Gerd Buntkowsky

Ruthenium Nanoparticles inside Porous [Zn4O(bdc)3] by Hydrogenolysis of Adsorbed [Ru(cod)(cot)]: A Solid-State Reference System for Surfactant-Stabilized Ruthenium Colloids

The gas-phase loading of [Zn4O(bdc)3] (MOF-5; bdc = 1,4-benzenedicarboxylate) with the volatile compound [Ru(cod)(cot)] (cod = 1,5-cyclooctadiene, cot = 1,3,5-cyclooctatriene) was followed by solid-state (13)C magic angle spinning (MAS) NMR spectroscopy. Subsequent hydrogenolysis of the adsorbed complex inside the porous structure of MOF-5 at 3 bar and 150 degrees C was performed, yielding ruthenium nanoparticles in a typical size range of 1.5-1.7 nm, embedded in the intact MOF-5 matrix, as confirmed by transmission electron microscopy (TEM), selected area electron diffraction (SAED), powder X-ray diffraction (PXRD), and X-ray absorption spectroscopy (XAS). The adsorption of CO molecules on the obtained Ru@MOF-5 nanocomposite was followed by IR spectroscopy. Solid-state (2)H NMR measurements indicated that MOF-5 was a stabilizing support with only weak interactions with the embedded particles, as deduced from the surprisingly high mobility of the surface Ru-D species in comparison to surfactant-stabilized colloidal Ru nanoparticles of similar sizes. Surprisingly, hydrogenolysis of the [Ru(cod)(cot)]3.5@MOF-5 inclusion compound at the milder condition of 25 degrees C does not lead to the quantitative formation of Ru nanoparticles. Instead, formation of a ruthenium-cyclooctadiene complex with the arene moiety of the bdc linkers of the framework takes place, as revealed by (13)C MAS NMR, PXRD, and TEM.

Pyridine-15N: A mobile NMR sensor for surface acidity and surface defects of mesoporous silica

The hydrogen bond interaction of pyridine with the silanol groups of the inner surfaces of MCM-41 and SBA-15 ordered mesoporous silica has been studied by a combination of solid-state NMR techniques. The pore diameters were varied between 3 and 4 nm for MCM-41 and between 7 and 9 nm for SBA-15. 1 H MAS experiments performed under magic angle spinning (MAS) conditions in the absence and the presence of pyridine-d 5 reveal that the large majority of silanol groups are located in the inner surfaces, isolated from each other but able to form hydrogen bonds with pyridine. On the other hand, low- and room-temperature 1 5 N CPMAS and MAS experiments (CP ≡ cross-polarization) performed on pyridine- 1 5 N show that at low concentrations all pyridine molecules are involved in hydrogen bonds with the surface silanol groups. In the presence of an excess of pyridine, a non-hydrogen-bonded pyridine phase is observed at 120 K in the slow hydrogen bond exchange regime and associates with an inner core phase. From these measurements, the number of pyridine molecules bound to the inner surfaces corresponding to the number of silanol groups could be determined to be n O H 3 nm - 2 for MCM-41 and 3.7 nm - 2 for SBA-15. At room temperature and low concentrations, the pyridine molecules jump rapidly between the hydrogen-bonded sites. In the presence of an excess of pyridine, the hydrogen-bonded binding sites are depleted as compared to low temperatures, leading to smaller apparent numbers n O H . Using a correlation established previously between the 1 5 N and 1 H chemical shifts and the NHO hydrogen bond geometries, as well as with the acidity of the proton donors, the distances in the pyridine-hydroxyl pairs were found to be about r H N = 1.68 A, r O H = 1.01 A, and r O N = 2.69 A. This geometry corresponds in the organic solid state to acids exhibiting in water a pK a of about 4. Roomtemperature 1 5 N experiments on static samples of pyridine- 1 5 N in MCM-41 at low coverage show a residual 1 5 N chemical shift anisotropy, indicating that the jumps of pyridine between different different silanol hydrogen bond sites is accompanied by an anisotropic reorientational diffusion. A quantitative analysis reveals that in this regime the rotation of pyridine around the molecular C 2 axis is suppressed even at room temperature, and that the angle between the Si-O axes and the OH axes of the isolated silanol groups is about 47°. These results are corroborated by 2 H NMR experiments performed on pyridine-4-d 1 . In contrast, in the case of SBA-15 with the larger pore diameters, the hydrogen bond jumps of pyridine are associated with an isotropic rotational diffusion, indicating a high degree of roughness of the inner surfaces. This finding is correlated with the finding by 2 9 Si CPMAS of a substantial amount of Si(OH) 2 groups in SBA-15. in contrast to the MCM-41 materials. The Si(OH) 2 groups are associated with surface defects, exhibiting not only silanol groups pointing into the pore center but also silanol groups pointing into other directions of space including the pore axes, leading to the isotropic surface diffusion. All results are used to develop molecular models for the inner surface structure of mesoporous silica which may be a basis for future simulations of the surfaces of mesoporous silica.

Triazole bridge: disulfide-bond replacement by ruthenium-catalyzed formation of 1,5-disubstituted 1,2,3-triazoles.

About one fourth of the peptidic macromolecular structures deposited in the protein data base (PDB) contain at least one disulfide bridge. In nature, disulfide bonds are formed in a milieu where oxidizing conditions prevail, for example, on the cell surface or in the extracellular matrix. Many proteins benefit from disulfide contributions to their conformational stability. In particular, the defined tertiary folding of oligopeptides smaller than 30 residues essentially relies on macrocyclization through the cystine motif because of the restricted number of noncovalent intramolecular interactions available. Moreover, formation of the disulfide pattern results in structural rigidity of the peptidic framework, as for example, in the family of cystine knot miniproteins, leading to conformationally constrained scaffolds with extraordinary thermal stability and resistance against proteolytic degradation. Hence, the discovery and development of disulfidebridged peptides suitable for diagnostic and therapeutic applications remains a field of intense research. The in-vitro generation of disulfide bonds in peptides is usually achieved post-synthetically and mediated by DMSO, air oxygen, or other oxidizing agents. Although this reaction step can be achieved under relatively mild conditions in solution, it remains one of the most demanding obstacles towards high-yield peptide synthesis, especially for disulfiderich species in which the controlled regiospecific formation of several disulfide bonds is not trivial to control. In addition, to suppress unwanted intermolecular reactions of the thiol groups of individual peptides, oxidative folding usually has to be conducted in highly diluted solutions. In spite of the use of gluthathione-based redox buffers, polymer-supported oxidation systems, macrocyclization on the solid support and/or orthogonal protecting groups, control over the topology of the disulfide bridges formed is still a challenge. 5] In view of these difficulties and to improve the redox stability of bridged peptides, several routes towards synthetic disulfide surrogates have been developed. Straightforward approaches usually employ thioether, olefin, or alkane-based isosters. However, cystathione bridges require multiple synthetic steps and careful choice of orthogonal protection, and dicarba bridges give cis/trans isomers during ring-closing metathesis (RCM). Only an additional purification step or the subsequent palladium-catalyzed hydrogenation of the unsaturated species to the corresponding alkane leads to a construct with defined configuration. In 2004, Meldal et al. described the utility of copper(I)catalyzed azide–alkyne cycloaddition (CuAAC) for a triazole-based disulfide replacement. Owing to the compelling characteristics of this prototypic “click” reaction, it has been extensively applied in peptide chemistry exploiting the almost perfect orthogonality to side-chain reactivities. The introduction of 1,4-disubstituted 1,2,3-triazoles into peptides has also been used to mimic and rigidify conformations of the amide backbone. Moreover, a variety of examples of CuAAC-based macrocyclizations of peptides in solution and on solid supports has been reported. Using the same azideand alkyne-functionalized buidling blocks, 1,5-disubstituted 1,2,3-triazoles can be generated in the ruthenium(II)-catalyzed variant (RuAAC) of the CuAAC. This reaction expands the range of peptidomimetic structures selectively accessible from the same precursor and having different biological activities governed by the architecture of the incorporated triazole. To our knowledge, 1,5-disubstitiuted 1,2,3-triazoles have not been taken into consideration as disulfide mimics to date. Herein, we report the facile introduction of 1,4and 1,5disubstituted 1,2,3-triazoles into a monocyclic variant of the sunflower trypsin inhibitor-I (SFTI-1[1,14], 1; Figure 1) and show that the macrocyclic peptidomimeticum 2 with the “1,5” substitution pattern retains nearly full biological activity in contrast to the “1,4” variants 3 and 4. The choice of 1 as the model peptide for the investigation of triazole-based disulfide replacements had several reasons. SFTI-1 is a small, though very potent, inhibitor of trypsin. Therefore, the influence of different modes of macrocyclization on the bioactivity of the corresponding synthetic variant can be routinely examined by serine protease inhibition assays. [*] M. Empting, Dr. O. Avrutina, Dr. R. Meusinger, S. Fabritz, M. Reinwarth, Prof. Dr. H. Kolmar Clemens-Sch pf-Institut f r Organische Chemie und Biochemie Technische Universit t Darmstadt Petersenstrasse 22, 64287 Darmstadt (Germany) Fax: (+49)6151-16-5399 E-mail: kolmar@biochemie-tud.de Homepage: http://www.chemie.tu-darmstadt.de/kolmar

Simulation and analysis of magnetic resonance elastography wave images using coupled harmonic oscillators and Gaussian local frequency estimation.

New methods for simulating and analyzing Magnetic Resonance Elastography (MRE) images are introduced. To simulate a two-dimensional shear wave pattern, the wave equation is solved for a field of coupled harmonic oscillators with spatially varying coupling and damping coefficients in the presence of an external force. The spatial distribution of the coupling and the damping constants are derived from an MR image of the investigated object. To validate the simulation as well as to derive the elasticity modules from experimental MRE images, the wave patterns are analyzed using a Local Frequency Estimation (LFE) algorithm based on Gauss filter functions with variable bandwidths. The algorithms are tested using an Agar gel phantom with spatially varying elasticity constants. Simulated wave patterns and LFE results show a high agreement with experimental data. Furthermore, brain images with estimated elasticities for gray and white matter as well as for exemplary tumor tissue are used to simulate experimental MRE data. The calculations show that already small distributions of pathologically changed brain tissue should be detectable by MRE even within the limit of relatively low shear wave excitation frequency around 0.2 kHz.

Air-Stable Gold Nanoparticles Ligated by Secondary Phosphine Oxides as Catalyst for the Chemoselective Hydrogenation of Substituted Aldehydes: a Remarkable Ligand Effect

Air-stable and homogeneous gold nanoparticles (AuNPs, 1a-5a) ligated by various secondary phosphine oxides (SPOs), [R(1)R(2)P(O)H] (R(1) = Naph, R(2) = (t)Bu, L1; R(1) = R(2) = Ph, L2; R(1) = Ph, R(2) = Naph, L3; R(1) = R(2) = Et, L4; R(1) = R(2) = Cy, L5; R(1) = R(2) = (t)Bu, L6), with different electronic and steric properties were synthesized via NaBH4 reduction of the corresponding Au(I)-SPO complex. These easily accessible ligands allow the formation of well dispersed and small nanoparticles (size 1.2-2.2 nm), which were characterized by the use of a wide variety of techniques, such as transmission electron microscopy, thermogravimetric analysis, UV-vis, energy-dispersive X-ray, X-ray photoelectron spectroscopy (XPS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR FT-IR), and cross polarization magic angle spinning (CP MAS) NMR spectroscopy. A pronounced ligand effect was found, and CP MAS NMR experiments enabled us to probe important differences in the polarity of the P-O bond of the SPOs coordinated to the nanoparticle surface depending on the type of substituents in the ligand. AuNPs containing aryl SPOs carry only SPO anions and are highly selective for aldehyde hydrogenation. AuNPs of similar size made with alkyl SPOs contain also SPOH, hydrogen bonded to SPO anions. As a consequence they contain less Au(I) and more Au(0), as is also evidenced by XPS. They are less selective and active in aldehyde hydrogenation and now show the typical activity of Au(0)NPs in nitro group hydrogenation.

Mechanism of nuclear spin initiated para-H2 to ortho-H2 conversion.

In this paper a quantitative explanation for a diamagnetic ortho/para H2 conversion is given. The description is based on the quantum-mechanical density matrix formalism originally developed by Alexander and Binsch for studies of exchange processes in NMR spectra. Only the nuclear spin system is treated quantum-mechanically. Employing the model of a three spin system, the reactions of the hydrogen gas with the catalysts are treated as a phenomenological rate process, described by a rate constant. Numerical calculations reveal that for nearly all possible geometrical arrangements of the three spin system an efficient spin conversion is obtained. Only in the chemically improbable case of a linear group H-X-H no spin conversion is obtained. The efficiency of the spin conversion depends strongly on the lifetime of the H-X-H complex and on the presence of exchange interactions between the two hydrogens. Even moderate exchange couplings cause a quench of the spin conversion. Thus a sufficiently strong binding of the dihydrogen to the S spin is necessary to render the quenching by the exchange interaction ineffective.

Braces for the peptide backbone: insights into structure-activity relationships of protease inhibitor mimics with locked amide conformations.

The architecture of protein macromolecules fundamentallydepends on the sequential arrangement of peptide backbonebonds in defined conformations. Among the three torsionangles(f,y,andw)presentateachaminoacid,itistheamidebond (w) which is intrinsically hindered as a result of itspartial double-bond character and it is thus more or lessrestricted to either a trans or a cis conformation (Figure 1).

Single-Source-Precursor Synthesis of Hafnium-Containing Ultrahigh-Temperature Ceramic Nanocomposites (UHTC-NCs)

Amorphous SiHfBCN ceramics were prepared from a commercial polysilazane (HTT 1800, AZ-EM), which was modified upon reactions with Hf(NEt2)4 and BH3·SMe2, and subsequently cross-linked and pyrolyzed. The prepared materials were investigated with respect to their chemical and phase composition, by means of spectroscopy techniques (Fourier transform infrared (FTIR), Raman, magic-angle spinning nuclear magnetic resonance (MAS NMR)), as well as X-ray diffraction (XRD) and transmission electron microscopy (TEM). Annealing experiments of the SiHfBCN samples in an inert gas atmosphere (Ar, N2) at temperatures in the range of 1300-1700 °C showed the conversion of the amorphous materials into nanostructured UHTC-NCs. Depending on the annealing atmosphere, HfC/HfB2/SiC (annealing in argon) and HfN/Si3N4/SiBCN (annealing in nitrogen) nanocomposites were obtained. The results emphasize that the conversion of the single-phase SiHfBCN into UHTC-NCs is thermodynamically controlled, thus allowing for a knowledge-based preparative path toward nanostructured ultrahigh-temperature stable materials with adjusted compositions.

Secondary phosphine oxides as pre-ligands for nanoparticle stabilization

The synthesis of ruthenium nanoparticles (RuNPs) using secondary phosphine oxides (SPOs) as ligands is reported. These easily accessible ligands allow the formation of small nanoparticles in the size range of 1–2 nm which display a high efficiency for hydrogenation of aromatics with TOFs up to 2700 mol h−1.

New Insight into the Mode of Action of Nickel Superoxide Dismutase by Investigating Metallopeptide Substrate Models

For the first time, the existence of a substrate adduct of a nickel superoxide dismutase (NiSOD) model, based on the first nine residues from the N terminus of the active form of Streptomyces coelicolor NiSOD, has been proven and the adduct has been isolated. This adduct is based on the cyanide anion (CN(-)), as a substrate analogue of the superoxide anion (O(2)(*-)), and the nickel metallopeptide H-HCDLPCGVY-NH(2)-Ni. Spectroscopic studies, including IR, UV/Vis, and liquid- and solid-state NMR spectroscopy, show a single nickel-bound cyanide anion, which is embedded in the metallopeptide structure. This complex sheds new light on the question of whether the mode of action of the NiSOD enzyme is an inner- or outer-sphere mechanism. Whereas discussion was previously biased in favor of an outer-sphere electron-transfer mechanism due to the fact that binding of cyanide or azide moieties to the nickel active site had never been observed, our results are a clear indication in favor of the inner-sphere electron-transfer mechanism for the disproportionation of the O(2)(*-) ion, whereby the substrate is attached to the Ni atom in the active site of the NiSOD.