bluuebunny/crossref_metadata_2025_split huggingface dataset:
Nε Functionalization of Metal and Organic Protected L‐Histidine for a Highly Efficient, Direct Labeling of Biomolecules with [Tc(OH2)3(CO)3]+
Authors: Pak Jae Kyoung , Benny Paul , Spingler Bernhard , Ortner Kirstin , Alberto Roger 
Published: May 2003

AbstractTwo different pathways for the introduction of an acetyl group at Nε in a Nα, Nδ, and ‐COO protected histidine to afford Nε‐(CH2COOH)‐histidine derivative 7 b are presented. The purpose of this study is the coupling of 7 b to amino groups in bioactive molecules such as peptides. After full deprotection of such a bioconjugate, histidine provides three coordination sites which efficiently coordinate to [99mTc(OH2)3(CO)3]+ or [Re(OH2)3(CO)3]+ in a facial geometry. This allows the development of novel radiopharmaceuticals. Selective derivatization at the Nε position has conveniently been achieved by concomitant protection of Nα and Nδ with a carbonyl group forming a six‐membered urea. Cyclic urea ring opening with Fm‐OH, coupling of phenylalanine as a model to 7 b through its primary amine and removing of all protecting groups in one step gave a histidine derivative of phenylalanine which could be labeled at 10−5 M with 99mTc in very high yield and even in about 50 % yield at 10−6 M. The Xray structure of a complex with [Re(CO)3]+ in which anilin is coupled to 7 b confirms the facial arrangement of histidine. A second pathway applies directly the [Re(CO)3]+ moiety as a protecting group. This is one of the rare examples in which a metal fragment is used as a protecting group for organic functionalities. The coordination to histidine protects the Nα, Nδ and COO group in one single step, subsequent alkylation with BrCH2COOH(R) at Nε, coupling to phenylalanine and oxidative deprotection of [Re(CO)3]+ to [ReO4]− gave the corresponding bioconjugate in which histidine is coupled to phenylalanine through an acetylamide at Nε. Both methods offer convenient pathways to introduce histidine in a biomolecule under retention of its three coordination sites. The procedures are adaptable to any biomolecule with pendant amines and allow the development of novel radiopharmaceuticals or inversed peptides.
Bonding Energetics in Alkaline Metal Alkoxides and Phenoxides
Authors: Nunes Paulo , Leal Joāo P. , Cachata Vera , Raminhos Henrique , Minas da Piedade Manuel E. 
Published: May 2003

AbstractThe bonding energetics in a variety of alkaline metal, alkoxides and phenoxides, MOR, was investigated based on the corresponding enthalpies of formation in the crystalline state determined by reaction‐solution calorimetry. The results obtained at 298.15 K were as follows: ΔfH$\rm{_{m}^{\,o}}$ (MOR, cr)/kJ mol−1 = 382.7±1.4 (LiOC6H5), 513.6±2.5 (NaO‐nC6H13), 326.4±1.4 (NaOC6H5), 375.2±3.4 (KOCH3), 434.5±2.7 (KOC2H5), 467.1±5.2 (KO‐nC3H7), 459.3±2.1 (KO‐nC4H9), 464.6±5.7 (KO‐tC4H9), 464.3±2.5 (KO‐nC6H13), 333.3±3.1 (KOC6H5), 380.6±2.9 (RbOCH3), 434.1±2.9 (RbOC2H5), 345.3±2.9 (LiOC6H5), 379.1±3.0 (CsOCH3), 432.3±3.1 (CsOC2H5), 466.9±5.0 (CsO‐nC3H7), 461.3±3.5 (CsO‐nC4H9), 461.9±2.5 (CsO‐tC4H9), 349.2±1.4 (CsOC6H5). These results together with revised ΔfH$\rm{_{m}^{\,o}}$ (MOR, cr) values from the literature, were used to derive a consistent set of lattice energies for the MOR compounds and discuss general trends in the structure–energetics relationship based on the Kapustinskii equation.
Diastereoselective Solution and Multipin‐Based Combinatorial Array Synthesis of a Novel Class of Potent Phosphinic Metalloprotease Inhibitors
Authors: Makaritis Anastasios , Georgiadis Dimitris , Dive Vincent , Yiotakis Athanasios 
Published: May 2003

AbstractThe solution‐phase synthesis and resolution of new phosphinopeptidic building blocks containing a triple bond was realized in high yields and optical purities (units 3 a–d). The absolute configuration of the target compounds was unambiguously established by NMR studies. A post‐assembly diversification strategy of these blocks was developed through 1,3‐dipolar cycloaddition of a variety of in situ prepared nitrile oxides. This strategy led to the rapid and efficient diastereoselective preparation of a novel class of isoxazole‐containing phosphinic peptides (peptides 5 a–i). Solid‐phase version of this strategy was efficiently achieved on multipin solid technology, by developing a new protocol for the coupling of Punprotected dipeptidic blocks with solid supported amino acids in a quantitative and diastereoselective manner. Optimization of dipolar cycloadditions onto pin‐embodied phosphinic peptides allowed the convenient preparation of this new class of pseudopeptides. The crude phosphinic peptides (9 a–k) were obtained in high yields and purity as determined by RP‐HPLC. Inhibition assays of some of these peptides revealed that they behave as very potent inhibitors of MMPs, outmatching previously reported phosphinic peptides, in terms of potency (Ki in the range of few nM).
Pyrene‐Derived Novel One‐ and Two‐Component Organogelators
Authors: Babu P. , Sangeetha N. M. , Vijaykumar P. , Maitra Uday , Rissanen Kari , Raju A. R. 
Published: May 2003

AbstractA new class of alkyl‐chain‐appended pyrene derivatives 4–14 were synthesized and evaluated for their gelation abilities. Depending on the nature of the linking group, these compounds gelated a number of organic solvents, either in the presence or in the absence of the acceptor molecule 2,4,7‐trinitrofluorenone (TNF). Compounds with ester, ether, or alkyl linkages gelated a number of hydroxylic and hydrocarbon solvents by means of a charge‐transfer interaction with TNF, while compounds with amide, urethane and urea linkers formed gels on their own in a variety of solvents by means of π–π stacking and hydrogen‐bonding interactions. The Xray crystal structure of urethane (S)‐12 showed hydrogen‐bonding and stacking features, as suggested by the model. The gels obtained were investigated by spectroscopic and electron microscopic techniques which provided structural insights.
Ternary Complexes between Cationic GdIII Chelates and Anionic Metabolites in Aqueous Solution: An NMR Relaxometric Study
Authors: Botta Mauro , Aime Silvio , Barge Alessandro , Bobba Gabriella , Dickins Rachel S. , Parker David , Terreno Enzo 
Published: May 2003

AbstractThe 1H and 17O NMR relaxometric properties of two cationic complexes formed by GdIII with a macrocyclic heptadentate triamide ligand, L1, and its Nmethylated analogue, L2, have been investigated in aqueous media as a function of pH, temperature and magnetic field strength. The complexes possess two water molecules in their inner coordination sphere for which the rate of exchange has been found to be sensibly faster for the Nmethylated derivative and explained in terms of electronic effects (decrease of the charge density at the metal center) and perturbation of the network of hydrogen‐bonded water molecules in the outer hydration sphere. The proton relaxivity shows a marked dependence from pH and decreases of about six units in the pH range 6.5 to 9.0. This has been accounted for by the displacement of the two water molecules by dissolved carbonate which acts as a chelating anion. The formation of ternary complexes with lactate, malonate, citrate, acetate, fluoride and hydrogenphosphate has been monitored by 1H NMR relaxometric titrations at 20 MHz and pH 6.3 and the value of the affinity constant, K, and of the relaxivity of the adducts could be obtained. Lactate, malonate and citrate interact strongly with the complexes (log K ≥3.7) and coordinate in a bidendate mode by displacing both water molecules. Larger affinity constants have been measured for GdL2. Acetate, fluoride and hydrogenphosphate form monoaqua ternary complexes which were investigated in detail with regard to their relaxometric properties. The NMR dispersion (NMRD) profiles indicate a large contribution to the relaxivity of the adducts from water molecules belonging to the second hydration shell of the complexes and hydrogen‐bonded to the anion. A VT 17O NMR study has shown a marked increase of the rate of water exchange upon binding which is explained by coordination of the anion in an equatorial site, thus leaving the water molecule in an apical position, more accessible for interactions with the solvent molecules of the second hydration shell which facilitate the exchange process.
Theoretical Evidence of Persistent Chirality in D3 Homoleptic Hexacoordinate Complexes with Monodentate Ligands
Authors: Alemany Pere , Alvarez Santiago , Avnir David 
Published: May 2003

AbstractA theoretical study of the enantiomer interconversion pathway relevant to racemization reactions of hexacoordinate transition‐metal complexes is presented based on density functional calculations. The potential‐energy surface for the trigonal twist pathway of the [Zr(SH)6]2− model compound has been explored. The optimum structure reproduces, to a very good approximation, the experimental geometry of the analogous compound in which the thiolato groups have C6H4‐4‐OMe substituents instead of H atoms. A barrier of about 19 kcal mol−1 is estimated for the racemization of [Zr(SH)6]2− and exploratory calculations for [Zr(SC6H4‐4‐OMe)6]2− indicate that a larger barrier should be expected. For the chiral homoleptic organometallic complexes [ZrMe6]2− and [RhMe6]3− no significant racemization barrier is expected.
Novel and Anti‐Inflammatory Constituents of Garcinia subelliptica
Authors: Weng Jing‐Ru , Lin Chun‐Nan , Tsao Lo‐Ti , Wang Jih‐Pyang 
Published: May 2003

AbstractFour novel phloroglucinol derivatives, garcinielliptones A (1), B (2), C (3), D (4), a novel triterpenoid, garcinielliptone E (5), and three known compounds were isolated from the seeds of Garcinia subelliptica. The structures, including relative configurations, were elucidated by means of spectroscopic data. Known compounds garsubellin A (6) and garcinielliptin oxide (7) showed potent inhibitory effects on the release of β‐glucuronidase, and β‐glucuronidase and histamine, respectively, from peritoneal mast cells stimulated with compound 48/80 in a concentration‐dependent manner with IC50 values of 15.6±2.5, and 18.2±3.6 and 20.0±2.7 μM, respectively. Compound 7 showed potent inhibitory effects on the release of β‐glucuronidase and lysozyme from neutrophils stimulated with formyl‐Met‐Leu‐Phe(fMLP)/cytochalasin B (CB) in a concentration‐dependent manner with IC50 values of 15.7±3.0 and 23.9±3.2 μM, respectively. Compound 7 also showed potent inhibitory effect on superoxide formation from neutrophils stimulated with fMLP/CB also in a concentration‐dependent manner with an IC50 value of 17.9±1.5 μM.
Gold Glyconanoparticles: Synthetic Polyvalent Ligands Mimicking Glycocalyx‐Like Surfaces as Tools for Glycobiological Studies
Authors: Barrientos África G , de la Fuente Jesús M. , Rojas Teresa C. , Fernández Asunción , Penadés Soledad 
Published: May 2003

AbstractA simple and versatile methodology is described for tailoring sugar‐functionalised gold nanoclusters (glyconanoparticles) that have 3D polyvalent carbohydrate display and globular shapes. This methodology allows the preparation of glyconanoparticles with biologically significant oligosaccharides as well as with differing carbohydrate density. Fluorescent glyconanoparticles have been also prepared for labelling cells in biological tests. The materials are water soluble, stable under physiological conditions and present an exceptional small core size. All of them have been characterised by 1H NMR, UV and IR spectroscopy, TEM and elemental analysis. Their highly polyvalent network can mimic glycosphingolipid clustering and interactions at the plasma membrane, providing an controlled system for glycobiological studies. Furthermore, they are useful building blocks for the design of nanomaterials.
Coordination Chemistry of the Soluble Metal Oxide Analogue [Mo5O13(OCH3)4(NO)]3− with Manganese Carbonyl Species
Authors: Villanneau Richard , Proust Anna , Robert Francis , Gouzerh Pierre 
Published: May 2003

AbstractThe reactions of neutral or cationic manganese carbonyl species towards the oxo‐nitrosyl complex [Na(MeOH){Mo5O13(OCH3)4(NO)}]2− have been investigated in various conditions. This system provides an unique opportunity for probing the basic reactions involved in the preparation of solid oxide‐supported heterogeneous catalysts, that is, mobility of transition‐metal species at the surface and dissolution–precipitation of the support. Under nitrogen and in the dark, the reaction of in situ generated fac‐{Mn(CO)3}+ species with (nBu4N)2[Na(MeOH)‐{Mo5O13(OMe)4(NO)}] in MeOH yields (nBu4N)2[Mn(CO)3(H2O){Mo5O13(OMe)4(NO)}] at room temperature, while (nBu4N)3[Na{Mo5O13(OMe)4(NO)}2{Mn(CO)3}2] is obtained under reflux. The former transforms into the latter under reflux in methanol in the presence of sodium bromide; this involves the migration of the fac‐{Mn(CO)3}+ moiety from a basal κ2O coordination site to a lateral κ3O site. Oxidation and decarbonylation of manganese carbonyl species as well as degradation of the oxonitrosyl starting material and reaggregation of oxo(methoxo)molybdenum fragments occur in non‐deareated MeOH, and both (nBu4N)4[Mn(H2O)2{Mo5O16(OMe)2}2{Mn(CO)3}2] and (nBu4N)4[Mn(H2O)2{Mo5O13(OMe)4(NO)}2] as well as (nBu4N)2[MnBr{Mo5O13(OMe)4(NO)}] have been obtained in this way. The rhenium analogue (nBu4N)2[Re(CO)3(H2O){Mo5O13(OMe)4(NO)}] has also been synthesized. The crystal structures of (nBu4N)2[Re(CO)3(H2O){Mo5O13(OMe)4(NO)}], (nBu4N)3[Na{Mo5O13(OMe)4(NO)}2{Mn(CO)3}2], (nBu4N)4[Mn(H2O)2{Mo5O16(OMe)2}2{Mn(CO)3}2], (nBu4N)4[Mn(H2O)2{Mo5O13(OMe)4(NO)}2] and (nBu4N)2[MnBr{Mo5O13(OMe)4(NO)}] have been determined.
Formation of Novel Sulfur‐Containing C60F16 Cycloadducts between Tetrathiafulvalene and C60F18; A Unique Six‐Electron Cycloaddition of a Fullerene Involving F2 Loss
Authors: Darwish Adam D. , Avent Anthony G. , Boltalina Olga V. , Gol′dt Ilya , Kuvytchko Igor , Da Ros Tatiana , Street Joan M. , Taylor Roger 
Published: May 2003

AbstractCo‐evaporation of solutions of C60F18 and tetrathiafulvalene in toluene produces an unsymmetrical C60F16:tetrathiafulvalene adduct through a unique six‐electron cycloaddition involving displacement of two fluorine atoms by a terminal CC double bond of the fulvalene. The adduct rearranges into two further adducts, one of which is characterised as a new type of fullerene derivative, a thiiranofullerene, formed by elimination of a thioketene moiety from the tetrathiafulvalene adduct. The initial addition also produces a bisadduct in which the addends comprise one tetrathiafulvalene molecule and one in which carbon disulfide has been eliminated. The latter adduct involves cycloaddition of an unsaturated aromatic dithiolactone moiety.
Mutations in Distant Residues Moderately Increase the Enantioselectivity of Pseudomonas fluorescens Esterase towards Methyl 3Bromo‐2‐methylpropanoate and Ethyl 3Phenylbutyrate
Authors: Horsman Geoff P. , Liu Andrew M. F. , Henke Erik , Bornscheuer Uwe T. , Kazlauskas Romas J. 
Published: May 2003

AbstractDirected evolution combined with saturation mutagenesis identified six different point mutations that each moderately increases the enantioselectivity of an esterase from Pseudomonas fluorescens (PFE) towards either of two chiral synthons. Directed evolution identified a Thr230Ile mutation that increased the enantioselectivity from 12 to 19 towards methyl (S)‐3‐bromo‐2‐methylpropanoate. Saturation mutagenesis at Thr230 identified another mutant, Thr230Pro, with higher‐than‐wild‐type enantioselectivity (E=17). Previous directed evolution identified mutants Asp158Asn and Leu181Gln that increased the enantioselectivity from 3.5 to 5.8 and 6.6, respectively, towards ethyl (R)‐3‐phenylbutyrate. In this work, saturation mutagenesis identified other mutations that further increase the enantioselectivity to 12 (Asp158Leu) and 10 (Leu181Ser). A homology model of PFE indicates that all mutations lie outside the active site, 12–14 Å from the substrate and suggests how the distant mutations might indirectly change the substrate‐binding site. Since proteins contain many more residues far from the active site than close to the active site, random mutagenesis is strongly biased in favor of distant mutations. Directed evolution rarely screens all mutations, so it usually finds the distant mutations because they are more common, but probably not the most effective.
Theoretical Study on the Mechanism of Iron Carbonyls Mediated Isomerization of Allylic Alcohols to Saturated Carbonyls
Authors: Branchadell Vicenç , Crévisy Christophe , Grée René 
Published: May 2003

AbstractThe conversion of allylic alcohols to enols mediated by Fe(CO)3 has been studied through density functional theoretical calculations. From the results obtained a complete catalytic cycle has been proposed in which the first intermediate is the [(allyl alcohol)Fe(CO)3] complex. This intermediate evolves to the [(enol)Fe(CO)3] complex through two consecutive 1,3‐hydrogen shifts involving a π‐allyl hydride intermediate. The highest Gibbs energy transition state corresponds to the partial decoordination ot the enol ligand prior to the coordination of a new allyl alcohol molecule that regenerates the first intermediate. Alternative processes for the [(enol)Fe(CO)3] complex such as [Fe(CO)3]‐mediated enol–aldehyde transformation and enol isomerization have also been considered. The results obtained show that the former process is unfavourable, whereas the enol isomerization may compete with the enol decoordination step of the catalytic cycle.
Improved One‐Pot Synthesis of Styryl Tetrahydrofurans and Cyclohexanes by Radical Addition to β‐Nitrostyrenes in the Presence of Benzoyl Peroxide
Authors: Jang Yeong‐Jiunn , Shih Yuh‐Kuo , Liu Jing‐Yuan , Kuo Wen‐Yu , Yao Ching‐Fa 
Published: May 2003

AbstractStereoselective styryl derivatives have been prepared based on radical substitution (addition–elimination) of heterocycles or cyclohydrocarbons units to (E)‐β‐nitrostyrenes 1 using a common radical initiator benzoyl peroxide. High reactivity and selectivity with wide substrate scope were attained by using this easy methodology. The reactions using easily obtained and one‐pot potential starting materials gave excellent trans‐selectivity with medium to high yields in all cases. Synthetic utility of this approach has been demonstrated by the preparation of various trans‐styryl derivatives.
Rational Chemical Strategies for Carbon Nanotube Functionalization
Authors: Banerjee Sarbajit , Kahn Michael G. C. , Wong Stanislaus S. 
Published: May 2003

AbstractWhereas the chemistry of fullerenes is well‐established, the chemistry of single‐walled carbon nanotubes (SWNTs) is a relatively unexplored field of research. Investigations into the bonding of moieties onto SWNTs are important because they provide fundamental structural insight into how nanoscale interactions occur. Hence, understanding SWNT chemistry becomes critical to rational, predictive manipulation of their properties. Among the strategies discussed include molecular metal complexation with SWNTs to control site‐selective chemistry in these systems. In particular, work has been performed with Vaska's and Wilkinson's complexes to create functionalized adducts. Functionalization should offer a relatively simple means of tube solubilization and bundle exfoliation, and also allows for tubes to be utilized as recoverable catalyst supports. Solubilization of oxidized SWNTs has also been achieved through derivatization by using a functionalized organic crown ether. The resultant adduct yielded concentrations of dissolved nanotubes on the order of 1 g L−1 in water and at elevated concentrations in a range of organic solvents, traditionally poor for SWNT manipulation. To further demonstrate chemical processability of SWNTs, we have subjected them to ozonolysis, followed by treatment with various independent reagents, to rationally generate a higher proportion of oxygenated functional groups on the nanotube surface. This protocol has been found to purify nanotubes. More importantly, the reaction sequence has been found to ozonize the sidewalls of these nanotubes. Finally, SWNTs have also been chemically modified with quantum dots and oxide nanocrystals. A composite heterostructure consisting of nanotubes joined to nanocrystals offers a unique opportunity to obtain desired physical, electronic, and chemical properties by adjusting synthetic conditions to tailor the size and structure of the individual sub‐components, with implications for self‐assembly.
Cycloadditions of Isobenzofuran to a Constrained Template Bearing Neighboring Dienophiles
Authors: Stoermer Martin J. , Butler Douglas N. , Warrener Ronald N. , Weerasuria K. D. V. , Fairlie David P. 
Published: May 2003

AbstractA high yielding synthesis of the pentacyclic diene–dione 1 has enabled investigation of its reactivity as a double dienophile in Diels–Alder [4+2] cycloadditions with isobenzofuran, leading to novel and highly symmetrical three‐sided cavitands 3 and 4.
Hydrogen–Hydrogen Bonding: A Stabilizing Interaction in Molecules and Crystals
Authors: Matta Chérif F. , Hernández‐Trujillo Jesús , Tang Ting‐Hua , Bader Richard F. W. 
Published: May 2003

AbstractBond paths linking two bonded hydrogen atoms that bear identical or similar charges are found between theortho‐hydrogen atoms in planar biphenyl, between the hydrogen atoms bonded to the C1–C4 carbon atoms in phenanthrene and other angular polybenzenoids, and between the methyl hydrogen atoms in the cyclobutadiene, tetrahedrane and indacene molecules corseted with tertiary‐tetra‐butyl groups. It is shown that each such H–H interaction, rather than denoting the presence of “nonbonded steric repulsions”, makes a stabilizing contribution of up to 10 kcal mol−1to the energy of the molecule in which it occurs. The quantum theory of atoms in molecules—the physics of an open system—demonstrates that while the approach of two bonded hydrogen atoms to a separation less than the sum of their van der Waals radii does result in an increase in the repulsive contributions to their energies, these changes are dominated by an increase in the magnitude of the attractive interaction of the protons with the electron density distribution, and the net result is a stabilizing change in the energy. The surface virial that determines the contribution to the total energy decrease resulting from the formation of the H–H interatomic surface is shown to account for the resulting stability. It is pointed out that H–H interactions must be ubiquitous, their stabilization energies contributing to the sublimation energies of hydrocarbon molecular crystals, as well as solid hydrogen. H–H bonding is shown to be distinct from “dihydrogen bonding”, a form of hydrogen bonding with a hydridic hydrogen in the role of the base atom.