bluuebunny/crossref_metadata_2025_split huggingface dataset:
On the Role of Structural Zinc in Bis(Cysteinyl) Protein Sequences
Authors: Meißner Axel , Haehnel Wolfgang , Vahrenkamp Heinrich 
Published: July 2007

AbstractBesides its functional role in many hydrolytic metalloenzymes, zinc acts as a structural component by being attached to bis(cysteinyl) protein sequences in some of the same enzymes, and in other metalloproteins and zinc fingers, and by being an essential constituent in metallothioneins. It is not always obvious whether the zinc‐binding proteins are pre‐organized for the incorporation of the metal or whether the zinc ion provides the structurizing power and stability for the observed peptide conformations. We have addressed the coordination chemistry aspects of this question by synthesizing zinc complexes of small model peptides and by determining their structures in solution by 2D NMR spectroscopy. The peptides chosen were of the terminally protected bis(cysteinyl) type: Cys‐Cys, Cys‐Gly‐Cys, Cys‐Phe‐Cys, and Cys‐Gly‐Ile‐Cys. The zinc ions fold these peptides into structures that can be superimposed on those of the natural proteins.
Phenoxyl Radical Complexes of Gallium, Scandium, Iron and Manganese
Authors: Adam Britta , Bill Eckhard , Bothe Eberhard , Goerdt Beatrix , Haselhorst Gabriele , Hildenbrand Knut , Sokolowski Achim , Steenken Steen , Weyhermüller Thomas , Wieghardt Karl 
Published: July 2007

AbstractThe hexadentate macrocyclic ligands 1,4,7‐tris(3,5‐dimethyl‐2‐hydroxybenzyl)‐1,4,7‐triazacyclononane (LH3), 1,4,7‐tris(3,5‐di‐tert‐butyl‐2‐hydroxybenzyl)‐1,4,7‐triazacyclononane (LBuH3) and 1,4,7‐tris(3‐tert‐butyl‐5‐methoxy‐2‐hydroxybenzyl)‐1,4,7‐triazacyclononane (L‐H3) form very stable octahedral neutral complexes LMIIIwith trivalent (or tetravalent) metal ions (GaIII, ScIII, FeIII, MnIII, MnIV). The following complexes have been synthesized: [LBuM], where M = Ga (1), Sc (2), Fe (3); [LBuMnIV]PF6(4′); [LM], where M = Ga (1 a), Sc (2 a), Fe (3 a); [LMnIV]PF6(4 a'); [LM], where M = Sc (2 b), Fe (3 b), MnIII(4 b); [LMnIV]2(ClO4)3(H3O)(H2O)3(4 b'). An electrochemical study has shown that complexes1, 2, 3, 1 a, 2 aand3 aeach display three reversible, ligand‐centred, one‐electron oxidation steps. The salts [LFeIII]ClO4and [LGaIII]ClO4, have been isolated as stable crystalline materials. Electronic and EPR spectra prove that these oxidations produce species containing one, two or three coordinated phenoxyl radicals. The Mössbauer spectra of3 aand [3 a]+show conclusively that both compounds contain high‐spin iron(III) central ions. Temperature‐dependent magnetic susceptibility measurements reveal that3 ahas anS= 5/2 and [3a]+anS= 2 ground state. The latter is attained through intramolecular antiferromagnetic exchange coupling between a high‐spin iron(III) (S1= 5/2) and a phenoxyl radical (S2= 1/2) (H= ‐ 2JS1S2;J= ‐ 80 cm−1). The manganese complexes undergo metal‐ and ligand‐centred redox processes, which were elucidated by spectroelectrochemistry; a phenoxyl radical MnIVcomplex [MnIVL]2+is accessible.
Chain Conformation and Metal…Metal Contacts in Dimers and Stacks of d8‐ML4Complexes: Electronic Effects
Authors: Aullón Gabriel , Alvarez Santiago 
Published: July 2007

AbstractA qualitative theoretical study is presented of the ligand effects on the strength of the M…︁M nonbonded interaction in dimers and stacks of8‐ML4chains. It is found that π‐acidic ligands enhance and π‐basic ligands weaken the M…︁M bonding interaction. A large amount of structural data can be rationalized with the help of a simple orbital model. Other features of the crystal structures of such compounds are correctly accounted for by semiempirical molecular orbital and electronic band calculations (extended Hückel level). Electronic effects control the deviation from linearity of stacks of complexes with mixed ligands, whereas coupled electronic and steric effects determine the conformational preference. The predictive capability of theoretical band calculations is found to be good.
Absolute Asymmetric Synthesis from Achiral Molecules in the Chiral Crystalline Environment
Authors: Sakamoto Masami 
Published: July 2007

AbstractThe current status of absolute asymmetric synthesis in a chiral crystalline environment has been reviewed. A number of topochemically controlled four‐center type photocycloadditions are described for a series of unsymmetrically substituted diolefin crystals and CT crystals. This concept has been applied to intramolecular photoreactions, and several successful absolute asymmetric syntheses have been achieved, involving Norrish Type II reaction, di‐π‐methane rearrangement, electrocyclization, thietane formation, oxetane formation, hydrogen abstraction by thiocarbonyl and alkenyl groups, and radical‐pair intermediates.
The Structure of α‐Gallium and Its Relationship to Deltahedral Clusters
Authors: Häuermann Ulrich , Lidin Sven , Simak Sergei I. , Abrikosov Igor A. 
Published: July 2007

AbstractWe present idealised geometrical models for the α‐Ga and β‐Ga structures based on two differently distorted, corrugated 36 nets. We investigated the structural stability of two sets of two‐dimensional and three‐dimensional model structures consisting of these corrugated nets as a function of the net puckering. We used the simple tight‐binding (TB) Hückel model with the structural energy difference theorem and the advanced full‐potential linear muffin‐tin orbital (FP‐LMTO) method in the framework of local‐density functional theory. Both methods show the existence of an optimum puckering angle that corresponds well to the situation in the experimental α‐Ga structure. The geometrical model of the α‐Ga structure follows the building principle of terminally coordinated deltahedral clusters, extended to two‐dimensional structures. The chemical bonding in α‐Ga is interpreted in terms of multicentre bonding within the corrugated 36 nets and two‐electron–two‐centre bonds connecting these nets.
Peptide Nucleic Acids with a Conformationally Constrained Chiral Cyclohexyl‐Derived Backbone
Authors: Lagriffoule Pierre , Eriksson Magdalena , Jensen Kristine Kilså , Nielsen Peter E. , Wittung Pernilla , Nordén Bengt , Buchardt Ole 
Published: July 2007

AbstractPeptide nucleic acid (PNA) is an achiral nucleic acid mimic with a backbone consisting of partly flexible aminoethyl glycine units. By replacing the aminoethyl portion of the backbone by an amino cyclohexyl moiety, either in the (S, S) or the (R, R) configuration, we have synthesized conformationally constrained PNA residues. PNA oligomers containing (S, S)‐cyclohexyl residues were able to form hybrid complexes with DNA or RNA, with little effect on the thermal stability (Tm = 1°C per (S, S) unit, depending on their number and the sequence). In contrast, incorporation of the (R, R) isomer resulted in a drastic decrease in the stability of the PNA‐DNA (or RNA) complex (Tm = −8°C per (R, R) unit). In PNA‐PNA duplexes, however, the (R, R)‐ and (S, S)‐cyclohexyl residues only exerted a minor effect on the stability, and the complexes formed with the two isomers are of opposite handedness, as evidenced from circular dichroism spectroscopy. In some cases the introduction of a single (S, S) residue in a PNA 15‐mer improves its sequence specificity for DNA or RNA. From the thermal stabilities and molecular modeling based on the solution structure of a PNA‐DNA duplex determined by NMR techniques, we conclude that the right‐handed helix can accommodate the (S, S) isomer more easily than the (R, R) isomer. Thermodynamic measurements of H and S upon PNA‐DNA duplex formation show that the introduction of an (S, S)‐cyclohexyl unit in the PNA does indeed decrease the entropy loss, indicating a more conformationally constrained structure. However, the more favorable entropic contribution is balanced by a reduced enthalpic gain, indicating that the structure constrained by the cyclohexyl group is not so well suited for DNA hybridization.
1‐Azafagomine: A Hydroxyhexahydropyridazine That Potently Inhibits Enzymatic Glycoside Cleavage
Authors: Bols Mikael , Hazell Rita G. , Thomsen Ib B. 
Published: July 2007

Abstract(3,4‐trans‐4,5‐trans)‐4,5‐dihydroxy‐3‐hydroxymethylhexahydropyrida‐zine (16) was synthesized in four steps from 2,4‐pentadienol (22) and 4‐phenyl‐triazolin‐3,5‐dione (18) in an overall yield of 32%. In the first step a Diels‐Alder reaction between 18 and 22 gave (π)‐2‐hydroxymethyl‐8‐phenyl‐1,6,8‐triazabicyclo[4.3.0]non‐3‐ene‐7,9‐dione (23c) in 88% yield. Epoxidation of 23c with trifluoromethyl(methyl)dioxirane, generated in situ, gave the trans epoxide 24c in 62% yield. Hydrolysis of the epoxide with perchloric acid gave stereoselectively (2,3‐trans‐3,4‐trans)‐3,4‐dihydroxy‐2‐hydroxy‐methyl‐8‐phenyl‐1,6,8‐triazabicyclo[4.3.0]‐nonane‐7,9‐dione (26) in 73% yield. In the fourth and final step, hydrazinolysis of 26 gave 16 in 84% yield. Pyridazine 16 was found to be a potent inhibitor of α‐and β‐glucosidase, isomaltase and glyco‐gen phosphorylase, while galactosidases and α‐mannosidase were not inhibited. The inhibition of β‐glucosidase is independent of pH, and was found to be due to unprotonated 16.
A Comprehensive Qualitative and Quantitative Molecular Orbital Analysis of the Factors Governing the Dichotomy in the Dinorcaradiene 1,6‐Methano[10]annulene system
Authors: Mealli Carlo , Ienco Andrea , Zoellner Robert W. , Hoyt Earle B. 
Published: July 2007

AbstractThe interconversion between the valence tautomers 1,6‐methano[10]‐annulene (1) and dinorcaradiene (2) was computationally investigated by qualitative extended Hückel molecular orbital methods (with CACAO visualization) and quantitative semiempirical and ab initio methods. The fundamental bonding interactions are described in terms of perturbation theory arguments for both tautomers, the influence of π‐acceptor or π‐donor substituents at the C11 position is rationalized, and bonding changes during the interconversion are monitored. The electronic molecular structural preferences can be modified by through‐space interactions, and residual C1–C6 σ bonding remains even at the >2.1 å separations of the annulenic structures. Hitherto unprecedented calculations of the geometries of C11 homo‐disubstituted derivatives of 1 and 2 have been carried out at semiempirical and ab initio levels to identify the more stable tautomer and find whether two stable minima may exist for each derivative. Searches at the AM 1 level to determine transition‐state structures for the interconversion of 1 to 2 and of their C11 homo‐disubstituted derivatives are also reported.
A New Family of Carbenerhodium(I) Complexes: Ligand Variation as The Key to Success
Authors: Werner Helmut , Schwab Peter , Bleuel Elke , Mahr Norbert , Steinert Paul , Wolf Justin 
Published: July 2007

AbstractA synthetic methodology to obtain square‐planar carbenerhodium(I) complexes of the general composition trans‐[RhCl(CRR′)(L)2] where L is a tertiary phosphane, arsane, or stibane has been developed. The starting material trans‐[RhCl(C2H4)(SbiPr3)2] (3) reacts with diazoalkanes RR′CN2 [RR′ = Ph2, Ph(C6H4X), (C6H4X)2, Ph(CF3), C12H8] under mild conditions to give the compounds trans‐[RhCl(CRR′)(SbiPr3)2] (4–11) almost quantitatively. On treatment of 3 with EtO2CCHN2 and PhC(N2)C(O)R, the olefinrhodium and diazoalkanerhodium compounds trans‐[RhCl{(E)‐C2H2(CO2Et)2}(SbiPr3)2] (12) and trans‐[RhCl{N2C(R)C(O)Ph}(SbiPr3)2] (13, 14) are obtained instead of carbene complexes. Displacement of the SbiPr3 ligands in 4 (R = R′ = Ph) by PiPr3, PiPr2Ph, PiPrPh2, PPh3, PPh2Me, AsiPr3, and SbEt3 leads to the corresponding carbene complexes trans‐[RhCl‐(CPh2)(L)2] (15–21) in high yields. The results of the X‐ray crystal structure analyses of 4 and 15 (L = PiPr3) illustrate that the different donor—acceptor properties of SbiPr3 and PiPr3 have little influence on the RhC bond length. The reactions of 4 and 15 with CO and CNtBu afford, by metal‐assisted CC coupling, diphenylketene Ph2CCO (23) and the corresponding imine Ph2CCNtBu (26). On treatment of 4 and 15 with ethene, however, two different olefinic products, 3,3‐diphenyl‐1‐propene (31) and 1,1‐diphenyl‐1‐propene (32), are formed. Compound 15 reacts with KBr, NaOPh, and NaC5H5 by substitution of the chloride to give trans‐[RhBr(CPh2)‐(PiPr3)2] (33), trans‐[Rh(OPh)(CPh2)‐(PiPr3)2] (34) and [C5H5Rh(CPh2)‐(PiPr3)] (35), and with HCl by oxidative addition to yield [RhCl2(CHPh2)‐(PiPr3)2](36).
The Role of Backbone Oxygen Atoms in the Organization of Nucleic Acid Tertiary Structure: Zippers, Networks, Clamps, and C‐H…︁O Hydrogen Bonds
Authors: Berger Imre , Egli Martin 
Published: July 2007

AbstractTight packing between structural elements is a prerequisite for molecular recognition and catalysis. In proteins, α‐helices and β‐sheets present the amino acid side chains on the surface while the polar amide bonds are buried. The opposite is found in double‐ and polystranded nucleic acids, where negatively charged phosphates occupy the surface and the side chains are hydrogen bonded in the core. Thus the question arises: How do densely packed nucleic acid molecules achieve close approach, despite the repulsion between phosphates that would appear to preclude tight contacts? One obvious answer is by mediating interstrand contacts through coordinated cations that can screen the negative charges. In this contribution, however, we highlight a variety of alternative direct interactions involving atoms of the sugar‐phosphate backbone that can promote tight packing of RNA and DNA into functional molecules. We have analyzed the existing nucleic acid crystal structures in terms of the presence of close contacts between backbone regions. In RNA, ribose 2′‐hydroxyl groups were observed to mediate such contacts in the majority of cases. However, their absence in DNA does not prevent oligodeoxynucleotides from packing tightly, aided by various interactions between backbone atoms.
A Contribution to the Asymmetric Synthesis of 3‐Amino β‐Lactams: The Diastereoselective [2+2] Cycloaddition Reaction of Chiral Aminoketene Equivalents with Enolizable Aldehyde‐Derived Imines
Authors: Palomo Claudio , Aizpurua Jesus M. , Legido Marta , Mielgo Antonia , Galarza Regina 
Published: July 2007

AbstractN‐[Bis(trimethylsilyl)methyl]imines 9 show unique chemical properties when compared with conventional imines. Their reaction with optically pure aminoketenes derived from dehydrochlorination of 14 and 15 affords the corresponding 3‐amino‐4‐alkyl‐β‐lactams 16 and 17 in good yields and high diastereoselectivities. The mild deprotection of bis(trimethylsilyl)methyl‐ and phenyloxazolidinone moieties with, respectively, cerium(IV) ammonium nitrate and lithium/ammonia or hydrogen/Pd(OH)2 allows the preparation of a variety of β‐lactam antibiotic building blocks.
A Square As4 and a Prismatic As6 Structure as Complex Ligands
Authors: von Hänisch Carsten , Fenske Dieter , Weigend Florian , Ahlrichs Reinhart , Ahlrichs R. , Weigend F. 
Published: July 2007

Abstract[As7(SiMe3)3] reacts with [(Cp*CoCl)2] to give black crystals of the ionic compound [(Cp*Co)3As6][CoCl4] (1). The cation [(Cp*Co)3As6]2+ consists of an As6 prism, with squares capped by [Cp*Co] fragments. When [(CptBuCoCl)2] is used instead of [(Cp*CoCl)2] in the reaction, dark red crystals of [(CptBuCo)3As6][CoCl3(thf)]2 (2) and green–black crystals of [(CptBuCo)2As4][Co3Cl8(thf)2] (3) can be obtained. The cation in 2 shows a structure very similar to that of 1. The [(CptBuCo)2As4]2+ cations in 3 can be described as a triple‐decker sandwich complex with two [CptBuCo]+ fragments bridged by an As4 ligand. Density functional calculations reproduce the experimental data of 1 and 3 and allow an interpretation of molecular electronic structure and bonding in these and related compounds with As replaced by P and Co by Fe.
1′,5′‐Anhydrohexitol Oligonucleotides: Hybridisation and Strand Displacement with Oligoribonucleotides, Interaction with RNase H and HIV Reverse Transcriptase
Authors: Hendrix Chris , Rosemeyer Helmut , De Bouvere Bart , Van Aerschot Arthur , Seela Frank , Herdewijn Piet 
Published: July 2007

AbstractHexitol nucleic acids (HNAs) with four natural bases form stable and sequence‐selective duplexes with RNA. This was investigated by Tm determinations and gel shift experiments. The CD spectra of an HNA‐RNA duplex show similarities with the CD spectra of the A‐form of dsRNA. Single‐stranded HNAs are able to induce strand displacement in a double‐stranded RNA sequence. An HNA‐RNA duplex is a poor substrate for RNase H, and can inhibit the RNase H‐mediated cleavage of a natural DNA‐RNA substrate. The HNA‐RNA hybrid enhances the activity of HIV reverse transcriptase.
Investigation of the Formation Reaction and Structural Characterization of the „Platinum Grignard Reagent”︁ [Pt(MgCl)2(THF)x] by Extended X‐ray absorption fine structure (EXAFS) and Other methods
Authors: Aleandri Lorraine E. , Bogdanović Borislav , Dürr Christine , Huckett Sara C. , Jones Deborah J. , Kolb Uwe , Lagarden Martin , Rozière Jacques , Wilczok Ursula 
Published: July 2007

AbstractThe „platinum Grignard reagent”︁ [Pt(MgCl)2(THF)x] (2), obtained by the reaction of PtCl2 and Et2Mg in a 1:2 molar ratio, as well as finely divided platinum (Pt*)—a possible intermediate formed during the preparation of 2—have been investigated by EXAFS spectroscopy at the PtLII edge. Parallel investigations were carried out on Pt* obtained from PtCl2 and (9,10–dihydro‐9,10‐an‐thracenediyl)tris(tetrahydrofuran)magnesium (MgA), and on 2 obtained from Pt*, MgA, and MgCl2. The EXAFS results suggest that Pt* consists of extremely small particles (≈ 5–11 Å) with strongly reduced Pt–Pt distances compared to bulk Pt (≈ 0.09 Å). The EXAFS spectra of 2 indicate the presence of Mg shells in addition to Pt shells in the Pt environment; Mg atoms are at a bonding distance from Pt atoms (2.78–2.80 Å). These results suggest that 2 consists of very small Pt–Mg clusters and confirm their formation from organomagnesium reagents and PtCl2 or Pt*.
Electronic and Geometrical Structures of Dialuminoxane, Diboroxane, and Their Sulfur Analogues: Ab Initio Study of H2X‐Y‐XH2 (X = Al, B; Y = O, S) Compounds
Authors: Boiteau Laurent , Demachy Isabelle , Volatron François 
Published: July 2007

AbstractThe structures of H2XYXH2 compounds (X = B, Al; Y = O, S) have been studied by means of ab initio calculations at the MP4/6‐311 G**/MP2/6‐311G** level. The potential energy surface (PES) of the aluminoxane species H2AlOAlH2 is rather flat: the energy differences between the various located extrema are less than 1.5 kcal mol−1, and this shows that the observed geometrical preference is not due to electronic factors. For the sulfur analogues H2XSXH2 (X = B, Al), three minima are located on the PES: a planar C2v structure, a Cs structure in which one hydrogen atom bridges the two X atoms, and a C2v structure with two bridging hydrogen atoms. For H2AlSAlH2, the Cs structure is the most stable, and the planar C2v and the dibridged structures lie 3.2 and 6.7 kcal mol−1 higher, respectively. Qualitatively similar results are obtained for H2BSBH2: the Cs and planar C2v structures are nearly isoenergetic, and the dibridged minimum lies 19.4 kcal mol−1 above the planar C2v minimum. These results are rationalized by analyzing the ability of these systems to bend, depending on the nature (oxygen or sulfur) of the central Y atom. The conjugation in these species is also discussed, and calculations on model systems H2XYH (X = B, Al; Y = O, S) are presented.
A Study of the Gas‐Phase Reactivity of Neutral Alkoxy Radicals by Mass Spectrometry: α‐Cleavages and Barton‐type Hydrogen Migrations
Authors: Hornung Georg , Schalley Christoph A. , Dieterle Martin , Schröder Detlef , Schwarz Helmut 
Published: July 2007

AbstractThe reactivity of neutral alkoxy radicals in the absence of any interfering intermolecular interactions is investigated by means of the recently introduced method of neutral and ion decomposition difference (NIDD) spectra. These are obtained from quantitative analysis of the corresponding neutralization–reionization (NR) and charge reversal (CR) mass spectra. The following trends emerge: alkoxy radicals with short (C1C3) or branched alkyl chains give rise to α‐cleavage products, whereas longer‐chained alkoxy radicals undergo 1,5‐hydrogen migrations from carbon to oxygen, that is, Barton‐type chemistry. This facile rearrangement has been studied in detail for n‐pentoxy radicals by isotopic labeling experiments and computation at the Becke 3 LYP/6‐31 G* level of theory. Further, the NIDD spectra of 3‐methylpentoxy radicals permit for the first time the identification of the diastereoselectivity of the gas‐phase hydrogen migrations. The results from the NIDD method are compared to those from earlier studies in the condensed phase. This new mass spectrometric approach is suggested as a tool for the examination of intramolecular reactions of free alkoxy radicals which can usefully complement theoretical studies.
Drug Discovery by Combinatorial Chemistry—the Development of a Novel Method for the Rapid Synthesis of Single Compounds
Authors: Terrett Nicholas K. , Gardner Mark , Gordon David W. , Kobylecki Ryszard J. , Steele John 
Published: July 2007

AbstractThrough the advantages conferred in the speed of synthesis, combinatorial chemistry is making a significant impact on the process of drug discovery. The mix‐and‐split paradigm has been an effective method for the production of compound mixtures, although there is now a need for new, fast library approaches to generate well‐characterised single compounds. Having already demonstrated the successful preparation and application of library mixtures, we have now developed a novel combinatorial method for the production of single compounds.
Metallic Hydrogen at High Pressures and Temperatures in Jupiter
Authors: Nellis W. J. 
Published: July 2007

AbstractElectrical conductivities of hydrogen were measured recently at 93–180 GPa (0.93–1.8 Mbar), 0.28–0.36 mol cm−3, and 2000–4000 K. Shock temperatures were measured up to values of 5200 K at 0.83 Mbar. The conductivities indicate a continuous transition from a semiconducting to metallic fluid at 140 GPa and 3000 K. The results indicate that hydrogen becomes metallic much closer to the surface of Jupiter than thought previously and that Jupiter has no core–mantle boundary.
Synthesis of Rotaxane Assemblies
Authors: Dünnwald Thomas , Jäger Ralf , Vögtle Fritz 
Published: July 2007

AbstractThe preparative syntheses of new nano‐sized rotaxane assemblies are described, namely, of the first tris[2]rotaxane,15, the first bis[2]rotaxane connected at the axles,4, and the first [3]rotaxane with two different axles,12(unsymmetrical “bis[2]rotaxane”). These syntheses were made possible by the directed incorporation of a sulfonamide group into the amide‐linked rotaxane building blocks. The sulfonamide moiety can be used as a functional group in alkylation reactions, allowing the further attachment of building blocks in good yields. The successful synthesis of these new rotaxane architectures demonstrates the synthetic potential of this sulfonamide strategy in the preparation of multirotaxane and dendritic rotaxane structures.