thr(tBu)‐(Z)‐Dhb‐OH. The didehydrodipeptide was synthesized by a water‐soluble carbodiimide‐induced β‐elimination of a protected dipeptide containing a residue of Thr with its free hydroxy side chain unprotected.
A New Cryptophane Receptor Featuring Three endo‐Carboxylic Acid Groups: Synthesis, Host Behavior and Structural Study
Authors: Roesky Christian E. O. , Weber Edwin , Rambusch Torsten , Stephan Holger , Gloe Karsten , Czugler Mátyás 
Published: February 2003

AbstractExamples of a new type of cryptophane molecule incorporating aromatic groups in the bridges (1–4) and, for the first time, being also supplied with three endo‐positional ionizable carboxylic acid functions (1) have been synthesized and characterized. The cryptophane triester 2 yielded a solvate (channel inclusion compound) with trichloromethane and water, the X‐ray crystal structure of which is reported. The complexation of 1 with low‐molecular‐weight alcohols in solution was studied, and the liquid–liquid extraction of different metal ions including alkali (Na+, Cs+), alkaline earth (Mg2+, Ca2+, Sr2+, Ba2+), and the lanthanide metal ions Eu3+ and Yb3+ in an extraction system containing metal nitrate buffer/H2O/1/CHCl3 was examined. Molecular modeling calculations of the cryptophanes 1 and 2, and of the Eu3+ complex of 1 were carried out contributing to the discussion.
An Insight into the Local Aromaticities of Polycyclic Aromatic Hydrocarbons and Fullerenes
Authors: Poater Jordi , Fradera Xavier , Duran Miquel , Solà Miquel 
Published: February 2003

AbstractIn this work we quantify the local aromaticity of six‐membered rings in a series of planar and bowl‐shaped polycyclic aromatic hydrocarbons (PAHs) and fullerenes. The evaluation of local aromaticity has been carried out through the use of structurally (HOMA) and magnetically (NICS) based measures, as well as by the use of a new electronically based indicator of aromaticity, theparadelocalization index (PDI), which is defined as the average of all the Bader delocalization indices betweenpara‐related carbon atoms in six‐membered rings. The series of PAHs selected includes C10H8, C12H8, C14H8, C20H10, C26H12, and C30H12, with benzene and C60taken as references. The change in the local aromaticity of the six‐membered rings on going from benzene to C60is analyzed. Finally, we also compare the aromaticity of C60with that of C70, open [5,6]‐ and closed [6,6]‐C60NH systems, and C60F18.
Instantaneous SmI2/H2O/Amine‐Mediated Reductions in THF
Authors: Dahlén Anders , Hilmersson Göran 
Published: February 2003

AbstractThe SmI2‐mediated reductions of ketones, imines, and α,β‐unsaturated esters have been shown to be instantaneous in the presence of H2O and an amine in THF. The SmI2‐mediated reductions are not only shown to be fast and quantitative by the addition of H2O and an amine, but the workup procedures are also simplified. Competing experiments with SmI2/H2O/amine confirmed that α,β‐unsaturated esters could be selectively reduced in the presence of ketones or imines. Comparison of analogue ligands showed that nitrogen and phosphorus ligands are superior to oxygen and sulfur ligands in these reductions. The trialkylphosphine 1,2‐bis(dimethylphosphino)ethane (DMPE) provided a primary kinetic isotope effect, yielding a kH/kD of 4.5.
The Total Synthesis of (−)‐Callystatin A
Authors: Kalesse Markus , Chary Khandavalli P. , Quitschalle Monika , Burzlaff Arne , Kasper Cornelia , Scheper Thomas 
Published: February 2003

AbstractCallystatin A is a prominent member of a class of natural products which display promising growth inhibition of cancer cells in their biological profile. The challenging structure and the interesting biological activity of (−)‐callystatin A fueled our interest in the synthesis of this marine natural product. We achieved the total synthesis using a highly convergent approach joining four subunits together with a Wittig olefination, a selective Heck reaction and an aldol reaction as the pivotal steps. The aldol reaction as one of the final transformations during the synthesis opens fast access to a variety of structural analogues and circumvents tedious protecting group manipulations. Here we report an improved synthesis utilizing a modified vinyl iodide which shortens the synthesis by two steps. Additionally, first biological results will be reported.
Highly Sensitive Amplified Electronic Detection of DNA By Biocatalyzed Precipitation of an Insoluble Product onto Electrodes
Authors: Patolsky Fernando , Lichtenstein Amir , Willner Itamar 
Published: February 2003

AbstractThe amplified detection of a target DNA, based on the alkaline phosphatase oxidative hydrolysis of the soluble 5‐bromo‐4‐chloro‐3‐indoyl phosphate to the insoluble indigo product as an amplification path, is addressed by two different sensing configurations. The accumulation of the insoluble product on Au electrodes or Au/quartz crystals alters the interfacial electron‐transfer resistance at the Au electrode or the mass associated with the piezoelectric crystal, thus enabling the quantitative transduction of the DNA sensing by Faradaic impedance spectroscopy or microgravimetric quartz crystal microbalance measurements, respectively. One sensing configuration involves the association of a complex consisting of the target DNA and a biotinylated oligonucleotide to the functionalized transducers. The binding of the avidin/alkaline phosphatase conjugate to the sensing interface followed by the biocatalyzed precipitation provides the amplification path for the analysis of the target DNA. This analysis scheme was used to sense the target DNA with a sensitivity limit that corresponds to 5×10−14 M. The second amplified detection scheme involves the use of a nucleic‐acid‐functionalized alkaline phosphatase as a biocatalytic conjugate for the precipitation of the insoluble product. Following this scheme, the functionalized transducers are interacted with the analyzed sample that was pretreated with the oligonucleotide‐modified alkaline phosphatase, followed by the biocatalyzed precipitation as the amplification route for the analysis of the target DNA. By the use of this configuration, a detection limit corresponding to 5×10−13 M was achieved. Real clinical samples of the Tay–Sachs genetic disorder were easily analyzed by the developed detection routes.
Structural Regulation of a Peptide‐Conjugated Graft Copolymer: A Simple Model for Amyloid Formation
Authors: Koga Tomoyuki , Taguchi Kazuhiro , Kobuke Yoshiaki , Kinoshita Takatoshi , Higuchi Masahiro 
Published: February 2003

AbstractThe self‐assembly of peptides and proteins into β‐sheet‐rich high‐order structures has attracted much attention as a result of the characteristic nanostructure of these assemblies and because of their association with neurodegenerative diseases. Here we report the structural and conformational properties of a peptide‐conjugated graft copolymer, poly(γ‐methyl‐L‐glutamate) grafted polyallylamine (1) in a water‐2,2,2‐trifluoroethanol solution as a simple model for amyloid formation. Atomic force microscopy revealed that the globular peptide 1 self‐assembles into nonbranching fibrils that are about 4 nm in height under certain conditions. These fibrils are rich in β‐sheets and, similar to authentic amyloid fibrils, bind the amyloidophilic dye Congo red. The secondary and quaternary structures of the peptide 1 can be controlled by manipulating the pH, solution composition, and salt concentration; this indicates that the three‐dimensional packing arrangement of peptide chains is the key factor for such fibril formation. Furthermore, the addition of carboxylic acid‐terminated poly(ethylene glycol), which interacts with both of amino groups of 1 and hydrophobic PMLG chains, was found to obviously inhibit the α‐to‐β structural transition for non‐assembled peptide 1 and to partially cause a β‐to‐α structural transition against the 1‐assembly in the β‐sheet form. These findings demonstrate that the amyloid fibril formation is not restricted to specific protein sequences but rather is a generic property of peptides. The ability to control the assembled structure of the peptide should provide useful information not only for understanding the amyloid fibril formation, but also for developing novel peptide‐based material with well‐defined nanostructures.
Complexation Behavior of a Highly Preorganized 7,7‐Diphenylnorbornane‐Derived Macrocycle: Towards the Design of Molecular Clocks
Authors: García Martínez Antonio , Osío Barcina José , del Rosario Colorado Heras María , de Fresno Cerezo Álvaro , del Rosario Torres Salvador María 
Published: February 2003

AbstractThe syntheses of two new cyclophane hosts, 4 and 6, are described. The main difference between them is the higher degree of preorganization of 4 as a consequence of the inclusion of the 7,7‐diphenylnorbornane (DPN) subunit. The inner cavity of 4 adopts a belt‐shaped structure, while 6 has a twisted geometry. In the solid state, the molecules of macrocycle 6 are stacked along an axis to form nanotubular structures. Compounds 4 and 6 form two of the strongest complexes between arene cyclophanes and Ag+ reported up to date. The silver cation is located inside the cavity of the macrocycles. The stability of 4⋅Ag+ is considerably higher than that of 6⋅Ag+. The additional stabilization of 4⋅Ag+ is attributed to higher preorganization of macrocycle 4. DNMR experiments as well as theoretical calculations carried out with 4⋅Ag+ show evidence of Ag+‐hopping between two different binding sites inside the macrocycle. This phenomenon could be the basis for the design of molecular clocks.
A Theoretical Study on the Complete Catalytic Cycle of the Hetero‐Pauson–Khand‐Type [2+2+1] Cycloaddition Reaction of Ketimines, Carbon Monoxide and Ethylene Catalyzed by Iron Carbonyl Complexes
Authors: Imhof Wolfgang , Anders Ernst , Göbel Angela , Görls Helmar 
Published: February 2003

AbstractThe [2+2+1] cycloaddition reaction of 1,4‐diazabutadienes, carbon monoxide and ethylene catalyzed by iron carbonyl complexes produces pyrrolidin‐2‐one derivatives. Only one of the two imine moieties is activated during the catalysis. The mechanism of this cycloaddition reaction is studied by density functional theory at the B3LYP/6‐311++G(d,p) level of theory. In accordance with experimental results, a [(diazabutadiene)Fe(CO)3] complex of square‐pyramidal geometry is used as the starting compound S of the catalytic cycle. Based on experimental experience, the reaction with ethylene is considered to take place before any interaction with carbon monoxide. According to the computational results, the reaction does not proceed by ligand dissociation followed by addition of ethylene and subsequent intramolecular activation steps but by the approach of an ethylene molecule from the base of the square‐pyramidal complex. This reaction yields an intermediate I4 in which ethylene is coordinated to the iron centre and a new CC bond between ethylene and one of the imine groups is formed. The insertion of a terminal carbon monoxide ligand into the metal–carbon bond between ethylene and iron produces the key intermediate I7. The reaction proceeds by metal‐assisted formation of a lactam P. The catalytic cycle is closed by a ligand‐exchange reaction in which the diazabutadiene ligand substitutes P with reformation of S. This reaction pathway is found to be energetically favored over a reductive elimination. It leads to the experimentally observed heterocyclic product P and a reactive [Fe(CO)3] fragment.
A Theoretical Case Study of Type I and Type II β‐Turns
Authors: Czinki Eszter , Császár Attila G. , Perczel András 
Published: February 2003

AbstractNMR chemical shielding anisotropy tensors have been computed by employing a medium size basis set and the GIAO‐DFT(B3LYP) formalism of electronic structure theory for all of the atoms of type I and type II β‐turn models. The models contain all possible combinations of the amino acid residues Gly, Ala, Val, and Ser, with all possible side‐chain orientations where applicable in a dipeptide. The several hundred structures investigated contain either constrained or optimized ϕ, ψ, and χ dihedral angles. A statistical analysis of the resulting large database was performed and multidimensional (2D and 3D) chemical‐shift/chemical‐shift plots were generated. The 1Hα‐13Cα, 13Cα‐1Hα‐13Cβ, and 13Cα‐1Hα‐13C′ 2D and 3D plots have the notable feature that the conformers clearly cluster in distinct regions. This allows straightforward identification of the backbone and side‐chain conformations of the residues forming β‐turns. Chemical shift calculations on larger For‐(L‐Ala)n‐NH2 (n=4, 6, 8) models, containing a single type I or type II β‐turn, prove that the simple models employed are adequate. A limited number of chemical shift calculations performed at the highly correlated CCSD(T) level prove the adequacy of the computational method chosen. For all nuclei, statistically averaged theoretical and experimental shifts taken from the BioMagnetic Resonance Bank (BMRB) exhibit good correlation. These results confirm and extend our previous findings that chemical shift information from selected multiple‐pulse NMR experiments could be employed directly to extract folding information for polypeptides and proteins.
Vilsmeier Formylation of 5,10,15‐Triphenylcorrole: Expected and Unusual Products
Authors: Paolesse Roberto , Nardis Sara , Venanzi Mariano , Mastroianni Marco , Russo Michele , Fronczek Frank R. , Vicente Maria Graça H. 
Published: February 2003

Abstract5,10,15‐Triphenylcorrole (1) reacts with the Vilsmeier reagent (POCl3/DMF) to give the corresponding 3‐formyl derivative 3 as the major product. The regioselectivity of the reaction was proven by X‐ray crystallography and only traces of the 2‐formyl isomer were observed. A more polar product is also observed and this compound becomes the major product when an excess of DMF is used for the preparation of the Vilsmeier reagent, while the formation of the 3‐formyl isomer is almost completely suppressed. X‐ray crystallography allowed us to identify this compound as the fully substituted N‐ethane bridged derivative 4, formed from the attack of the Vilsmeier reagent at the inner core of the macrocycle. This compound is unique among porphyrinoid macrocycles, and further confirms the peculiarity of corrole chemistry.
Supramolecular Control of Two‐Dimensional Phase Behavior
Authors: De Feyter Steven , Larsson Mattias , Schuurmans Norbert , Verkuijl Bas , Zoriniants George , Gesquière André , Abdel‐Mottaleb Mohamed M. , van Esch Jan , Feringa Ben L. , van Stam Jan , De Schryver Frans 
Published: February 2003

AbstractWe have used directed two‐component self‐assembly to “pattern” organic monolayers on the nanometer scale at the liquid/solid interface. The ability of the scanning tunneling microscope to investigate structural details in these adlayers was used to gain insight into the two‐component two‐dimensional phase behavior. The components are symmetrically alkylated bisurea derivatives (R1‐urea‐spacer‐urea‐R2; R1, R2=alkyl, spacer=alkyl or bisthiophene). The bisthiophene unit acts as a marker and its bisurea derivative (T2) is a component in all the mixtures investigated. By varying the position of the hydrogen‐bond forming urea groups along the molecule and the length of the alkyl chains of the other components, the effect of 1) hydrogen bonding, 2) molecule length, 3) odd–even effects, and 4) shape complementarity on the two‐dimensional phase behavior was investigated. Insight into the effect of these parameters leads to the control of the two‐dimensional patterning: from randomly intermixed systems to phase separation.
Atomic Properties of Selected Biomolecules: Quantum Topological Atom Types of Hydrogen, Oxygen, Nitrogen and Sulfur Occurring in Natural Amino Acids and Their Derivatives
Authors: Popelier Paul L. A. , Aicken Fiòna M. 
Published: February 2003

AbstractMolecular electron densities are generated at B3LYP/6‐311+G(2d,p)//HF/6‐31G(d) level for 57 molecules, including one conformation of each naturally occurring amino acid and smaller derived molecules. The electron densities are partitioned into atomic fragments according to the approach of quantum chemical topology (QCT). A set of 547 unique topological atoms is obtained, containing 421 hydrogens, 63 oxygens, 57 nitrogens and 6 sulfurs. Each atom is described by seven properties: volume, kinetic energy, monopole, dipole, quadrupole, octupole and hexadecapole moment. Cluster analysis groups atoms into atom types based on their similarity expressed in the discrete 7D space of atomic properties. Using a separation criterion we distinguish seven hydrogen, six oxygen, two nitrogen and six sulfur atom types.