bluuebunny/crossref_metadata_2025_split huggingface dataset:
(9Z)‐ and (11Z)‐8‐Methylretinals for Artificial Visual Pigment Studies: Stereoselective Synthesis, Structure, and Binding Models
Authors: Alvarez Rosana , Domínguez Marta , Pazos Yolanda , Sussman Fredy , de Lera Angel R. 
Published: December 2003

AbstractArtificial visual pigment formation was studied by using 8‐methyl‐substituted retinals in an effort to understand the effect that alkyl substitution of the chromophore side chain has on the visual cycle. The stereoselective synthesis of the 9‐cis and 11‐cis isomers of 8‐methylretinal, as well as the 5‐demethylated analogues is also described. The key bond formations consist of a thallium‐accelerated Suzuki cross‐coupling reaction between cyclohexenylboronic acids and dienyliodides (C6C7), and a highly stereocontrolled Horner–Wadsworth–Emmons or Wittig condensation (C11C12). The cyclohexenylboronic acid was prepared by trapping the precursor cyclohexenyllithium species with B(OiPr)3 or B(OMe)3. The cyclohexenyllithium species is itself obtained by nBuLi‐induced elimination of a trisylhydrazone (Shapiro reaction), or depending upon the steric hindrance of the ring, by iodine–metal exchange. In binding experiments with the apoprotein opsin, only 9‐cis‐5‐demethyl‐8‐methylretinal yielded an artificial pigment; 9‐cis‐8‐methylretinal simply provided residual binding, while evidence of artificial pigment formation was not found for the 11‐cis analogues. Molecular‐mechanics‐based docking simulations with the crystal structure of rhodopsin have allowed us to rationalize the lack of binding displayed by the 11‐cis analogues. Our results indicate that these isomers are highly strained, especially when bound, due to steric clashes with the receptor, and that these interactions are undoubtedly alleviated when 9‐cis‐5‐demethyl‐8‐methylretinal binds opsin.
Novel Heterotopic Colloids of Anionic Porphyrins Entangled in Cationic Amphiphilic Cyclodextrins: Spectroscopic Investigation and Intracellular Delivery
Authors: Mazzaglia Antonino , Angelini Nicola , Darcy Raphael , Donohue Ruth , Lombardo Domenico , Micali Norberto , Sciortino Maria Teresa , Villari Valentina , Scolaro Luigi Monsù 
Published: December 2003

AbstractThe entanglement process between water‐soluble 5,10,15,20‐tetrakis(4‐sulfonatophenyl)‐21H,23H‐porphine and the amphiphilic cyclodextrin (CD) heptakis(2‐ω‐amino‐O‐oligo(ethylene oxide)‐6‐hexylthio)‐β‐CD and the occurrence of various species at different porphyrin:CD ratios were studied by a combination of UV/Vis absorption, fluorescence anisotropy, time‐resolved fluorescence, resonance light scattering, and circular dichroism. The effect of the entanglement process on the mean vesicle diameter was investigated over a wide concentration range by quasielastic light‐scattering techniques. The experimental results indicate that the presence of porphyrins in this colloidal system promotes some structural rearrangements, essentially driven by charge interaction, which are responsible for a sensitive change of vesicle dimensions. In the range of porphyrin:CD molar ratios between 1:10 and 1:50, the porphyrin is solubilized in monomeric form (τ1=11.5 ns) and photosensitizes the production of singlet oxygen (1O2). At the same molar ratio the ability of this amphiphilic cyclodextrin to transport porphyrins into tumor cells indicates specificity at the nuclear‐compartment level. These findings may be of potential interest for the of development agents for photodynamic therapy of tumors.
Theoretical Studies on the Mode of Inhibition of Ribonucleotide Reductase by 2′‐Substituted Substrate Analogues
Authors: Fernandes Pedro Alexandrino , Ramos Maria João 
Published: December 2003

AbstractSeveral 2′‐substituted‐2′‐deoxyribonucleotides are potent time‐dependent inactivators of the enzyme ribonucleotide reductase (RNR), which function by destructing its essential tyrosil radical and/or by performing covalent addition to the enzyme. The former leads to inhibition of the R2 dimer of RNR and the latter to inhibition of the R1 dimer. Efforts to elucidate the mechanism of inhibition have been undertaken in the last decades, and a general mechanistic scheme has emerged. Accordingly, two alternative pathways lead either to the inhibition of R1 or R2, for which the 2′‐chloro‐2′‐deoxynucleotides serve as the model for the inhibition of R1 and the 2′‐azido‐2′‐deoxynucleotides the model for the inhibition of R2. However, the underlying reason for the different behavior of the inhibitors has remained unknown until now. Moreover, a fundamental mechanistic alternative has been proposed, based on results from biomimetic reactions, in which the 2′‐substituents would be eliminated as radicals, and not as anions, as previously assumed. This would lead to further reactions not predicted by the existing mechanistic scheme. To gain a better understanding we have performed high‐level theoretical calculations on the active site of RNR. Results from this work support the general Stubbe's paradigm, although some changes to that mechanism are necessary. In addition, a rational explanation of the factors that determine which of the dimers (R1 or R2) will be inactivated is provided for the first time. It has been demonstrated also that the 2′‐substituents are indeed eliminated as anions, and not as radicals. Biomimetic experiments have led to different results because they lack a basic group capable of deprotonating the 3′‐HO group of the substrate. It has been found here that the chemical character of the leaving group (radical or anionic) can be manipulated by controlling the protonation state of the 3′‐HO group.
Calenzanane Sesquiterpenes from the Red Seaweed Laurencia microcladia from the Bay of Calenzana, Elba Island: Acid‐Catalyzed Stereospecific Conversion of Calenzanol into Indene‐ and Guaiazulene‐Type Sesquiterpenes
Authors: Guella Graziano , Skropeta Danielle , Mancini Ines , Pietra Francesco 
Published: December 2003

AbstractIt is shown here that calenzanane sesquiterpenes (1 and 6) can be isolated from organic extracts from the red seaweed Laurencia microcladia Kützing from the Bay of Calenzana, Elba Island, provided contact with acidic media is minimized. Such contact induces rearrangements of 1 in dry solvents to indene‐type 5 and the blue‐colored guaiazulenium‐type ion 17, via spectrometrically (NMR) characterized indene‐type transient intermediates 10, 14, and 12. Addition of NEt3 to the reaction mixture at appropriate stages allowed the isolation of 12 (and 8 on workup on SiO2), and guaiazulene (18). Prolonged contact with silica gel led to complete degradation of 1, giving calenzanane‐type epimeric enones 20 a/20 b as well as indene‐type epimeric carbinols 22 a/22 b and fulvene 7. The latter was also formed during silica‐gel flash chromatography of the algal extracts. A unifying mechanistic view of these branching and cascade transformations may have both heuristic value, suggesting possible artefact origin of azulenoids, and synthetic applications.
A Phthalocyanine Dendrimer Capable of Forming Spherical Micelles
Authors: Uchiyama Takamichi , Ishii Kazuyuki , Nonomura Taro , Kobayashi Nagao , Isoda Seiji 
Published: December 2003

AbstractA novel and intrinsically spherical micelle has been prepared by utilizing a silicon phthalocyanine ((WG3)SiPc) that has a thin hydrophobic alkyl chain and a bulky hydrophilic poly(aryl ether) dendrimer with terminal carboxyl groups, as its two axial ligands. Gel‐permeation chromatography and cryo‐transmission electron microscopic experiments indicate that (WG3)SiPcs self‐assemble to form a spherical micelle at very low concentrations in aqueous solution. Depending on the pH of the aqueous phase, (WG3)SiPc shuttles between aqueous and organic phases. In the presence of hydrophobic molecules, this transfer is accompanied by the inclusion of these (guest) molecules, indicating that the micelle acts as a molecular capsule with a nanospace surrounded by functional phthalocyanine planes.
Molecular Dynamics Simulations of Small Peptides: Can One Derive Conformational Preferences from ROESY Spectra?
Authors: Peter Christine , Rueping Magnus , Wörner Hans Jakob , Jaun Bernhard , Seebach Dieter , van Gunsteren Wilfred F. 
Published: December 2003

AbstractFolding properties of β‐peptides were investigated by means of NMR experiments and MD simulations of β‐dipeptides, which serve as small test systems to study the influence of stereocenters and side chains on hydrogen‐bond and consequently on secondary‐structure formation. Two stereoisomers, SR and SS, of a Val‐Phe dipeptide, and of the corresponding Ala‐Ala dipeptide, and a Gly‐Gly dipeptide were simulated in methanol for 40 ns. In agreement with experiment, the isomers of the Val‐Phe dipeptide adopt quite different conformers at 298 K, the differences being reduced at 340 K. Interestingly, the SR isomer shows enhanced hydrogen bonding at the higher temperature. The adopted conformations are primarily determined by the R or S side chain substitution, and less by the type of side chain. Back‐calculation of 1H ROESY spectra and 3J coupling constants from the MD simulations and comparison with the experimental data for the Val‐Phe dipeptides shows good agreement between simulation and experiment, and reveals possible problems and pitfalls, when deriving structural properties of a small and extremely flexible molecule from NMR data only. Inclusion of all aspects of internal dynamics is essential to the correct prediction of the NMR spectra of these small molecules. Cross comparison of calculated with experimental spectra for both isomers shows that only a few out of many ROESY peaks reflect the sizeable conformational differences between the isomers at 298 K.
LaSeTe2—Temperature Dependent Structure Investigation and Electron Holography on a Charge‐Density‐Wave‐Hosting Compound
Authors: Doert Thomas , Fokwa Boniface Polequin Tsinde , Simon Paul , Lidin Sven , Söhnel Tilo 
Published: December 2003

AbstractSingle crystals of LaSeTe2 have been prepared by reaction of the elements in a LiCl/RbCl flux at 970 K for seven days. Satellite reflections observed in diffraction experiments indicate the presence of an incommensurate lattice distortion, which is of the charge‐density‐wave (CDW) type. The modulated structure has been solved from X‐ray data at 173, 293, and 373 K. LaSeTe2 crystallizes in the 3+1‐dimensional orthorhombic superspace group Cmcm(00γ)s00 (No. 63.2) with lattice parameters of a=4.295(1), b=25.371(4), c=4.306(1) Å (173 K), a=4.297(1), b=25.408(4), c=4.309(1) Å (293 K), and a=4.309(1), b=25.481(6), c=4.321(1) Å (373 K). The modulation vector q=(0, 0, 0.288) does not change over the temperature interval. Electron holographic investigations confirm the existence of the modulation and help to visualize the charge‐density wave.
Lanthanide(III) Complexes of Novel Mixed Carboxylic‐Phosphorus Acid Derivatives of Diethylenetriamine: A Step towards More Efficient MRI Contrast Agents
Authors: Kotek Jan , Lebdušková Petra , Hermann Petr , Vander Elst Luce , Muller Robert N. , Geraldes Carlos F. G. C. , Maschmeyer Thomas , Lukeš Ivan , Peters Joop A. 
Published: December 2003

AbstractThree novel phosphorus‐containing analogues of H5DTPA (DTPA = diethylenetriaminepentaacetate) were synthesised (H6L1, H5L2, H5L3). These compounds have a ‐CH2‐P(O)(OH)‐R function (R = OH, Ph, CH2NBn2) attached to the central nitrogen atom of the diethylenetriamine backbone. An NMR study reveals that these ligands bind to lanthanide(III) ions in an octadentate fashion through the three nitrogen atoms, a PO oxygen atom and four carboxylate oxygen atoms. The complexed ligand occurs in several enantiomeric forms due to the chirality of the central nitrogen atom and the phosphorus atom upon coordination. All lanthanide complexes studied have one coordinated water molecule. The residence times (τ${{298\hfill \atop {\rm M}\hfill}}$) of the coordinated water molecules in the gadolinium(III) complexes of H6L1 and H5L2 are 88 and 92 ns, respectively, which are close to the optimum. This is particularly important upon covalent and noncovalent attachment of these Gd3+ chelates to polymers. The relaxivity of the complexes studied is further enhanced by the presence of at least two water molecules in the second coordination sphere of the Gd3+ ion, which are probably bound to the phosphonate/phosphinate moiety by hydrogen bonds. The complex [Gd(L3)(H2O)]2− shows strong binding ability to HSA, and the adduct has a relaxivity comparable to MS‐325 (40 s−1 mM−1 at 40 MHz, 37 °C) even though it has a less favourable τM value (685 ns). Transmetallation experiments with Zn2+ indicate that the complexes have a kinetic stability that is comparable to—or better than—those of [Gd(dtpa)(H2O)]2− and [Gd(dtpa‐bma)(H2O)].
Spectroscopic and Computational Investigations of Stable Radical Anions of Triosmium Benzoheterocycle Clusters
Authors: Nervi Carlo , Gobetto Roberto , Milone Luciano , Viale Alessandra , Rosenberg Edward , Rokhsana Dalia , Fiedler Jan 
Published: December 2003

AbstractThe variable temperature 1H and 13C NMR and EPR spectra of the stable radical anions [Os3(CO)9(μ3‐η2‐L)(μ‐H)] (LH=phenanthridine, 1; 5,6‐benzoquinoline, 2), and [Os3(CO)10(μ3‐η2‐L)(μ‐H)] (LH=quinoxaline, 3) are reported. The radical anions 1−, 2−, and 3− can be prepared by both exhaustive electrolysis and partially by chemical reduction with cobaltocene and with sodium dispersion (only with sodium dispersion in the case of 3−). DFT calculations on 1–3 reveal that the LUMO for the electron‐deficient compounds 1 and 2 involves significant contributions from both the heterocyclic ligand and the two metal atoms bridged by the ligand and the μ‐hydride. The character of this orbital rationalizes the previously observed regioselective reactions of these complexes with nucleophiles. In contrast, the LUMO for the electron precise 3 involves only ligand‐based orbitals. Partial chemical reduction of 1 and 2 requires an excess of either cobaltocene or sodium, and their 1H and 13C NMR spectra reveal selective line broadening of those proton resonances that are predicted by DFT calculations to bear the greatest amount of free spin density. The variable temperature behavior of the partially chemically reduced species of 1 and 2 indicates that electron transfer between the reduced/unreduced cluster pair and between the cobaltocene/cobaltocenium pair occurs on the NMR timescale. The radical anions of 1 and 2 prepared by exhaustive electrolysis show an EPR signal at room temperature, while the NMR signals are uniformly broadened. Compound 3 appears to be partially reduced by sodium at room temperature and shows uniformly broadened 1H NMR resonances at room temperature that sharpen significantly at −80 °C. The temperature dependence of the spectra are discussed in terms of the effects of relative electron nuclear relaxation processes, chemical exchange, and the results of the DFT calculations.
Continuous Flow Techniques in Organic Synthesis
Authors: Jas Gerhard , Kirschning Andreas 
Published: December 2003

AbstractAs part of the dramatic changes associated with the need for preparing compound libraries in pharmaceutical and agrochemical research laboratories, the search for new technologies that allow automation of synthetic processes has become one of the main topics. Despite this strong trend for automation high‐throughput chemistry is still carried out in batches, whereas flow‐through processes are rather restricted to production processes. This is far from understandable because the main advantages of that approach are facile automation, reproducibility, safety, and process reliability, because constant reaction parameters can be assured. Indeed, methods and technologies are missing that allow rapid transfer from the research level to process development without time‐consuming adaptation and optimization of methods from the laboratory scale to production plant scale. Continuous‐flow processes are considered as a universal lever to overcome these restrictions and, only recently, joint efforts between synthetic and polymer chemists and chemical engineers have resulted in the first continuous‐flow devices and microreactors; these allow rapid preparation of compounds with minimum workup. Many of these approaches use immobilized reagents and catalysts, which are embedded in a structured flow‐through reactor. It is generally accepted, that for achieving best reaction and kinetic parameters for convective‐flow processes monolithic materials are ideally suited as solid phases or polymer supports. In addition, immobilization techniques have to be developed that allow facile regeneration of the active species in the reactor.
Asymmetric Dearomatization of the Furan Ring Promoted by Conjugate Organolithium Addition to (Menthyloxy)(3‐furyl)carbene Complexes of Chromium
Authors: Barluenga José , Nandy Sandip K. , Laxmi Y. R. Santosh , Suárez José Ramón , Merino Isabel , Flórez Josefa , García‐Granda Santiago , Montejo‐Bernardo José 
Published: December 2003

AbstractThe sequential low‐temperature addition reaction of an organolithium compound and methyl triflate to (menthyloxy)(3‐furyl)carbene complexes of chromium and tungsten proceeded with excellent regioselectivity (1,4‐addition) and diastereoselectivity (2,3‐trans disposition of the nucleophile and electrophile groups) to afford new 2,3‐disubstituted (2,3‐dihydro‐3‐furyl)carbene complexes. In addition, a high degree of diastereofacial selectivity was achieved by employing alkenyllithium compounds. After detachment of both the metal fragment and the chiral auxiliary group, trisubstituted 2,3‐dihydrofuran derivatives containing a quaternary stereogenic center at the C3 position were obtained. The characterization, including X‐ray crystallography, of a novel type of stable four‐membered chelate (η2‐alkene)tetracarbonylcarbene complex of chromium is also reported.
Homo‐ and Heterometallic [2×2] Grid Arrays Containing RuII, OsII, and FeII Subunits and their Mononuclear RuII and OsII Precursors: Synthesis, Absorption Spectra, Redox Behavior, and Luminescence Properties
Authors: Bassani Dario M. , Lehn Jean‐Marie , Serroni Scolastica , Puntoriero Fausto , Campagna Sebastiano 
Published: December 2003

AbstractThe absorption spectra, redox behavior, and luminescence properties (both at 77 K in rigid matrices and at room temperature in fluid solution) of a series of [2×2] molecular grids have been investigated. The latter were prepared either by means of sequential self‐assembly, or by a stepwise protection/deprotection procedure, and are based on a ditopic hexadentate ligand 1 in which two terpyridine‐like binding sites are fused together in a linear arrangement. The molecular grids studied include the homometallic species [{Fe(1)}4]8+ (Fe2Fe2), and the heterometallic species [{Ru(1)}2{Fe(1)}2]8+ (Ru2Fe2) and [{Os(1)}2{Fe(1)}2]8+ (Os2Fe2). For comparison purposes, the properties of the mononuclear complexes [Ru(1)2]2+ (1‐Ru) and [Os(1)2]2+ (1‐Os) have been studied. All these compounds exhibit very intense absorption bands in the UV region (ε in the 105–106 M−1 cm−1 range, attributed to spin‐allowed ligand‐centered (LC) transitions), as well as intense metal‐to‐ligand charge‐transfer (MLCT) transitions (ε in the 104–105 M−1 cm−1 range) that extend to the entire visible region. The mononuclear species 1‐Ru and 1‐Os exhibit relatively intense luminescence, both in acetonitrile at room temperature (τ=59 and 18 ns, respectively) and in butyronitrile rigid matrices at 77 K. In contrast, the tetranuclear molecular grids do not exhibit any luminescence, either at room temperature or at 77 K. This is attributed to fast intercomponent energy transfer from the Ru‐ or Os‐based subunits to the low‐lying metal‐centered (MC) levels involving the FeII centers, which leads to fast radiationless decay. The redox behavior of the compounds is characterized by several metal‐centered oxidation and ligand‐centered reduction processes, most of them reversible in nature (as many as twelve for Fe2Fe2). Detailed assignment of each redox process has been made, and it is apparent that these systems can be viewed as multilevel molecular electronic species capable of reversibly exchanging a number of electrons at accessible and predetermined potentials. Furthermore, it is shown that the electronic interaction between specific subunits depends on their location in the structure and on the oxidation states of the other components.
Aerosole und ihre technische Bedeutung
Authors: Heidenreich S. , Büttner H. , Ebert F. 
Published: December 2003

AbstractAerosole – gezielt erzeugt oder unerwünscht gebildet – treten in vielen technischen Prozessen auf. So werden zum Beispiel Aerosole gezielt erzeugt, wenn Produkte, die aus feindispersen Feststoffen bestehen, in einem Gasphasenprozess (im weitesten Sinn) hergestellt werden. Partikelbildung und ‐wachstum im Aerosolzustand prägen dabei die Größe, Form und Struktur der Partikel und damit entscheidend die Produkteigenschaften. Aerosole können sich auch unerwünscht in Prozessen bilden. Dies führt zur Verunreinigung von Prozessgasen oder Produkten beziehungsweise zu erhöhten Beladungen von Abluftströmen. Kann die unerwünschte Aerosolbildung nicht vermieden werden, müssen wirkungsvolle Abscheideverfahren eingesetzt werden. Effiziente Abscheider sind auch zur Gewinnung von Produktpartikeln aus der Gasphase notwendig. Ob gezielt erzeugt oder unerwünscht gebildet, Aerosole müssen hochauflösend erfasst und charakterisiert werden. Dies erfordert eine entsprechend leistungsfähige Aerosolmesstechnik. Der Beitrag vermittelt einen Überblick über die technische Bedeutung von Aerosolen.
Nassmahl‐Technologien mit Mahlkörpern und Lösungsansätze für Zentrifugenmühlen zur Herstellung nanoskaliger Suspensionspartikeln
Authors: Vock F. 
Published: December 2003

AbstractAnhand der Entwicklung der Nassmahltechnologie mit Mahlkörpern in Rührwerksmühlen zum Zerkleinern und Dispergieren von organischen und anorganischen Feststoffpartikeln in Suspensionen mit Praxiserfahrung in der Chemie‐, Lackfarben‐ und Druckfarben‐Industrie wird gezeigt, dass ein weiterführender Entwicklungsschritt nötig ist, um die künftigen Anforderungen bei der Herstellung nanoskaliger Partikeln mit speziellen funktionalen Eigenschaften befriedigen zu können. Es werden Lösungsansätze beschrieben und erste Ergebnisse dargestellt.
MALDI‐TOF‐Massenspektrometrie: Risikoanalyse im Flug
Authors: Humeny Andreas 
Published: December 2003

AbstractDurch ihre hohe Genauigkeit, molekulare Auflösung und Sensitivität ist die Matrix‐gestützte Laser Desorptions Ionisations Flugzeit Massenspektrometrie (MALDI‐TOF‐MS) eine sehr effiziente Methode zur Analyse von allen Biomolekülen wie Nukleinsäuren, Proteinen/Peptiden, Kohlenhydraten und Lipiden. Die MALDI‐TOF‐MS basierte Hochdurchsatz‐Genotypisierung von genetischen Heterogenitäten besitzt das Potential, eine Routinemethode zu werden. MALDI‐TOF‐MS kann für die Identifizierung von Proteinen und posttranslationalen Modifizierungen eingesetzt werden. Zusammenfassend ist die MAL‐DI‐TOF‐MS eine integrative Plattform‐Technologie in der Bioanalytik und Molekularen Medizin.
Die molekulare Welt des Lebensmittelgenusses: Auf den Geschmack gekommen
Authors: Schieberle Peter , Hofmann Thomas 
Published: December 2003

AbstractGeruch und Geschmack sind stofflich objektivierbare Qualitätsparameter bei Lebensmitteln. Originär im Lebensmittel vorkommende bzw. beim Herstellungsprozess aus Vorläufern entstehende geruchs‐ bzw. geschmackaktive Verbindungen weisen z. T. extreme Unterschiede in ihrer Geruchs‐ bzw. Geschmackswirksamkeit auf. Anhand verschiedener Beispiele wird der Einsatz aktivitätsorientierter Screeningverfahren zur Entschlüsselung wichtiger originärer Geruchs‐ und Geschmacksstoffe von Lebensmitteln vorgestellt. Diese als „Molekulare Sensorik”︁ bezeichneten Techniken haben kürzlich zur Identifizierung sehr potenter Naturstoffe mit interessanten sensorischen Eigenschaften geführt. Solche sensorisch aktiven Biomoleküle sind zudem gute Marker zur gezielten Steuerung industrieller Lebensmittelproduktionsprozesse, um optimale Geschmacksprofile zu erhalten.