Long-range exciton-coupled circular dichroism: application for determination of the absolute configuration of oligonaphthalenes

Hexadecanaphthalenes (S,S,S,S,S,S,S,S,S,S,S,S,S,S,S)-4b and (S,S,S,S,S,S,S,R,S,S,S,S,S,S,S)-4b that possess two tetraphenylporphyrins (TPP) on the upper and lower naphthalene rings were synthesized. Long-range exciton-coupled CD in the Soret region of TPP (about 66 A) was observed. [structure: see text]     

Investigations on the effect of Mn ions on the local structure and photocatalytic activity of Cu(I)-ZSM-5 catalysts

The introduction of Mn ions into Cu(I)-ZSM-5 was found to lead to an enhancement of the photocatalytic activity for the direct decomposition of N2O into N2 and O2 at 298 K. Various in-situ techniques such as ESR, photoluminescence, XAFS as well as a combination of CO-FT-IR and CO-TPD measurements revealed that the accommodation of Mn ions within ZSM-5 zeolite cavities significantly affects the location sites of the ion-exchanged Cu(II) ions as well as the local structure of the Cu(I) ion species formed by evacuation at high temperatures. Moreover, the introduction of Mn ions into ZSM-5 led to an increase in the amount of 3-coordinated Cu(I) species at the main channel of the zeolite, playing a major role as the active species for the photocatalytic decomposition of N2O into N2 and O2.     

Soft-UV photolithography using self-assembled monolayers

We report thiol-on-gold self-assembled monolayers (SAMs) that can be photodeprotected using soft UV irradiation (lambda = 365 nm) to yield CO(2)H functionalized surfaces complementing those reported previously, which yielded NH(2) functionalized surfaces. The photolysis of these SAMs were monitored using a combination of surface sensitive techniques. In the SAM environment the photodeprotection yields are lower than those obtained for equivalent reactions in dilute solution. The protected carboxylic acids SAMs are shown to have a low yield approximately 50% due to competing photoreduction reactions of the nitro group. The results from infrared studies show that, as the photolysis progresses, the long chain protected residues reorganize and shield the functional COOH groups, thereby reducing the hydrophilic character of the surface.     

Structural confirmation of ostreocin-D by application of negative-ion fast-atom bombardment collision-induced dissociation tandem mass spectrometric methods

Negative-ion fast-atom bombardment collision-induced dissociation tandem mass spectrometric (FAB-CID-MS/MS) methodology was successfully applied to verify the highly complex structure of ostreocin-D (MW 2633), a new palytoxin analog isolated from the marine dinoflagellate Ostreopsis siamensis and proposed to be 42-hydroxy-3,26-didemethyl-19,44-dideoxypalytoxin based on NMR data. The charge-remote fragmentations were facilitated by a negative charge introduced to a terminal amino group or to a hydroxyl group at the other terminus by a reaction with 2-sulfobenzoic acid cyclic anhydride. Product ions generated from the [M - H](-) ions provided information on the structural details of ostreocin-D. Comparisons between the spectral data for ostreocin-D and palytoxin also provided a rational basis for the assignments of product ions.     

NMR-based reappraisal of the coordination of a metal ion at the pro-Rp oxygen of the A9/G10.1 site in a hammerhead ribozyme

In the identification of a metal-binding site within enzymes, kinetic analyses based on thio-effects and Cd(2+)-rescues are widely used. In those analyses, kinetic studies using a phosphorothioate have been discussed on the premise that the substitution by a sulfur atom does not change the conformation of a ribozyme. However, our present NMR structural analysis demonstrates the change of the conformation at the metal-binding site by Rp-sulfur but not by Sp-sulfur substitution and warns against incautious interpretations of thio-effects and rescue phenomena in kinetic studies using a phosphorothioate. Our analysis further demonstrates that, in solution, a Cd(2+) ion can interact with an Rp-phosphorothioate (in support of the controversial McKay's structure, Nature 1994, 372, 68-74) and with an Sp-phosphorothioate (in support of the controversial Scott's structure, Cell 1995, 81, 991-1002) at the metal-binding A9/G10.1 site and that, in the former case, the bound Cd(2+) ion can return the ribozyme to an active conformation and rescue its enzymatic activity.     

Remarkable effect of pore size on the catalytic activity of mesoporous silica for the acetalization of cyclohexanone with methanol

The catalytic activity of mesoporous silica (MCM-41) for the acetalization of cyclohexanone shows a volcano-shaped dependence on the pore diameter. The diameter of the cyclohexanone molecule is ca. 0.75 nm, while its acetalization with methanol was well accelerated on MCM-41 with a pore diameter of approximately 1.9 nm, and smaller or larger pores were not suitable for the catalysis. The catalytic activity was independent of the Al concentration.     

On the problem of potential shifts at glassy carbon electrodes: Part II. Voltammetric analysis of the surface interactions involved in the electrode processes of nitrobenzenes at freshly polished and electrochemically pretreated electrodes

The contribution of the heterogeneous (involving non-adsorbed solution species) and surface path (involving adsorbed species) to the electrode processes of aromatic nitro compounds has been studied as a function of the surface conditions of glassy carbon (GC-20) by linear sweep voltammetry (LSV) and cyclic voltammetry (CV). The influence of adsorption was negligible at the freshly polished electrodes. Oxidative electrochemical pretreatment led to the formation of active sites at which the adsorption of the reactant resulted in enhanced electron-transfer rates. The nature of the adsorption process and the characteristics of the electrode reaction in the adsorbed state were studied in detail for p-nitrotoluene. The adsorption followed the Frumkin isotherm with an interaction parameter of 1.2, indicating moderate attractive interactions between the adsorbed molecules. The rate constant for the adsorbed species was of the order of 10⁵, higher than the apparent heterogeneous rate constant of p-nitrotoluene in the solution phase. The magnitude of the potential shift and the strength of adsorption varied with the relative position and electron-withdrawing power of the substituent in the aromatic ring of nitrobenzene. Free radical coupling was selectively catalysed at electrochemically pretreated electrodes.     

Total synthesis of pyridomycin

The first total synthesis of pyridomycin is described including the stereocontrolled construction of the exocyclic (Z)-s-butylidene moiety in the 12-membered ring system.     

Electroactive supramolecular self-assembled fibers comprised of doped tetrathiafulvalene-based gelators

New electroactive supramolecular fibers have been formed by self-assembly of the derivatives of tetrathiafulvalene (TTF) in liquid crystals. These derivatives are designed and prepared by introducing the TTF moiety to the scaffold derived from amino acids such as L-isoleucine whose derivatives function as organogelators. These TTF-based gelators form stable fibrous aggregates in liquid crystals. These fibers are the first example of hydrogen-bonded one-dimensional aggregates having electroactive moieties whose electrical conductivities were measured after doping. Their electronic states have also been characterized by spectroscopic methods. Unidirectionally aligned fibers are formed in the oriented liquid crystal solvents on the rubbed polyimide surface for further functionalization of the fibers.     

Fluorescent Molecular Sensing System Based on Tri-Pyrenes-Labeled γ-Cyclodextrin at the Hetero Rim

Fluorescent active host labeled at the upper and lower rims of -cyclodextrin, namely, mono-3^A-deoxy-3^A-pyrenebutylamido-6^X,Y-O-bis-pyrenebutylate-mono-altro--cyclodextrin (-3) has been synthesized as a chemosensor for steroidal guests using fluorescence spectra change upon a guest addition. The -3 shows monomer and excimer fluorescence, which results in an increase of the intensity of monomer and decrease of excimer fluorescence with a host–guest binding in the cyclodextrin cavity. The extent of monomer and excimer fluorescence variations of -3 was used as an indication for the sensing ability for the guests examined. The guest-induced fluorescence changes were measured for 10^–7 M solutions of -3. The sensing parameters (I_ex/I⁰_ex and I_mon/I⁰_mono) were used to describe the sensing ability of -3. The values of I_ex/I⁰_ex describe that -3 shows less selectivity for guests than that of mono-3^A-deoxy-3^A-pyrenebutylamido-6^X-O-mono-pyrenebutylate-mono-altro--cyclodextrin (-2), and the values of I_mon/I⁰_mono show that -3 shows higher sensitivity and selectivity than that of -2.