Nucleophilic additions of phosphines to a π-cyclobutadiene ligand

Nucleophilic addition of phosphines to the cyclobutadiene ring in [(h⁴-C₄H₄)Fe(CO)₂ NO]⁺PF₆^? leads to the formation of (exo-phosphonium-h³-cyclobutenyl) dicarbonylnitrosyliron hexafluorophosphate.     

Coexistence and coupling of superconductivity and magnetism in thin film structures

Superconducting and magnetic order are usually mutually exclusive, and are found to coexist in relatively few materials. We have obtained direct evidence for a spin-density wave (SDW) coexisting with bulk superconductivity in a ferromagnetic-superconducting trilayer. In the superconducting state the amplitude of the SDW is enhanced and modeling the data also suggests a pi/2 phase shift of one component of the SDW, implying a profound coupling of these two forms of order.     

Mimics of ganglioside GM1 as cholera toxin ligands: replacement of the GalNAc residue

Two new cholera toxin (CT) ligands (4 and 5) are described. The new ligands were designed starting from the known GM1 mimics 2 and 3 by replacement of their GalNAc residue with the C4 isomer GlcNAc. As predicted by molecular modelling, the conformational properties of the equivalent pairs 2-4 and 3-5 are very similar and their affinity for CT is of the same order of magnitude. NMR experiments have also proved that 5 occupies the GM1-binding site of the toxin and have revealed its bound conformation.     

Scattering in optical fibres: analysis of mode degeneration

Mode degeneration and mode conversion in multimode optical fibres are the main subjects of this work. In particular it is shown how fluctuations of the core shape can cause a rapid degeneration among the modes with the same azimuthal mode number. This allows a rigorous single variable approach to the second problem obtaining evident simplifications. The formalism proposed is applied to the study of mode distribution in the general case of microbending, of core-radius fluctuations in fibres with power law profiles and for simultaneous presence of microbending and ellipticity in parabolic fibres with differential mode attenuation.     

Real-time holograms generated by second-harmonic cross correlation of object and reference optical wave fields

Three-dimensional holographic images of extended diffusing objects are simultaneously recorded and reconstructed by optical cross correlation in a second-order nonlinear crystal. An interaction geometry in which the phase-matched object and reference fields propagate slightly noncollinearly is particularly convenient for producing these second-harmonic-generated holograms.     

Structural, infrared, and density functional theory studies of N,N,N',N'-Tetramethylimidazolidinium dichloride: a model for cation-anion association of headgroups and counterions in the interfacial regions of gemini micelles

N,N,N',N'-Tetramethylimidazolidinium dichloride (1-Im-1 2Cl) has been studied as a model system for cation-anion interactions in the interfacial regions of gemini micelles by X-ray crystallography, density functional theory (DFT) calculations, and infrared spectroscopy. Single crystals of 1-Im-1 2Cl contain 1-Im-1 dications, whose five-membered rings adopt a distorted envelope conformation. Eight chloride anions surround each dication, two of which are cradled above and below the five-membered ring (apical) and six of which are dispersed about the periphery of the ring (equatorial). The cations and anions are linked in the solid state by an extensive network of weak C-H...Cl hydrogen bonds that involve all of the H atoms of the dication. The calculated (DFT at the 6-31+G(d) level) structure of the asymmetric unit, which consists of a dication and two apical chloride ions, closely resembles the equivalent unit in the crystal structure with respect to bond distances and angles, the conformation of the 1-Im-1 ring, and the nature and location of the C-H...Cl hydrogen bonds. The calculated IR spectrum predicts a number of absorptions in the 3000 cm(-1) region, assigned as C-H...Cl stretching modes, which are consistent with the presence of an intense band in the observed IR spectrum of the crystals. Over all, this study supports the notion that apical chloride ions interact more strongly with gemini surfactant headgroups by forming multiple hydrogen bonds in ion pairs of a type that cannot be present in the corresponding ion pairs of quaternary headgroups with counterions of single-chain surfactants.     

Solid-state changes in ligand-to-metal charge-transfer spectra of (NH3)5Ru(III)(2,4-dihydroxybenzoate) and (NH3)5Ru(III)(xanthine) chromophores

Five distorted-octahedral complexes containing (NH3)5Ru(III)L ions, where L = 2,4-dihydroxybenzoate or a xanthine, have been studied using a combination of X-ray crystallography, solution and polarized single-crystal electronic absorption spectroscopy, and first principles electronic structure computational techniques. Both yellow (2) and red (3) forms of the complex (NH3)5Ru(III)L, where L = 2,4-dihydroxybenzoate, as well as three xanthine complexes in which L = hypoxanthine-kappaN(7) (4), 7-methylhypoxanthine-kappaN(9) (5), and 1,3,9-trimethylxanthine-kappaN(7) (6) were examined. In the solid state, some of these complexes exhibit split low-energy ligand-to-metal charge-transfer (LMCT) bands. Traditional solid-state effects, such as ligand pi-pi overlap or hydrogen bonding that might lead to splitting of electronic absorption bands, were probed in an attempt to identify the origins of these unusual observations. For comparison, companion studies were carried out for spectroscopically normal reference complexes of the same ligands. Time-dependent density-functional theory (TD-DFT) calculations, employing modified B3LYP-type functionals with increased contributions of exact exchange, attribute the color change in the crystalline complexes 2 and 3 to pi(ligand) --> Ru[d(pi)] LMCT bands which, in the red form (3), arise from ligand donor pi-orbitals split by strongly overlapping phenyl moieties in centrosymmetric (NH3)5Ru(III)(2,4-dihydroxybenzoate) dimers. Complex 5 does not show split visible absorptions, whereas both the polarizations and energies of the split visible absorptions shown by 4 and 6 also suggest assignment as LMCT. No support is found for relating the split absorptions of 4 and 6 to the details of pi-pi xanthine overlap in the solid state; indeed, complex 4 enjoys considerably less pi-stacking overlap than does 5. We feel compelled to attribute the split absorptions in crystalline 4 and 6 to the emergence of a LMCT transition originating in the carbonyl lone pair, potentially deriving intensity from the significant intramolecular N-H...O hydrogen bonding present in both 4 and 6 (but not in 5). The electronic structure calculations suggest an O(n) --> Ru[d(sigma*)] LMCT transition; however, this novel assignment must be considered tentative.     

A NMR and computational study of Smac mimics targeting both the BIR2 and BIR3 domains in XIAP protein

In this paper we report an extensive NMR analysis of small ligands (Smac mimics) complexed with different constructs of XIAP. The mimics-binding site of XIAP is known as the BIR3 domain - primary, and the linker BIR2 region - secondary site. Interactions between the BIR3 domain and Smac mimics have been extensively studied by X-ray but, as of today, there are scarce data about the interaction between BIR2, or the whole linker-BIR2-BIR3 construct, and Smac mimics. In order to characterize our Smac mimics, we performed a STD NMR study between our 4-substituted, 1-aza-2-oxobicyclo[5.3.0]decane scaffold-based molecules and three different XIAP fragments: single BIR2 and BIR3 domains, and bifunctional linker-BIR2-BIR3. The results were integrated with docking calculations and molecular dynamics simulations. NMR data, which are consistent with biological tests, indicated that the two BIR subunits interact differently with our Smac mimics and suggest that the ligands enter into more intimate contact with the linker-BIR2-BIR3. In conclusion, we observe that the SMAC mimics showed with the construct linker-BIR2-BIR3 a series of NOE contacts that were not observed in the mono-domain ligand:BIR2 or :BIR3 complexes. So, in agreement with the computational models we believe that the linker moieties of the binding site play a key role in the stability of the protein complex.     

Cyclic RGD peptidomimetics containing bifunctional diketopiperazine scaffolds as new potent integrin ligands

The synthesis of eight bifunctional diketopiperazine (DKP) scaffolds is described; these were formally derived from 2,3-diaminopropionic acid and aspartic acid (DKP-1-DKP-7) or glutamic acid (DKP-8) and feature an amine and a carboxylic acid functional group. The scaffolds differ in the configuration at the two stereocenters and the substitution at the diketopiperazinic nitrogen atoms. The bifunctional diketopiperazines were introduced into eight cyclic peptidomimetics containing the Arg-Gly-Asp (RGD) sequence. The resulting RGD peptidomimetics were screened for their ability to inhibit biotinylated vitronectin binding to the purified integrins α(v)β(3) and α(v)β(5), which are involved in tumor angiogenesis. Nanomolar IC(50) values were obtained for the RGD peptidomimetics derived from trans DKP scaffolds (DKP-2-DKP-8). Conformational studies of the cyclic RGD peptidomimetics by (1)H?NMR spectroscopy experiments (VT-NMR and NOESY spectroscopy) in aqueous solution and Monte Carlo/Stochastic Dynamics (MC/SD) simulations revealed that the highest affinity ligands display well-defined preferred conformations featuring intramolecular hydrogen-bonded turn motifs and an extended arrangement of the RGD sequence [Cβ(Arg)-Cβ(Asp) average distance ≥8.8??]. Docking studies were performed, starting from the representative conformations obtained from the MC/SD simulations and taking as a reference model the crystal structure of the extracellular segment of integrin α(v)β(3) complexed with the cyclic pentapeptide, Cilengitide. The highest affinity ligands produced top-ranked poses conserving all the important interactions of the X-ray complex.