Luminescence techniques and characterization of the morphology of polymer latices 2. Fluorescence lifetime, phosphorescence and fluorescence anisotropy studies

Five poly(n-butyl methacrylate), PBMA, latex dispersions have been prepared, each incorporating a different fluorescent label, via a two-stage seeded emulsion polymerization. The resultant latices contain ca. 35% by weight total solids and are of 80 (+/-10) nm diameter as determined by photon correlation spectrometry. Luminescence spectroscopic techniques, namely fluorescence (and phosphorescence) excited state lifetime measurements in addition to time-resolved anisotropy experiments have provided useful information regarding the morphology, microviscosity and water permeability of the resultant particles. A picture of the PBMA colloid emerges of an interior which is highly viscous and water impermeable in nature. Indeed, the environment is protective enough to sustain room temperature stabilized phosphorescence from both an acenaphthylene and 9-phenanthrylmethyl methacrylate labeled dispersion through simple nitrogen purging of the solutions. However, the current spectroscopic measurements should be viewed with the knowledge that each luminescent label may fashion its own distinctive microenvironment within the latex during polymerization.     

Proteasome-inhibitory and cytotoxic constituents of Garcinia lateriflora: absolute configuration of caged xanthones

A new biflavonoid (1), a new xanthone enantiomer (2), five new caged xanthones (3-7), and several known compounds were isolated from the stem bark of Garcinia lateriflora, collected in Indonesia. The structures of the new compounds were determined by analysis of spectroscopic data, and the absolute configuration of the caged xanthones was shown for the first time at carbons 5, 7, 8, 8a, 10a, and 27, by analysis of COSY and NOESY NMR and ECD spectra. The biflavonoids exhibited proteasome-inhibitory activity, and the known compound, morelloflavone (8) was found to have the greatest potency (IC₅₀=1.3 μM). The caged xanthones were cytotoxic toward HT-29 cells, with the known compound, morellic acid (10) being the most active (ED₅₀=0.36 μM). However, when tested in an in vivo hollow fiber assay, it was inactive at the highest dose tested (20 mg/kg).     

Intricate role of water in proton transport through cytochrome c oxidase

Cytochrome c oxidase (CytcO), the final electron acceptor in the respiratory chain, catalyzes the reduction of O(2) to H(2)O while simultaneously pumping protons across the inner mitochondrial or bacterial membrane to maintain a transmembrane electrochemical gradient that drives, for example, ATP synthesis. In this work mutations that were predicted to alter proton translocation and enzyme activity in preliminary computational studies are characterized with extensive experimental and computational analysis. The mutations were introduced in the D pathway, one of two proton-uptake pathways, in CytcO from Rhodobacter sphaeroides . Serine residues 200 and 201, which are hydrogen-bonded to crystallographically resolved water molecules halfway up the D pathway, were replaced by more bulky hydrophobic residues (Ser200Ile, Ser200Val/Ser201Val, and Ser200Val/Ser201Tyr) to query the effects of changing the local structure on enzyme activity as well as proton uptake, release, and intermediate transitions. In addition, the effects of these mutations on internal proton transfer were investigated by blocking proton uptake at the pathway entrance (Asp132Asn replacement in addition to the above-mentioned mutations). Even though the overall activities of all mutant CytcO's were lowered, both the Ser200Ile and Ser200Val/Ser201Val variants maintained the ability to pump protons. The lowered activities were shown to be due to slowed oxidation kinetics during the P(R) → F and F → O transitions (P(R) is the "peroxy" intermediate formed at the catalytic site upon reaction of the four-electron-reduced CytcO with O(2), F is the oxoferryl intermediate, and O is the fully oxidized CytcO). Furthermore, the P(R) → F transition is shown to be essentially pH independent up to pH 12 (i.e., the apparent pK(a) of Glu286 is increased from 9.4 by at least 3 pK(a) units) in the Ser200Val/Ser201Val mutant. Explicit simulations of proton transport in the mutated enzymes revealed that the solvation dynamics can cause intriguing energetic consequences and hence provide mechanistic insights that would never be detected in static structures or simulations of the system with fixed protonation states (i.e., lacking explicit proton transport). The results are discussed in terms of the proton-pumping mechanism of CytcO.     

Differential non-destructive image current detection in a fourier transform quadrupole ion trap

Dual-detector differential non-destructive Fourier transform detection in a quadrupole ion trap is shown to improve signal intensity and reduce noise compared with spectra recorded using a single detector. A larger area detector in each end-cap electrode is machined to fit its hyperbolic shape and so minimize field imperfections on the z-axis. Argon, acetophenone and bromobenzene spectra were recorded to allow a comparison between single- and dual-detector (differential) modes of detection and to demonstrate the improvement achieved with differential detection. Copyright 1999 John Wiley & Sons, Ltd.     

Gold-catalyzed rearrangement of substituted allyl aryl ethers

Triphenylphosphinegold(I) complexes catalyze the Claisen-type rearrangement of aryl allyl ethers to the corresponding branched and linear products. The product distribution depends on the olefin geometry of the allylic ether. Stereochemical transfer experiments support an ionic mechanism.     

Derivations and the integral closure of ideals

Let be a complete local domain containing the rationals. Then there exists an integer such that for any ideal , if , , then there exists a derivation of with .

     

Pseudo-steady-state solutions for solidification in a wedge

Polubarinova-Kochina's analytical differential equation methodis used to determine the pseudo-steady-state solution to problemsinvolving the freezing (solidification) of wedges of liquidwhich are initially at their fusion temperature. In particular,we consider four distinct problems for wedges which are: freezingwith the same constant boundary temperature, freezing with thesame constant boundary heat fluxes, freezing with distinct constantboundary temperatures and freezing with distinct constant fluxesat the boundaries. For the last two problems, a Heun's differentialequation with an unknown singularity is derived, which in bothcases admits a particularly elegant simple solution for thespecial case when the wedge angle is . The moving boundariesobtained are shown pictorially.     

Luminescence Characterization of a Novel, Hydrophobically Modified and Heavy Atom-Functionalized, Water-Soluble Polymer

A hydrophobically modified water-soluble polymer, based upon acrylic acid and styryl derivatives, was synthesized. A proportion (ca. 75 mol%) of the styryl residues in the copolymer contain a bromine substituent as a heavy-atom functionality. It has been shown that room-temperature phosphorescence (RTP) can be induced in an acenaphthylene (ACE) label, covalently bound to the polymer chain, through intramacromolecular interactions in dilute solutions of the copolymer. This is the first instance in which RTP has been generated in either label or solubilized guest, in such a fashion. The conformational behavior of the functionalized copolymer, BrSTY/STY/AA, has been studied using RTP, fluorescence lifetime, and time-resolved anisotropy measurements and compared to that of both its unbrominated, styrene-modified analogue, STY/AA, and poly(acrylic acid) PAA itself. The conformation transition which accompanies conversion of each polyacid into the corresponding fully neutralized polysalt is much more dramatic in either hydrophobically modified species than in poly(acrylic acid). Intramacromolecular aggregation leading to the creation of hydrophobic domains within the coils of the macromolecules is enhanced at a low pH and severely impedes segmental motion in the two styrene-modified polyacids. The effect is greater in the bromine-containing polymer, which suggests that more densely packed domains are formed in this species than in STY/AA. In addition to altering the magnitude of the effect that neutralization has upon the molecular dynamics of the polyacid in aqueous media, hydrophobic modification raises the pH range over which the change in conformational behavior of the macromolecule is apparent.