Stereochemistry of protected ornithine side chains of gramicidin S derivatives: X-ray crystal structure of the bis-Boc-tetra-N-methyl derivative of gramicidin S

An X-ray crystallographic analysis of the bis-Ndelta-Boc-tetra-Nalpha-methyl derivative of gramicidin S, cyclo(-Val-MeOrn(Boc)-Leu-d-MePhe-Pro-)2, was undertaken successfully (R-factor = 0.088). As expected, the main chain adopts an antiparallel pleated beta-sheet conformation, but the pleated sheet is slightly twisted, and the sense of twisting is opposite to that found in the reported crystal structures of the gramicidin S-urea complex and the bis-Ndelta-(trichloroacetyl) and bis-Ndelta-(m-bromobenzoyl) derivatives of gramicidin S. In agreement with the observed resistance toward N-methylation, the urethane NH groups of the protected Orn side chains are hydrogen bonded to the carbonyl groups of the d-Phe residues. However, the side-chain-main-chain hydrogen bonding is in the i --> i - 3 mode, although hydrogen bonding in the i --> i + 2 mode was deduced from a 1H NMR study of protected gramicidin S derivatives and was actually found in the crystal structures of the diacylated gramicidin S.     

3 + 2] Cycloreversion of bicyclo[m.3.0]alkan-3-on-2-yl-1-oxonium ylides to alkenyloxyketenes. Stereospecific aspect

Rhodium(II)-catalyzed intramolecular reaction of diazoketones 1 bearing a cyclic ethereal moiety transiently formed bicyclo[m.3.0]octan-3-one-1-oxonium-2-ylides (2), which underwent sigmatropic and stereospecific [3 + 2] cycloreversion reaction to form alkenyloxyketenes 3. The ketenes were efficiently trapped by methanol to form the corresponding esters 4. Mechanistic studies revealed that the size of ethereal ring can be variable at least from THF to the THP, oxepane, and oxocane moiety, i.e., m = 3-6. On the other hand, the size of the ylide ring containing the carbonyl unit is limited to a five-membered ring. The cycloreversion was found to be stereospecific as was proven by the reactions of diastereoisomeric pairs bearing a methyl group at the bond-cleaving position. From threo isomers 7, (E)-alkenyloxyacetates 15 were exclusively formed (77-84%), whereas from erythro isomers 8, (Z)-isomers 16 were formed (80-88%). Mechanism of the cleavage from diazoacetonyl-substituted cyclic ethers to alkenyloxyketenes via bicyclic oxonium ylides was analyzed on the basis of calculations employing the hybrid density functional B3LYP and the highly correlated quadratic configuration interaction QCISD method to reveal that the concerted [3 + 2] cycloreversion is the key step of this reaction.     

Application of an Al-Ti alloy target in surface analysis by X-ray photoelectron spectroscopy

An X-ray target of an Al-Ti alloy was used in the determination of the composition of the surface layer (0-200 A thick) of a Cu-Zn alloy.     

Effects of titanium addition on the microstructure of carbon/copper composite materials

Carbon(C)/copper(Cu)-based materials with high thermal conductivity and good stability at high temperatures were developed by adding a small amount of titanium, which has a low enthalpy of alloy formation with C and Cu. The isotropic fine-grained nuclear grade graphite and felt type C/C composite, which were impregnated by Cu and titanium, provided 1.3 times higher thermal conductivity at 1200 K than the original carbon materials. Microstructural analysis showed that the increase of thermal conductivity is due to the formation of titanium compounds at the C/Cu interface. These carbon-based materials could be a candidate material for the plasma facing components of fusion devices.     

Numerical and experimental investigation of laser propulsion

Aiming at the application of laser propulsion to stratospheric flight, we propose here a repetitive propulsion system, friction-less environment and metal-free ablation in a water cannon target. A special reservoir of water is proposed to supply a definite amount of water only at the time of laser irradiation. This system is tested for the object levitated on the airflow, which is called air-slider in order to simulate friction-less environment. In order to avoid the evaporation and freezing of water in lower atmospheric pressures, we propose here an air curtain system. We have performed simulations and experiments and confirmed that a small amount of air flow can sustain the high pressure inside the water reservoir. PACS 42.62.Cf; 52.38.Mf; 07.87.+v     

The First Structure Determination of Nanosized Colloidal Particles of Pd3P by High-Resolution Electron Microscopy

The exact three-dimensional structure of a nanosized colloidal particle of Pd₃P was determined directly from high-resolution transmission electron microscopy (HRTEM) images that were recorded in several crystallographic directions. An HRTEM image recorded along [011] is shown on the right. The cores of the particles were excluded from the analysis because of severe multiple diffraction.     

Formation and Structure of Cluster-Included Carbon Nanocapsules Prepared by Polymer Pyrolysis

Abstract

Formation of carbon nanocapsules with various clusters (SiC, Au, Fe. Co. Ge. and GeO₂) by polymer pyrolysis was investigated, and nanocapsules with SiC and Au nanoparticles were produced by thermal decomposition of polyvinyl alcohol at ?500°C in Ar gas atmosphere. The formation mechanism of nanocapsules and a structural model for the nanocapsule/SiC interface were proposed. In addition, carbon clusters were formed at the surface of carbon nanocapsules, and carbon onions were produced by electron irradiation of amorphous carbon produced from polyvinyl alcohol. The present work indicates that the pyrolysis of polymer materials with clusters is a useful fabrication method for the mass-production of carbon nanocapsules and onions at low temperatures compared to the ordinaly are discharge method.