Use of cw-CRDS for studying the atmospheric oxidation of acetic acid in a simulation chamber

The coupling between cavity ring-down spectroscopy (CRDS) and an environmental chamber in the investigation of photo-induced reaction mechanisms is demonstrated for the first time. The development of the CRDS device and the corresponding analytical performances are presented. The first application is devoted to the investigation of the branching ratio of the ^?OH radical reaction of CH₃C(O)OH and CH₃C(O)OD under tropospheric conditions. An environmental chamber coupled to two complementary detection systems is used:

gas chromatography with FTIR spectroscopy for quantitative detection of acetic acid;

CRDS for quantitative detection of CO₂.

Investigation of the reaction kinetics of ^?OH+CH₃C(O)OH gives a rate constant of (6.5±0.5)×10^-13 cm³?molecule^-1?s^-1 (296 K) and shows good agreement with literature data. The product study indicates that the H-abstraction channel from the acid group is the dominant pathway with a branching ratio of (78±13)%, whereas the corresponding D-abstraction channel in the ^?OH+CH₃C(O)OD reaction represents only (36±7)%. This result could be attributed to a strong kinetic isotope effect. Glyoxylic acid has also been detected for the first time as by-product. These results illustrate the high interest of the CRDS technique in the investigation of atmospheric relevant problems.     

Synthesis of BODIPY Derivatives Substituted with Various Bioconjugatable Linker Groups: A Construction Kit for Fluorescent Labeling of Receptor Ligands

The goal of the present study was to design small, functionalized green-emitting BODIPY dyes, which can readily be coupled to target molecules such as receptor ligands, or even be integrated into their pharmacophores. A simple two-step one-pot procedure starting from 2,4-dimethylpyrrole and ω-bromoalkylcarboxylic acid chlorides was used to obtain new ω-bromoalkyl-substituted BODIPY fluorophores (1a–1f) connected via alkyl spacers of different length to the 8-position of the fluorescent dye. The addition of radical inhibitors reduced the amount of side products. The ω-bromoalkyl-substituted BODIPYs were further converted to introduce various functional groups: iodo-substituted dyes were obtained by Finkelstein reaction in excellent yields; microwave-assisted reaction with methanolic ammonia led to fast and clean conversion to the amino-substituted dyes; a hydroxyl-substituted derivative was prepared by reaction with sodium ethylate, and thiol-substituted BODIPYs were obtained by reaction of 1a–1f with potassium thioacetate followed by alkaline cleavage of the thioesters. Water-soluble derivatives were prepared by introducing sulfonate groups into the 2- and 6-position of the BODIPY core. The synthesized BODIPY derivatives showed high fluorescent yields and appeared to be stable under basic, reducing and oxidative conditions. As a proof of concept, 2-thioadenosine was alkylated with bromoethyl-BODIPY 1b. The resulting fluorescent 2-substituted adenosine derivative 15 displayed selectivity for the A₃ adenosine receptor (ARs) over the other AR subtypes, showed agonistic activity, and may thus become a useful tool for studying A₃ARs, or a lead structure for further optimization. The new functionalized dyes may be widely used for fluorescent labeling allowing the investigation of biological targets and processes.     

Formation of HO2 radicals from the 248 nm two-photon excitation of different aromatic hydrocarbons in the presence of O2

The excitation energy dependence of HO(2) radical formation from the 248 nm irradiation of four different aromatic hydrocarbons (benzene, toluene, o-xylene, and mesitylene) in the presence of O(2) has been studied. HO(2) has been monitored at 6638.20 cm(-1) by cw-CRDS, and the formation of a short-lived, unidentified species, showing broad-band absorption around the HO(2) absorption line, has been observed. For all four hydrocarbons, the same HO(2) formation pattern has been observed: HO(2) is formed immediately on our time scale after the excitation pulse, followed by a formation of more HO(2) on a much longer time scale. Taking into account the absorption of the short-lived species, the yields of both types of HO(2) radicals are in agreement with a formation following 2-photon absorption by the aromatic hydrocarbons. The yields do not much depend on the nature of the aromatic hydrocarbon. For practical use in past and future experiments on aromatic hydrocarbons, an empirical value is given, allowing the estimation of the total concentration of HO(2) radicals formed at 40 Torr He in the presence of around [O(2)] = 1 × 10(17)cm(-3) as a function of the 248 nm excitation energy: [HO(2)]/[aromatic hydrocarbon] ≈ 2 × 10(-6) × E(2) (with E in mJ cm(-2)).     

A Thermodynamic Study of 1-Propanol–Glycerol–H2O at 25°C: Effect of Glycerol on Molecular Organization of H2O

The excess chemical potential, partial molar enthalpy, and volume of 1-propanol were determined in ternary mixtures of 1-propanol–glycerol–H₂O at 25°C. The mole fraction dependence of all these thermodynamic functions was used to elucidate the effect of glycerol on the molecular organization of H₂O. The glycerol molecules do not exert a hydrophobic effect on H₂O. Rather, the hydroxyl groups of glycerol, perhaps by forming clusters via its alkyl backbone with hydroxyl groups pointing outward, interact with H₂O so as to reduce the characteristics of liquid H₂O. The global hydrogen bond probability and, hence, the percolation nature of the hydrogen bond network is reduced. In addition, the degree of fluctuation inherent in liquid H₂O is reduced by glycerol perhaps by participating in the hydrogen bond network via OH groups. At infinite dilution, the pair interaction coefficients in enthalpy were evaluated and these data suggest a possibility that the interaction is mediated through H₂O.     

Excess partial molar enthalpies,entropies, Gibbs energies,and volumes in aqueous dimethylsulfoxide

The excess partial molar enthalpies, the vapor pressures, and the densities of dimethylsulfoxide (DMSO)–H₂O mixtures were measured and the excess partial molar Gibbs energies and the partial molar volumes were calculated for DMSO and for H₂O. The values of the excess partial molar Gibbs energies for both DMSO and H₂O are negative over the entire composition range. The results for the water-rich region indicated that the presence of DMSO enhances the hydrogen bond network of H₂O. Unlike monohydric alcohols, however, the solute-solute interaction is repulsive in terms of the Gibbs energy. This was a result of the fact that the repulsion among solutes in terms of enthalpy surpassed the attraction in terms of entropy. The data in the DMSO-rich region suggest that DMSO molecules form clusters which protect H₂O molecules from exposure to the nonpolar alkyl groups of DMSO.     

STIMULATION OF APPEARANCE OF EXTRAPLASTIDIC TETRAPYRROLES BY A PHOTOOXIDATIVE TREATMENT OF THE PLASTIDS*

In the mustard seedling (Sinapis alba L.) the appearance of photodetectable phytochrome and synthesis of relatively abundant cytosolic hemoproteins (nitrite reductase, ascorbate peroxidase) are stimulated rather than impaired by a photooxidative treatment of the plastids. While the ability to synthesize protoporphyrin IX from exogenous 5-aminolevulinic acid was preserved in the photooxidatively damaged plants, protochlorophyll and chlorophyll accumulation was no longer possible. It appears from our data that in higher plants the pathway of tetrapymole synthesis up to protoporphyrin IX is not adversely affected by a photooxidative treatment of the plastids that destroys the capacity of the organelle to synthesize chlorophyll.     

H2O adsorption on Ni(100): Evidence for oriented water dimers

H₂O adsorption on clean Ni(110) surfaces at T ≦ 150 K leads at coverages below $\text{θ ? 0.5}$ to the formation of chemisorbed water dimers, bound to the Ni substrate via both oxygen atoms. The linear hydrogen bond axis is oriented parallel to the [001] surface directions. With increasing H₂O coverage $\text{(θ ≧ 0.5)}$, the accumulation of further hydrogen bonded water molecules induces some modification of the dimer configuration, producing at $\text{θ ? 1}$ a two-dimensional hydrogen bonded network with a slightly distorted ice lattice structure and long range order.     

Heterologous Expression,Biosynthetic Studies,and Ecological Function of the Selective Gq‐Signaling Inhibitor FR900359

The cyclic depsipeptide FR900359 (FR), isolated from the tropical plant Ardisia crenata, is a strong and selective inhibitor of Gq proteins, making it an indispensable pharmacological tool to study Gq‐related processes, as well as a promising drug candidate. Gq inhibition is a novel mode of action for defense chemicals and crucial for the ecological function of FR, as shown by in vivo experiments in mice, its affinity to insect Gq proteins, and insect toxicity studies. The uncultured endosymbiont of A. crenata was sequenced, revealing the FR nonribosomal peptide synthetase (frs) gene cluster. We here provide a detailed model of FR biosynthesis, supported by in vitro enzymatic and bioinformatic studies, and the novel analogue AC‐1, which demonstrates the flexibility of the FR starter condensation domains. Finally, expression of the frs genes in E. coli led to heterologous FR production in a cultivable, bacterial host for the first time.     

The Hydrolysis of Ethyl 1-Methyl-2,4-cyclopentadiene-1-carboxylate by Nonenzymatic and Enzymatic Methods. Carbon-Carbon vs. Carbon-Oxygen Bond Cleavage

The title ester 5 is shown to undergo C–C bond cleavage under the conditions of basic ester hydrolysis (KOH/EtOH) with formation of potassium ethyl carbonate ( 6 ) and the tautomeric methylcyclopentadienes 7 and 8 . In contrast, porcine liver esterase (PLE, EC 3.1.1.1) cleanly hydrolyses 5 to give the isolable 1-methyl-2,4-cyclopentadiene-1-carboxylic acid ( 13 ). The latter undergoes thermal dimerization with conservation of the geminal-substitution pattern. The configuration of the Diels-Alder adduct 17 is ascertained by it photochemical transformation into bishomocubane dicarboxylic acid 12 , easily distinguished by its C₂ symmetry. Under the conditions of acid-catalyzed hydrolysis, dimerization of ester 5 and polymerization prevail, unless low acid concentration is used. The dimer 9 of 5 has one ester function that is reluctant to undergo basic hydrolysis.