bluuebunny/crossref_metadata_2025_split huggingface dataset:
Oxidation‐Reduction Physical Chemistry of the Acceptor Quinone Complex in Bacterial Photosynthetic Reaction Centers: Evidence for a New Model of Herbicide Activity
Authors: Wraight C. A. 
Published: November 2013

AbstractThe oxidation‐reduction midpoint potential (Em) of the primary quinone (QA) of the acceptor quinone complex of bacterial photosynthetic reaction centers has been measured as a function of pH in the presence and absence of ubiquinone and o‐phenanthroline (o‐Phen). Reaction centers, isolated from Rhodopseudomonas sphaeroides, were incorporated into egg phosphatidylcholine vesicles. Contrary to earlier reports, the Em was found to exhibit a pH‐dependence very similar to that observed in chromatophores, with a slope of ‐ 60 mV/pH up to a p K for Q−A/Q−A(H+) at pH 9.5–10.0. In the presence of ubiquinone to reconstitute the secondary quinone (QB), the Em/pH curve of QA was shifted to lower potentials, indicating that the binding of Qn (actually QBH2) was suppressed by reduction of QA. o‐Phen, an inhibitor of electron transport from QA to QB, raised the pK of Q−A/Q−A(H+) and, at pH‐values below but not above this pK, reversed the effects of QB. In the absence of QB, o‐Phen lowered the Em of QA above the pK but had no effect below it. These results are discussed in terms of interactions between the binding sites for QA and QB (A‐ and B‐sites). It is suggested that ubiquinone and o‐Phen compete for the B‐site in a mutually exclusive fashion, and that their relative binding strengths are modulated by the redox and protonation state of QA. In preparations with low quinone content, o‐Phen inhibits photochemistry suggesting that it can also compete with ubiquinone at the A‐site. Competitive displacement of quinone from the B‐site by o‐Phen and other inhibitors is suggested as the primary mode of action of a broad class of herbicides active in Photosystem II of plants. The relative binding affinities of the various redox states of QB are also discussed and it is concluded that the order of binding strength is: Q−B → QB → QBH2. This accounts for the atypical stability of the semiquinone and the lower average Em for reduction to the quinol, compared to free ubiquinone in the quinone pool. It may also be significant in the functioning of quinones in communicating reducing equivalents from the reaction center to other electron transport complexes in the intact membrane.
Rearrangements of Methyl Substituted N‐Neopentylmethyleneaziridines in Methanol
Authors: Wiering Petrus G. , Steinberg Herbert 
Published: November 2013

AbstractOn heating the N‐neopentylalkylideneaziridine1or4, a mixture of1, 4and N‐neopentyl‐2,2‐dimethylcyclopropanimine3is obtained. The imine3does not rearrange, but breaks down into the fragments isobutene and neopentylisocyanide. The reaction between1(or4) and methanol gives the ring‐opened products5and6and the cyclopropylamino ether7. Addition of acid gives rise to only ring‐opened products5, 6and8. It is believed that the reactions proceed via the common intermediate 2‐N‐neopentylaminoallyl cation2, followed by a rapid ring closure to the protonated isomeric aziridines1and4and 2,2‐dimethylcyclopropanimine3.
Circular Dichroism. LXXVI. CD of Chiral Vinyl—Cyclopropanes
Authors: Itô Shô , Kuriyama Kaoru , Norin Torbjörn , Snatzke Günther 
Published: November 2013

AbstractThe sign of the CD within the first absorption band (200–220 nm) of optically active vinyl‐cyclopropanes has been correlated with the absolute conformation of the chromophore in a non‐empirical way. A positive torsional angle between the C=C and the bisectrix of the cyclopropane ring between 0° and ≃ + 70° gives a positive Cotton effect; between ≃ + 110° and 180° it gives a negative one. The rule has been tested mainly on models of terpenoid origin.
Reactions of Photoexcited Triplet States of Zinc Porphyrin with Transient Radicals in Aqueous Solutions
Authors: Neta P. , Levanon Haim 
Published: November 2013

AbstractThe technique of simultaneous pulse radiolysis and photolysis, PRAP, has been utilized to study the reactions of various radicals with ground state ZnTPPS and the triplet state ZnTPPST in aqueous solutions. The radicals H and OH add to both states with k ∼ 1 × 1010 M−1 s−1. The CH2C(CH3)2OH radical from t‐BuOH is relatively inert toward ZnTPPS but reacts rapidly (k = 1.8 × 109 M−1 s−1) with ZnTPPST to form an adduct. Electron transfer reactions are found to be about an order of magnitude faster with the triplet than with the ground state. The (CH3)2COH radical reduces both ZnTPPS (k = 1 × 108 M−1 s−1) and ZnTPPST (k = 3 × 109 M−1 s−1) to the anion radical (ZnTPPS)−. The radical Br−2 oxidizes both states to the cation radical (ZnTPPS)+ with k = 8 × 108 M−1 s−1 for the ground state and 5 × 109 M−1 s−1 for the triplet. The transient cation Cd+ reduces both states with a diffusion‐controlled rate (k = 1 × 1010 M−1 s−1) to produce the anion radical. The above mechanisms of radical addition and electron transfer are also supported by the product spectra.
Photoinduced Charge Separation and Recombination in Exciplex Systems in Lipid Bilayer Vesicles
Authors: Neumann S. , Korenstein R. , Barenholz Y. , Ottolenghi M. 
Published: November 2013

AbstractPulsed laser photolysis experiments are carried out in lipid bilayer vesicles in which the fluorescence of excited pyrene (P), pyrene‐ethanol (P‐CH2OH) and pyrene‐monosulphonate (P‐SO−3) are quenched by N.N‐diethylaniline (DEA). The observed photochemical patterns are basically different from those encountered in homogeneous solutions: No detectable exciplex emission is observed, and the yield of the separated ions P− + DEA+ is small. Two alternative explanations are given for the complete lack of an exciplex fluorescence in the vesicles. One involves the requirement of a highly polar environment around the reacting species in the membrane interior. More plausibly, serious limitations imposed by the “liquid crystal” membrane microenvironment on the relative orientation of the 1P* acceptor and the DEA donor, must be assumed. The quenching process leads to the formation of the P− + DEA+ radical ion pair and of the pyrene triplet state 3P*. An analysis of the submicrosecond ions decay process, including its behaviour around the phase transition point of the lipids, indicates that the ions do not escape the vesicle and recombine by an intravesicle diffusion process. The relatively small yield of P− and DEA+ is attributed to a microviscosity effect, inhibiting the primary separation of the solvent‐shared ion pair {P− … DEA+}. An analysis of temperature (phase transition) and external magnetic field effects indicates that the pyrene triplet state, 3P*, is generated by geminate recombination in a geometrically correlated radical pair [P− … DEA+]. The results bear on the general problem of photoinduced charge separation in nonisotropic media and on the structure of biomembranes.
Semiconductor‐Oxide Heterojunctions as Electrodes in Photoelectrochemical Cells
Authors: Decker F. , Melsheimer J. , Gerischer H. 
Published: November 2013

AbstractThe use of semiconductor‐oxide Schottky barriers as electrodes in photoelectrochemical cells has been investigated. The conducting oxide has the function of protecting the semiconductor surface from corrosion and of driving the electrochemical reaction. Since the oxide layer is only a few hundred Å thick, the ohmic resistance of this layer is low and nearly 90% of the incoming light is transmitted through it. Regenerative photoelectrochemical cells based on n‐Si‐SnO2junctions have shown solar efficiencies around 4%. Further improvements will require the addition of a catalyst to the oxide surface in order to increase the speed of the electrochemical reaction.
The Application of the Method of Pseudopotentials to Hydrides of Silicon and their Methyl Analogs
Authors: Janoschek Rudolf , Sax Alexander , Halevi E. Amitai 
Published: November 2013

AbstractThe method of local pseudopotentials is applied to the calculation of several ground‐ and excited‐state properties of selected silicon compounds. The calculated ground‐state geometries and ionization potentials of the silicon hydrides: SiH, SiH2, SiH3 and SiH4 and of the ions SiH+, SiH−, SiH+3 and SiH−3 are in good agreement with the best theoretical and experimental results — whenever these are available. All the singly excited states of SiH2 originating from a given occupied orbital are evaluated in a single calculation. For those few valence states for which reliable experimental and theoretical data exist, agreement is good. The conceptual and computational advantages of transforming the basis set of silicon into a set of atomic pseudoorbitals are demonstrated by repeating the latter computations with increasingly large sets of atomic orbitals, ranging from [3s, 3p] to [3s, 3p, 3d, 4s, 4p]. The utility of this procedure for reliable and economical calculations on organosilicon compounds is illustrated with a similar set of computations on the ground‐state properties and vertical excitation energies of Si(CH3)2. The effect of methyl substitution on the ground‐state properties of silylene and on the energetic ordering of its excited states is discussed.
On the Chemistry of Pyrrole Pigments. 461 Phytochrome Model Studies: The Tautomerism at N22—N23 of Unsymmetrically Substituted Bilatrienes‐abc and 2,3‐Dihydrobilatrienes‐abc
Authors: Falk Heinz , Grubmayr Karl , Magauer Karl , Müller Norbert , Zrunek Ulrich 
Published: November 2013

AbstractA set of model compounds substituted with methyl groups in positions 7,8,12 and 13 were prepared to elucidate the tautomerism within the pyrromethene moiety of unsymmetrical bilatrienes and of 2,3‐dihydrobilatrienes‐abc. Configurational and 1H‐NMR signal assignments were achieved using NOE difference spectra, coupling phenomena and comparison to the spectra of compounds substituted with ethyl groups in strategic positions. By application of resolution enhancement and the COSY‐technique a characteristic coupling pattern between the methyl groups of a pyrrole ring which is different from the one observed for the pyrrolenin ring could be documented using derivatives with fixed tautomeric structure (N‐methylation). All unsymmetrical bilatrienes and 2,3‐dihydrobilatrienes‐abc investigated are shown to be single tautomers — the position of the proton (N22, N23) obviously is governed by an optimisation of intramolecular hydrogen bonding.
Picosecond Kinetics of Excited State Relaxation in Biliverdin Dimethyl Ester
Authors: Holzwarth Alfre D. , Wendler Joachim , Schaffner Kurt , Sundström Villy , Sandström Ake , Gillbro Tomas 
Published: November 2013

AbstractThe relaxation processes of biliverdin IXα dimethyl ester have been studied by time‐resolved absorption and fluorescence detection, using pump‐probe and single‐photon timing techniques, respectively. The results complement and extend those of earlier investigations.1,3–5In 2‐methyltetrahydrofuran, chloroform, ethanol, and ethyl lactate, four emitting species, two short‐lived (picosecond) and two long‐lived ones (nanosecond), have been detected. In methanol, the number of emitters is possibly reduced to three. In the cases of four emitters, these can be attributed to either three or four ground state forms. We conclude that the radiationless relaxation of the short‐lived excited states is simultaneously induced both by proton transfer processes and by conformational changes, as previously discussed.3The spectroscopic properties of the long‐lived species point to the presence of relatively rigid conformations of the bilatriene in liquid solutions at room temperature. The possible relevance of this observation with regard to the influence of protein interactions on the conformational equilibrium of tetrapyrroles in phycobiliproteins is discussed.
Determination of Bilirubin in Serum by Fast Scan Square Wave Voltammetry
Authors: Saar Jacob , Yarnitzky Chaim 
Published: November 2013

AbstractThe use of fast scan square wave voltammetry for the determination of bilirubin in serum is described. The reduction of bilirubin in alkaline solutions with or without serum albumin results in three current peaks. The peak potential and current are related to bilirubin and albumin concentrations and allow the determination of the bilirubin concentrations after the addition of KOH to the sample. The effect of light and time on peak current was also checked. Comparison of results achieved by electroanalytical and spectrophotometric methods prove the feasibility of measuring total bilirubin in serum by electrochemical methods.
NMR Studies of Sodium Cholate–Lecithin Mixed Micelles
Authors: Eriksson Per‐Olof , Arvidson Gösta , Lindblom Göran 
Published: November 2013

AbstractT1 and T2 2H‐NMR relaxation times have been measured for 2H‐labelled phosphatidylcholine in the aqueous solution phase of the ternary system lecithin–sodium cholate–water. In this phase aggregates are formed by a mixture of cholate and lecithin. Information about the dimension of these micellar aggregates has been obtained from a simple model of the relaxation times in which two modes of molecular motion are considered. The results obtained accord well with recent investigations using laser‐light scattering techniques.
Structure and Dynamics of Liquid Crystalline Polymers from Deuteron NMR
Authors: Boeffel Ch. , Hisgen B. , Pschorn U. , Ringsdorf H. , Spiess H. W. 
Published: November 2013

AbstractThe molecular structures of thermotropic liquid crystalline polymers, which have been synthesized systematically in recent years, are described according to the model of decoupling the molecular motions of the mesogenic groups and the polymer chain by inserting a flexible spacer. The molecular structure is related to the properties of these systems, e.g., phase transition temperatures, mesophases formed, etc. The structure and mobility of a specific side chain, liquid crystalline polyacrylate with phenyl benzoate as a mesogenic group is investigated by deuteron NMR and compared with the behaviour of low molecular weight analogues selectively deuterated at equivalent positions, i.e., at the mesogenic group and at the spacer. The temperature dependence of the order parameter of the mesogenic group indicates significant differences between the intermolecular potential in polymeric liquid crystals and their low molecular weight analogues. The order parameter of the spacer (methylene group adjacent to the polymer chain) is reduced to 50% of the value of the mesogenic group, whereas the equivalent methylene group in the low molecular weight system exhibits a reduction by only 25%. This indicates a substantially higher fraction of gauche‐conformers in the spacer of the polymeric liquid crystal than in the alkyl chains of low molecular weight liquid crystals. In the glassy state the order parameter of the mesogenic group is high, 0.85 and 0.65 for a frozen smectic‐A and a frozen nematic system, respectively. In the glassy state rapid rotational jumps by 180° are observed. This process can be followed by 2H NMR line‐shape analysis down to temperatures 100K below the glass transition.
Oxidation of Diphenyl Sulfide by Singlet Oxygen Generated Carbonyl Oxides
Authors: Agarwal Sudhir K. , Murray Robert W. 
Published: November 2013

AbstractDiphenyl sulfide has been oxidized to the sulfoxide by a series of 4,4′‐disubstituted benzophenone oxides. The carbonyl oxides were produced by photosensitized oxidation of the corresponding diazo compounds. Treatment of the data by a Hammett ρΣ approach indicates that the oxygen atom transfer is electrophilic in nature with ρ = +0.90. The results are discussed in terms of analogous reactions which are mediated by flavin‐dependent monooxygenase and particularly those oxidations believed to be involved in producing hepatotoxins.
Mechanisms and Products of Photosensitized Degradation of Nucleic Acids and Related Model Compounds
Authors: Cadet J. , Decarroz C. , Wang S. Y. , Midden W. R. 
Published: November 2013

AbstractThe mechanisms and products of photosensitized and singlet oxygen oxidation of nucleic acid model compounds have been investigated using kinetics analysis and product isolation and identification. A competitive kinetics analysis is presented as a simple yet quantitative method for the determination of the role of singlet oxygen in homogeneous chemical reactions. This method is compared to other techniques that have been used in the past including determination of the solvent isotope effect and the effect of singlet oxygen quenchers and variations in oxygen and substrate concentration. The results of competitive kinetics experiments indicate that the photooxidation of dGuo (2′‐deoxyguanosine) at pH 7–10 in buffered aqueous solution sensitized by Rose Bengal, Methylene Blue, Thionine or several other dyes proceeds predominantly by a pathway requiring intermediate formation of singlet oxygen. In contrast, dGuo photooxidation under the same conditions sensitized by flavin or pterin derivatives proceeds predominantly by pathways that do not involve singlet oxygen. Most of the primary products of dGuo photooxidation are very unstable and consequently are difficult, if not impossible, to isolate and characterize. However, two compounds have now been isolated that correspond to the addition of a single molecule of oxygen to the purine ring of guanine with the purine ring system still intact. Some of the products formed in the singlet oxygen oxidation of 2′‐deoxyguanosine appear to be different than some of the products formed in flavin photosensitized reactions in agreement with kinetic results that indicate that the flavin sensitized reactions proceed by mechanisms that do not involve intermediacy of singlet oxygen. If the unique nature of the products of 2′‐deoxyguanosine oxidation by singlet oxygen can be firmly established it may be possible to detect singlet oxygen reactions in vivo by product analysis.
Microwave Discharge of O2 and CO2 as a Synthetic Tool for Oxidations of Unsaturated Compounds Adsorbed on Solid Supports
Authors: Zadok Elazar , Rubinraut Sara , Mazur Yehuda 
Published: November 2013

AbstractCis and trans stilbene, styrene, 1‐decene and 6‐dodecene adsorbed on silica gel were reacted with oxygen atoms produced by microwave discharge of O2 and CO2. At 0°C these compounds gave mainly epoxides and carbonyl compounds as in neat liquid reactions. Below — 60°C ozonolysis products were formed; cis and trans stilbene and styrene gave benzaldehyde while 1‐decene and 6‐dodecene resulted mainly in ozonides.
Synthetic Utility of (Propargyl)dicobalt Hexacarbonyl Cations. A New Cyclopentenone Annelation
Authors: Saha Manasi , Nicholas Kenneth M. 
Published: November 2013

AbstractThe regiospecific alkylation of cyclic ketones and TMS enol ethers by the cobalt‐stabilized propargyl cations 1 followed by demetalation and regiospecific acetylene hydration proceeds in good overall yield to produce 1,4‐diketones. The latter are efficiently cyclized in the presence of base to annelated cyclopentenones.
Alkenyl Copper Derivatives 251: Synthesis of E or Z Enol Ethers by Carbocupration of Alkoxy‐Allenes
Authors: Alexakis A. , Normant J. F. 
Published: November 2013

AbstractAlkoxy‐allenes react smoothly with organocopper and cuprate derivatives to afford almost pure E or Z substituted enol ethers according to the reaction conditions. Lithium diorganocuprate reagents in diethyl ether show the greatest E selectivity, while lithium organocopper reagents in THF are completely Z selective. The obtained alkenyl copper or cuprate derivatives may react further with various electrophiles leading to polysubstituted enol ethers of known configuration.
Organoselenium Chemistry. Preparation of Allyl and Homoallyl Amines by Aminomethylation of Phenylseleno‐Substituted Allyl Tin Reagents
Authors: Reich Hans J. , Schroeder Mel C. , Reich Ieva L. 
Published: November 2013

AbstractMetalation of methallyl phenyl selenide, allyl phenyl selenide, and methallyl phenyl sulfide followed by tri‐n‐butylstannylation gave exclusively the γ‐stannylated sulfide or selenide (e.g., 1‐phenylseleno‐2‐methyl‐3‐tri‐n‐butylstannyl‐1‐propene (5)). Compound 5 reacts with a series of immonium electrophiles to give products of aminomethylation α‐ to selenium. Electrophiles have included N‐(bromomethyl)phthalimide/ZnBr2, Eschenmoser salt (dimethylmethyleneammonium iodide), and Mannich reagents generated in situ from diethylamine, piperidine, isopropyl sarcosinate and benzylmethylamine. The selenide from the last of these amines (N, 3‐dimethyl‐N‐benzyl‐2‐phenylseleno‐3‐butenamine, 10) has been subjected to further transformations as follows: (1) treatment of 10 with trimethylstannyllithium results in replacement of phenylseleno by stannyl; this allyltin (11) can then again be aminomethylated giving compound 12 or 13; (2) oxidation of 10 gives amino alcohol 14 by [2, 3] sigmatropic rearrangement of the allyl selenoxide; (3) photolysis of 10 results in 1,3‐rearrangement of the phenylseleno group; oxidative rearrangement of this allyl selenide gives amino alcohol 16. The phthalimidomethylation product (6) is converted to a precursor for the side chain of the cytokinin zeatin by oxidation and [2, 3] sigmatropic rearrangement.
Temperature Jump Measurements in Gas Kinetics
Authors: Gozel P. , Calpini B. , van Den Bergh H. 
Published: November 2013

AbstractThe temperature jump relaxation method is applied to kinetic measurements in the gas phase. The system N2O4⇄2NO2is studied. Fast T‐jumps are induced by IR absorption from a TEA CO2laser. Temperature jumps of less than 1 K lead to readily detectable concentration changes which are monitored by UV absorption in a simple single shot experiment.krec(Ar) is found to be 1.05 × 1010cm3mol−1s−1atPAr= 1 bar andT= 258 K, in agreement with recent experimental data.
Photoresponsive Crown Ethers. Part 11. Azobenzene‐Pillared Cylindrical Macrocycle As a Photoresponsive Receptor
Authors: Shinkai Seiji , Honda Yoshihiro , Ueda Kaori , Manabe Osamu 
Published: November 2013

AbstractA cylindrical ionophore (1) in which two diaza‐crown ethers are linked by two azobenzene pillars serves as a photoresponsive host molecule. In alkaline solution containing K+, (1) forms a cascade complex with dianionic guest molecules, but photoirradiated (1) which has a shortened cylinder cannot bind the guest molecules so effectively as (1). (1) solubilised in acidic aqueous solution by protonation of four tertiary nitrogens provides a hydrophobic cavity to bind several anionic azo dyes. The association is also suppressed by photoirradiation. The results indicate that parallel array of the azobenzene segments in (1) provides a host cavity and the photoinduced structural change is readily reflected by the association tendency. This is the first example for the photoresponsive inclusion complex.
Polymerized Surfactant Vesicles
Authors: Endler Janos H. 
Published: November 2013

AbstractIrradiation of surfactant vesicles prepared from (C18H37)2 N+(CH3)C6H4‐CHCH2p,CT−, 1, [C15H31CO2(CH2)2]2N+(CH3)CH2C6H4CHCH2,CL− 2, and (C18‐H37)2N+(CH3)CH2CH2OCOC6H4CHCH2p,Br−, 3, by ultraviolet light or by bursts of 266 nm laser pulses have resulted in the loss of styrene absorbances. This process has been accounted for in terms of a model which considers intravesicular surface reactions to give polymers with average chainlength of 22. Degreees of photopolymerization have been determined in vesicles prepared from 3 subsequent to separating the polystryrene, formed in the photolysis, from the surfactants. Vesicle surface photopolymerizations result in aqueous cleft formation and in enhanced stabilities. Polymerized vesicles provide media for in situ generation of colloidal catalysts and semiconductors.