TIME-RESOLVED FLUORESCENCE STUDIES OF SPINACH CHLOROPLASTS–EVIDENCE FOR THE HETEROGENEOUS BIPARTITE MODEL

Abstract— Fluorescence lifetimes of spinach chloroplasts were measured with a modelocked dye laser and time-correlated single photon counting. Information about energy transport and functional organization of the chloroplasts is revealed by such time-resolved fluorescence studies. Quenching experiments using treatment with UV light or the chemical agent dibromothymoquinone are consistent with the notion that there is heterogeneity associated with PS II units and that such heterogeneity is reflected over the entire time range of fluorescence decay, not just in a single component. Phosphorylation experiments were also carried out which permit us to relate these kinetic studies to previous steady state observations.     

Short Communication: Diffusion of Simple Molecules in 4A Zeolite

Recently reported diffusivity data for N₂, CH₄ and Kr in 4A zeolite pelleted adsorbent are compared with earlier data for the same sorbates in several different 4A samples. It is shown that, although there are large differences in diffusivity between the different adsorbent samples the activation energies are essentially constant. The data can be reconciled if it is assumed that the samples contain different fractions of open windows as a result of non-ideal distribution of the Na⁺ cations.     

Rapid,clean, and mild O-acetylation of alcohols and carbohydrates in an ionic liquid

Archetypal O-acetylation reactions of alcohols and carbohydrates proceed rapidly in high yield under mild conditions in a dicyanamide based ionic liquid, that is not only an effective solvent but also an active base catalyst.     

Exploring privileged structures: the combinatorial synthesis of cyclic peptides

Head-to-tail cyclic peptides have been reported to bind to multiple, unrelated classes of receptor with high affinity. They may therefore be considered to be privileged structures. This review outlines the strategies by which both macrocyclic cyclic peptides and cyclic dipeptides or diketopiperazines have been synthesised in combinatorial libraries. It also briefly outlines some of the biological applications of these molecules, thereby justifying their inclusion as privileged structures.     

Quantitative capillary gel electrophoresis assay of phosphorothioate oligonucleotides in pharmaceutical formulations

Quantitative capillary gel electrophoresis (QCGE) has been developed for the accurate quantitation of a 21-mer phosphorothioate oligonucleotide, ISIS 2922, and its degradation products in an intravitreal formulation. The electrokinetic mode of injection employed by CGE necessitates formulation of the external reference standard in a sample matrix similar to that of the drug product and the use of an internal standard for improved accuracy and precision. The analytical method detailed in this paper has demonstrated the necessary accuracy, precision, linearity, range, selectivity and ruggedness for use in routine drug product analysis and stability monitoring of phosphorothioate oligonucleotides.     

Use of Sucrose‐based Epoxy ­Formulations and Cellulosic Fibers in ­the Design,Preparation, and Screening ­of New Composite Insulation Materials

Non‐woven composite insulation materials were generated from cotton, kenaf, jute, polyester, polypropylene, sucrose‐based epoxy formulations, and aluminum foil. The needlepunched fiber batts were rendered flame resistant by use of inorganic reagents and urea. To discover suitable epoxy formulations to bind the cellulose fibers to themselves or to dissimilar surfaces and to make flexible composites, a comparison of the performance of the known epoxy allyl sucroses (EAS), epoxy crotyl sucroses (ECS), and diglycidyl ether of bisphenol‐A (DGEBA) was made. The epoxies were cured with commercial diethylenetriamine (DETA), and UNIREZs‐2142 and 2355®, to discover a formulation with the following characteristics: (a) low cure temperature; (b) low Young's moduli and glass transition temperatures of cured thermosets for flexible composites; (c) ample bond strength between the fabric and the bonded surfaces; and (d) non‐cytotoxicity and non‐mutagenicity of the epoxies. Based on results following these criteria, EAS was selected, and the formulation comprising EAS and UNIREZ‐2355® was deemed suitable to bind fiber batts to surfaces of any type and geometry. ASTM guidelines were used to construct a wooden frame cube (heat box) for the simultaneous rapid screening of cellulosic fiber batts and composites. The new materials were compared against R‐19 fiberglass insulation for their ability to resist heat flow (denoted by relative R‐values) and time taken to approach thermal equilibrium. Plain non‐woven cellulosic fiber batts showed relative R‐values of 4.0 °F ft² hr/Btu per inch thickness (0.27 K m²/W per cm), and took about 2 hr to establish equilibrium heat flow. Commercial fiberglass batts showed relative R‐values­of 2.2 per in (0.15 per cm) and took 1 hr to attain equilibrium heat flow. When 6.25 in (15.9 cm) thick batts of fiberglass were needle punched to a thickness of 1 in (2.54 cm), relative R‐values and equilibrium heat flow times were 4.0 per in (0.27 per cm) and 2 hr, respectively. This denoted that the densities and thermal resistances of non‐conducting materials are raised concurrently. Anisotropic heat flow behavior was observed in cellulosic fiber composites with aluminum foil (shiny side out) bonded on one side. It depended upon whether the aluminum foil side or the fibers side faced the heat source. In the latter orientation the aluminum acted as a heat sink, and in the former orientation the foil acted as a poor heat reflector. The poor performance of these insulation composites was related to the fact that aluminum was directly bonded to the fiber batts and was acting as a heat conductor. When cellulose fiber shims (spacers) were placed between the fiber batts and the aluminum foil, the R‐values of the composites were comparable to those of plain batts but the times taken to approach thermal equilibrium increased to >3 hr, denoting that the foil was acting more as a reflector and less as a conductor. Copyright © 2001 John Wiley & Sons, Ltd.     

Critical reevaluation of proximity effects in the barton oxidation and related intramolecular reactions

The limiting factor to account for the increased rate of intramolecular reactions between functional groups as opposed to their intermolecular counterparts can and has been explained both by proximity effects and by activation energy. Neither explanation has emerged as the single most important reason in all or even the majority of cases studied. We have therefore reexamined the spatiotemporal hypothesis of Menger and the transition-state energy approach of Houk on a consistent set of compounds subjected to the Barton oxidation or related reactions in an effort to more clearly define the reasons for the proximity effect. For the 26 structures studied, neither hypothesis provides a consistent, quantitative explanation although the transitionstate energy hypothesis offered the most promise. © John Wiley & Sons, Inc.     

Stability of nitrogen-oxygen cages N12O2, N14O2, N14O3, and N16O4

Molecules consisting entirely of nitrogen have been studied extensively for their potential as high energy density materials (HEDM). However, many such molecules are too unstable to serve as practical energy sources. This has prompted many studies of molecules that are mostly nitrogen but which incorporate heteroatoms into the structure to provide additional stability. In the current study, cages of three-coordinate nitrogen are viewed as candidates for stabilization by insertion of oxygen atoms into the nitrogen framework. Cages of N12, N14, and N16 with four-membered rings are studied because four-membered rings have been previously shown to be a destabilizing influence. Insertion of oxygen atoms, which converts N-N bonds to N-O-N bonding groups, relieves ring strain and can potentially result in stable molecules. These molecules are studied by theoretical calculations, using Hartree-Fock and Moller-Plesset (MP3 and MP4) theories, to determine the dissociation energies of the molecules. The primary result of the study is that stable molecules can result from oxygen insertion but that oxygen-oxygen proximity destabilizes the insertion products.