Proton transfers in aromatic and antiaromatic systems. How aromatic or antiaromatic is the transition state? An ab initio study

An ab initio study of six carbon-to-carbon identity proton transfers is reported. They refer to the benzenium ion/benzene (C6H7(+)/C6H6), the 2,4-cyclopentadiene/cyclopentadienyl anion (C5H6/C5H5(-)), and the cyclobutenyl cation/cyclobutadiene (C4H5(+)/C4H4) systems and their respective noncyclic reference systems, that is, [structure: see text], [structure: see text] and [structure: see text]. For the aromatic C6H7(+)/C6H6 and C5H6/C5H5(-) systems, geometric parameters and aromaticity indices indicate that the transition states are highly aromatic. The proton-transfer barriers in these systems are quite low, which is consistent with a disproportionately high degree of transition-state aromaticity. For the antiaromatic C4H5(+)/C4H4 system, the geometric parameters and aromaticity indices indicate a rather small degree of antiaromaticity of the transition state. However, the proton-transfer barrier is higher than expected for a transition state with a low antiaromaticity. This implies that another factor contributes to the barrier; it is suggested that this factor is angle and torsional strain in the transition state. The question whether charge delocalization at the transition state might correlate with the development of aromaticity was also examined. No such correlation was found, that is, charge delocalization lags behind proton transfer as is commonly observed in nonaromatic systems involving pi-acceptor groups.     

Crystalline protein domains and lipid bilayer vesicle shape transformations

Cellular membranes can take on a variety of shapes to assist biological processes including endocytosis. Membrane-associated protein domains provide a possible mechanism for determining membrane curvature. We study the effect of tethered streptavidin protein crystals on the curvature of giant unilamellar vesicles (GUVs) using confocal, fluorescence, and differential interference contrast microscopy. Above a critical protein concentration, streptavidin domains align and percolate as they form, deforming GUVs into prolate spheroidal shapes in a size-dependent fashion. We propose a mechanism for this shape transformation based on domain growth and jamming. Osmotic deflation of streptavidin-coated GUVs reveals that the relatively rigid streptavidin protein domains resist membrane bending. Moreover, in contrast to highly curved protein domains that facilitate membrane budding, the relatively flat streptavidin domains prevent membrane budding under high osmotic stress. Thus, crystalline streptavidin domains are shown to have a stabilizing effect on lipid membranes. Our study gives insight into the mechanism for protein-mediated stabilization of cellular membranes.     

Synthesis of crosslinked nanostructured saccharidic vinyl copolymers and their functionalization

Saccharidic monomers and a macromonomer were synthesized and copolymerized in the presence of divinylbenzene (DVB) as crosslinker in conditions of separation of phases to give hydrophilic nanostructured sugar-based vinyl copolymers. Appropriate model molecules such as N-benzyl-d-gluconamide for the saccharidic copolymers and 4-(4-chlorobutoxy)benzaldehyde and (E)-4-(4-chloro-2-butenyloxy)benzaldehyde for electrophilic reagents prefiguring possible copper amine oxidase inhibitors allowed identification of conditions for useful monofunctionalizations mainly at the position 2 of the saccharidic units. The examined samples of the nanostructured copolymers from one of the monomers proved to be stable enough to tolerate the functionalization reactions without loss of morphology.     

2-substituted benzofuran fragment ion formation in the electron ionization mass spectra of 6-alkyl- and 6-aryldibenzo(d,f)(1,3)dioxepine derivatives. 1. Spirocyclization of the molecular ions in the gas phase

The formation of 2-substituted benzo[b]furan ions in the electron ionization (EI) mass spectra of a series of 6-alkyl- and 6-aryldibenzo(d,f)(1,3)dioxepines has been studied by means of exact mass measurements and multiple-stage mass spectrometry conditions using an ion trap mass spectrometer. The proposed mechanism of formation of benzo[b]furan ions requires the formation of a spirocyclic cyclohexadienone system, which undergoes elimination of a cyclopentadienone molecule. A parallel with the chemical conversion of arylmethyl-substituted dibenzo(d,f)(1,3)dioxepines into an analogous spirocyclic system was also underlined.     

Kinetics of the reactions of [p-nitrophenoxy(phenyl)carbene]pentacarbonylchromium(0) with aryloxide ions, hydroxide ion, and water in aqueous acetonitrile. Concerted or stepwise?

The main question addressed in this paper is whether the nucleophilic substitution of the p-nitrophenoxy group in (CO)5Cr=C(OC6H4-4-NO2)Ph (1-NO2) by a series of substituted phenoxide ions is concerted or stepwise. Rate constants, kArO, for these substitution reactions were determined in 50% MeCN-50% water (v/v) at 25 degrees C. A Br?nsted plot of log kArO versus pKa(ArOH) s consistent with a stepwise mechanism. This contrasts with reactions of aryl oxide ions with p-nitrophenyl acetate and with similar acyl transfers which are concerted. The reason for the contrast is that the tetrahedral intermediates formed in the reactions of 1-NO2 are much more stable than those in acyl transfers and the intrinsic barriers to their decomposition are higher than for the ester reactions. The points on the Br?nsted plots for which pKa(ArOH) > or = pKa(PNP) define a straight line with beta(nuc) = -0.39, suggesting that bond formation has made very little progress at the transition state and that partial desolvation of the nucleophile is part of the activation process. The hydrolysis of 1-NO2 and of the unsubstituted analogue (1-H) has also been studied over a wide pH range, providing rate constants for nucleophilic attack by hydroxide ion (kOH), by water (kH2O), and by general base-catalyzed reaction with water (kB). Furthermore, kH2O values were obtained for the hydrolysis of (CO)5Cr=C(OC6H4X)Ph (1-X) as a byproduct of the reactions of 1-NO2 with aryl oxide ions. Structure-reactivity relationships for these reactions are discussed in terms of inductive, pi-donor, and steric effects.     

Off-Bragg-angle light diffraction and structure of dynamic interband photorefractive gratings

Steady-state and time-resolved off-Bragg-angle diffraction experiments are used to determine the structure and the dynamics of photorefractive gratings induced by interband photoexcitation. In potassium niobate, we identify in such gratings basically a two-layer structure. Close to the surface, we find a space-charge electric field generated by a charge modulation stored directly in the bands. This grating component is typically 50 μm thick, the amplitude of the refractive index modulation is larger than 10^-4, and the response time is a few μs for resonant intensities of 100 mW cm^-2. This component is also robust under non-resonant illumination. Deeper in the crystal, a second holographic layer extends over a few hundreds of μm, its amplitude is smaller, and its slower response time is in the ms range. The mutual phase shift between the grating components is also determined. Received: 23 November 1998 / Revised version: 14 January 1999 / Published online: 12 April 1999     

Amidine- and Amidoxime-Substituted Heterocycles: Synthesis,Antiproliferative Evaluations and DNA Binding

The novel 1,2,3-triazolyl-appended N- and O-heterocycles containing amidine 4–11 and amidoxime 12–22 moiety were prepared and evaluated for their antiproliferative activities in vitro. Among the series of amidine-substituted heterocycles, aromatic diamidine 5 and coumarine amidine 11 had the most potent growth-inhibitory effect on cervical carcinoma (HeLa), hepatocellular carcinoma (HepG2) and colorectal adenocarcinoma (SW620), with IC₅₀ values in the nM range. Although compound 5 was toxic to non-tumor HFF cells, compound 11 showed certain selectivity. From the amidoxime series, quinoline amidoximes 18 and 20 showed antiproliferative effects on lung adenocarcinoma (A549), HeLa and SW620 cells emphasizing compound 20 that exhibited no cytostatic effect on normal HFF fibroblasts. Results of CD titrations and thermal melting experiments indicated that compounds 5 and 10 most likely bind inside the minor groove of AT-DNA and intercalate into AU-RNA. Compounds 6, 9 and 11 bind to AT-DNA with mixed binding mode, most probably minor groove binding accompanied with aggregate binding along the DNA backbone.     

Production of Cellulolytic Enzymes by <Emphasis Type="Italic">Aspergillus phoenicis</Emphasis> in Grape Waste using Response Surface Methodology

The production of cellulolytic enzymes by the fungus Aspergillus phoenicis was investigated. Grape waste from the winemaking industry was chosen as the growth substrate among several agro-industrial byproducts. A 2 × 2 central composite design was performed, utilizing the amount of grape waste and peptone as independent variables. The fungus was cultivated in submerged fermentation at 30 °C and 120 rpm for 120 h, and the activities of total cellulases, endoglucanases, and β-glucosidases were measured. Total cellulases were positively influenced by the linear increase of peptone concentration and decrease at axial concentrations of grape waste and peptone. Maximum activity of endoglucanase was observed by a linear increase of both grape waste and peptone concentrations. Concentrations of grape waste between 5 and 15 g/L had a positive effect on the production of β-glucosidase; peptone had no significant effects. The optimum production of the three cellulolytic activities was observed at values near the central point. A. phoenicis has the potential for the production of cellulases utilizing grape waste as the growth substrate.     

Fast sequential multi-element determination of Ca,Mg, K,Cu, Fe,Mn and Zn for foliar diagnosis using high-resolution continuum source flame atomic absorption spectrometry: Feasibility of secondary lines,side pixel registration and least-squares background correction

The fast sequential multi-element determination of Ca, Mg, K, Cu, Fe, Mn and Zn in plant tissues by high-resolution continuum source flame atomic absorption spectrometry is proposed. For this, the main lines for Cu (324.754 nm), Fe (248.327 nm), Mn (279.482 nm) and Zn (213.857 nm) were selected, and the secondary lines for Ca (239.856 nm), Mg (202.582 nm) and K (404.414 nm) were evaluated. The side pixel registration approach was studied to reduce sensitivity and extend the linear working range for Mg by measuring at wings (202.576 nm; 202.577 nm; 202.578 nm; 202.580 nm; 202.585 nm; 202.586 nm; 202.587 nm; 202.588 nm) of the secondary line. The interference caused by NO bands on Zn at 213.857 nm was removed using the least-squares background correction. Using the main lines for Cu, Fe, Mn and Zn, secondary lines for Ca and K, and line wing at 202.588 nm for Mg, and 5 mL min^− 1 sample flow-rate, calibration curves in the 0.1–0.5 mg L^− 1 Cu, 0.5–4.0 mg L^− 1 Fe, 0.5–4.0 mg L^− 1 Mn, 0.2–1.0 mg L^− 1 Zn, 10.0–100.0 mg L^− 1 Ca, 5.0–40.0 mg L^− 1 Mg and 50.0–250.0 mg L^− 1 K ranges were consistently obtained. Accuracy and precision were evaluated after analysis of five plant standard reference materials. Results were in agreement at a 95% confidence level (paired t-test) with certified values. The proposed method was applied to digests of sugar-cane leaves and results were close to those obtained by line-source flame atomic absorption spectrometry. Recoveries of Ca, Mg, K, Cu, Fe, Mn and Zn in the 89–103%, 84–107%, 87–103%, 85–105%, 92–106%, 91–114%, 96–114% intervals, respectively, were obtained. The limits of detection were 0.6 mg L^− 1 Ca, 0.4 mg L^− 1 Mg, 0.4 mg L^− 1 K, 7.7 µg L^− 1 Cu, 7.7 µg L^− 1 Fe, 1.5 µg L^− 1 Mn and 5.9 µg L^− 1 Zn.     

Photoinitiation rate profiles during polymerization of a dimethacrylate-based resin photoinitiated with camphorquinone/amine. Influence of initiator photobleaching rate

The photodecomposition of camphorquinone (CQ)/amine during the photo polymerization of a dimethacrylate-based resin under continuous irradiation was investigated in thick samples. The global CQ photoconsumption was measured by monitoring the decrease in light absorption as a function of irradiation time and the kinetics were satisfactory fitted to a first order expression where the rate constant of photobleaching was proportional to the irradiation intensity. In a thick sample, the photobleaching of the photoinitiator is accompanied by a deeper penetration of the light through the underlying layers. These gradients of photoinitiator concentration, light intensity and photoinitiation rate along the path of irradiation were calculated. The photodecomposition reaction was spatially inhomogeneous and the degree of nonuniformity increased with increased initial sample absorbance. The influence of the photobleaching process on the polymerization reaction was examined. The photobleaching rate of CQ was much slower than the polymerization rate and only 20% of the initial amount of CQ was consumed before the polymerization reaction had almost ceased. Results obtained in this research highlight the inherent interlinking of light attenuation and photobleaching rate in bulk polymerizing systems.