Total synthesis of the duocarmycins

The total synthesis of (+)-duocarmycin A and SA through a common indoline intermediate is described. The key reactions include selective lithiation of a 2,6-dibromoiodobenzene derivative and diastereoselective addition to a chiral nitroalkene, copper-mediated aryl amination, and addition of aryllithium to azlactones.     

Trimethylsilyl-group containing polyphenylacetylenes for oxygen and ethanol permselective membranes

Phenylacetylenes having one or two trimethylsilyl groups at their benzene ring were synthesized, and polymerized by [Rh(cyclooctadiene) (PPh₃)₂]PF₆, [Rh(norbornadiene)Cl]₂, or WCl₆ to afford high molecular-weight polymers in high yields. These poly(phenylacetylene)s were soluble in many kinds of solvents and were fabricated to tough membranes by the solvent casting method. The oxygen permselectivities of these membranes were very good. The oxygen permeability coefficients (P_o2) and oxygen separation factors (α = P_o2/P_N2) of poly[2,4-(o,p)-bis(trimethylsilyl)phenylacetylene] [poly ( o-1-p-1 )] and poly[(4(p)-trimethylsilyl)phenylacetylene] [poly( p-1 )] membranes were 4.73 × 10^?8 cc(STP) cm/cm² s cmHg and 2.65, and 1.52 × 10^?8 cc(STP) cm/cm² s cmHg and 3.39, respectively. In the case of poly( o-1-p-1 ), P_o2 was comparable to that of polydimethylsiloxane (PDMS) and α was higher than that of PDMS. However, the P_o2 value reduced to 48% of its initial value in about 1 year. In the case of poly( p-1 ), the P_o2 value did not change in about 1 year. Ethanol permeated preferentially through these membranes (α^EtOH > 1) in pervaporation of aqueous ethanol solution, whereas poly(phenylacetylene) [poly( PhA )] showed water permselectivity (α^EtOH < 1). These favorable effects of trimethylsilyl groups on the oxygen and ethanol permselectivities were discussed on the basis of comparison with those of poly( PhA ), other poly(substituted phenyl-acetylene)s, and trimethylsilyl-group containing polystyrenes. © 1994 John Wiley & Sons, Inc.     

Multi-frequency ESR study of the polycrystalline phenoxyl radical of alpha-(3,5-di-tert-butyl-4-hydroxyphenyl)-N-tert-butylnitrone in the diamagnetic matrix

Multifrequency (X-, Q-, and W-band) electron spin resonance (ESR) spectroscopy has been used to characterize the phenoxyl radical produced from alpha-(3,5-di-tert-butyl-4-hydroxyphenyl)-N-tert-butylnitrone, which is a new spin-trapping reagent. The X-band measurement did not resolve the powder-pattern ESR spectrum. Because of its higher resolution with g value, the Q-band ESR study revealed that the g factor has an axial-like symmetry and that the observed hyperfine structure in the Z-direction is caused by the nitrogen nucleus at the para-position. Furthermore, the results of the W-band ESR experiment more clearly distinguished the perpendicular components from the parallel component, resolving the perpendicular components into x and y components. The X-band powder spectrum was similar to the X-band ESR spectrum of the radical in a frozen solution of toluene. The computer simulation spectra performed using the obtained parameters fitted the experimental spectra well. A comparison of the amplitude of g( perpendicular)(gx, gy) with that of gz showed that the unpaired electron is delocalized over the pi-conjugated framework. Considering the hyperfine coupling constant, it was concluded that about 16% of the unpaired electron distributed over the nitrogen nucleus at the para-position. This study thus showed the significant potential of a multifrequency ESR approach to a powder sample radical in terms of its high resolution with g value.     

High-yield synthesis of single-wall carbon nanotubes on MCM41 using catalytic chemical vapor deposition of acetylene

High-quality single-wall carbon nanotubes (SWNTs) with narrow diameter distribution have been grown on Fe/Co-loaded MCM41 by using acetylene as the carbon source within a short reaction period, typically 10 min or less. The optimum temperature for SWNTs synthesis is 850 degrees C. Longer reaction time (i.e., 30 min) favors the formation of multiwall carbon nanotubes (MWNTs) and graphitic carbon. When the reaction time is reduced to less than 10 min, formation of MWNTs and graphitic carbon is greatly suppressed, and high-quality SWNTs dominates the yield. The surface of the as-grown SWNTs is found to be free from amorphous carbon, as observed from high-resolution transmission electron microscope (HRTEM) analysis. Raman spectral data show a G/D ratio above 10, indicating that the as-grown SWNTs have very few defects. Furthermore, radial breathing mode (RBM) analysis reveals that the diameter distribution of the current SWNTs is narrow and ranges from 0.64 to 1.36 nm.     

Infrared spectroscopic study on the reaction mechanism of CO hydrogenation over Pd/CeO2

In situ infrared spectroscopy was applied to elucidate the reaction mechanism of CO hydrogenation over Pd/CeO₂. Instead of direct dissociation of CO, a new reaction pathway is proposed for methane formation, involving geminal dicarbonyl intermediates and (HCO)₂(a) intermediates, which may be located on the surface of Pd covered with thin layers of reduced ceria (SMST effect). Transformation of methane formation sites into methanol formation ones by the oxidation with water vapor formed during the CO-H₂ reaction is proposed, which may be located on the Pd (111) planes adjacent to ceria support.     

High-resolution analysis of (Sc3C2)@ C80 metallofullerene by third generation synchrotron radiation X-ray powder diffraction

The X-ray structure of Sc(3)C(82) is redetermined by the MEM/Rietveld method by using synchrotron radiation powder data at SPring-8, where the C(2) encapsulated structure available to discuss the Sc-Sc interatomic distances has been determined. The encapsulated three scandium atoms form a triangle shape. A spherical charge distribution originating from the C(2) molecule is located at the center of the triangle. Interatomic distances between Sc and Sc are 3.61(3) A in the triangle. The distance between Sc and the center of the C(2) molecule is 2.07(1) A.     

pH‐Triggered Self‐Assembly of Cellulose Oligomers with Gelatin into a Double‐Network Hydrogel

Multicomponent systems for self‐assembled molecular gels provide huge opportunities to generate collective or new functions that are not inherent in individual single‐component gels. However, gelation tends to require careful and complicated procedures, because, among a myriad of kinetically trapped structures related to the degree of mixing of multiple components over a wide range of scales, from molecular level to macroscopic scale, a limited number of structures that exhibit the desired function need to be constructed. This study presents a simple method for the construction of double‐network (DN) hydrogels with improved stiffness composed of crystalline cellulose oligomers and gelatin. The pH‐triggered self‐assembly of cellulose oligomers leads to the formation of robust networks composed of crystalline nanofibers in the presence of dissolved gelatin, followed by cooling to allow for the formation of soft gelatin networks. The resultant DN hydrogels exhibit improved stiffness; the improvement in gel stiffness with double networking is comparable to that of previously reported DN hydrogels produced via a time‐consuming enzymatic reaction.     

Control of Intramolecular Hydrogen Bonding in a Conformation-Switchable Helical-Spring Polymer by Solvent and Temperature

A substituted poly(phenylacetylene) derivative (PPA_HB) with two hydroxymethyl groups at the meta position of the side phenyl ring was examined as a conformation-switchable helical spring polymer that responds to solvent and heat stimuli in a precisely controlled manner. Intramolecular hydrogen bonds, which cause the helical structure of the polymer, were broken and re-formed by adjusting the hydrogen-bonding strength values (pK_HB) of various combinations of solvents or by varying the temperature. In this process, a reversible conformational change from cis–cisoid to cis–transoid, accompanied by a phase transition in the form of a helix-coil transformation occurred, with the polymer exhibiting critical changes of color fading and recovery in specific environments. These results demonstrate that PPA_HB can be used as either a pK_HB indicator or a thermometer. The color changes of the polymer solution are described in detail based on spectroscopic analyses and thermodynamic considerations.     

Label-Free Imaging of Intracellular Temperature by Using the O−H Stretching Raman Band of Water

We propose a label-free method for measuring intracellular temperature using a Raman image of a cell in the O−H stretching band. Raman spectra of cultured cells and the medium were first measured at various temperatures using a Raman microscope and the intensity ratio of the two regions of the O−H stretching band was calculated. The intensity ratio varies linearly with temperature in both the medium and cells, and the resulting calibration lines allow simultaneous visualization of both intracellular and extracellular temperatures in a label-free manner. We applied this method to the measurement of temperature changes after the introduction of FCCP (carbonyl cyanide-p-trifluoromethoxyphenylhydrazone) in living cells. We observed a temperature rise in the cytoplasm and succeeded in obtaining an image of the change in intracellular temperature after the FCCP treatment.     

Preparation of Graft Copolymers By Means of Uv Photolysis of Poly(Methyl Vinyl Ketone) and Reverse Osmosis Performance of the Membranes from the Oximes of the Copolymers

ABSTRACT

An attempt was made to prepare a graft copolymer consisting of poly(methyl vinyl ketone) (PMVK) as a backbone chain and polyacrylonitrile, poly(4-vinylpyridine), or polystyrene as a graft chain by UV irradiation of a solution of PMKV in the presence of acrylonitrile, 4-vinylpyridine, or styrene. the influence of reaction conditions on the yield, composition, and viscosity of the resulting graft copolymers was investigated. It was suggested from NMR and gel permeation chromatography that those graft copolymers contained a high molecular weight fraction of narrow distribution and block copolymers as well. the reverse osmosis membranes derived from the oxime and amidoxime of the graft copolymers showed a characteristic performance of exhibiting a maximal difference between rejections against NaCl and CoCl₂ at a certain addition ratio of crosslinking agent, which was not observed in the membranes from copolymers by conventional radical copolymerization. the relationship between these phenomena and the branching structure of the graft copolymers was discussed.