Sequence peptide polymers: 3. Synthesis and conformational study in solution of poly (l-leucyl-l-leucyl-ε-N-carbo-benzyloxy-l-lysine

The synthesis and characterization of poly[Leu-Leu-(ε-Z)-Lys] and its conformational study in organic solvents are reported. The sequential polypeptide has been obtained with a satisfactory optical purity by the self-condensation method of the p-nitrophenyl ester of the corresponding tripeptide derivative according to the general procedure of De Tar et al. Poly[Leu-Leu-(ε-Z)-Lys] showed a right-handed α-helical conformation in solution of helicogenic solvents, as displayed by circular dichroism measurements and ultra-violet absorption spectra. The presence of the α-helix has been also detected in the solid state by infra-red spectroscopy. The helix stability of the polypeptide in chloroform—dichloroacetic acid solution is considerably high. This property may be related to the ordered sequence of the bulky leucyl residues which give rise to a screening effect, protecting the chain backbone from the acid solvent molecules.     

NMR, multi-spectroscopic and molecular modeling approach to investigate the complexes between C.I. Acid Orange 7 and human serum albumin in vitro

In this study, the interaction between C.I. Acid Orange 7 (AO7) and human serum albumin (HSA) was firstly investigated using nuclear magnetic resonance (NMR) spectroscopy in combination with fluorescence quenching spectroscopy, three-dimensional fluorescence spectroscopy, UV-vis absorption spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, circular dichroism (CD) spectroscopy and molecular modeling method in vitro. The results of NMR data confirmed that AO7 indeed interacted with HSA, and the hydrophobic portion of AO7 should be embedded to the hydrophobic pocket of HSA. The fluorescence quenching analysis revealed that AO7 can bind to HSA. The conformational change of HSA in the presence of AO7 was confirmed by synchronous fluorescence, three-dimensional fluorescence, UV-vis absorption, FT-IR and CD spectra. The binding distance between AO7 and tryptophan residue of HSA was calculated by the efficiency of fluorescence resonance energy transfer. Molecular modeling showed that hydrophobic force and hydrogen bonds were the major interaction between AO7 and HSA.     

The measurement of catalyst acidity: I. Titration measurements

The n-butylamine titration method has been widely used for the measurement of acid strength distributions on solid catalysts. An implicit assumption with this technique is that both the n-butylamine titrant and the indicators are in adsorption equilibrium with acid sites on the surfaces. This assumption has been tested and found invalid. Serious doubt is therefore cast on the utility of such methods for the assessment of surface acidity. It is shown that instead of determining an acid site distribution, the total acidity of the sample is usually obtained. Having ascertained the problems involved, an alternative method has been devised for characterizing the site energy distribution. A series of bases of varying pK_a dissolved in one of several solvents was employed. The strong irreversible chemisorption of these yielded distributions quite different from those obtained by the n-butylamine technique.     

Solvent motion and excited state decay in solutions of Cr(III) complexes

Thermal quenching of ²E in Cr(bpy)₃³⁺ (bpy = 2,2^′-bipyridine) solutions where back-intersystem crossing is not important is compared with the quenching of Cr(NCS)₆³⁻ and trans-Cr(NH₃)₂(NCS)₄⁻ in several solvents. Solvent motion enhances excited state relaxation whether or not back-intersystem crossing is a significant relaxation pathway. The relationship between motionally induced decay and the exponential or nonexponential character of the decay is discussed.     

Desaktivierungsverhalten von Arenen und Heteroarenen. XXXIII. Zur statischen Löschung der Fluoreszenz von Azinium-Anthracenen in Lösungsmitteln mit n-Donatoreigenschaften

Deactivation Behaviour of Arenes and Heteroarenes. XXXIII. Static Fluorescence Quenching of Azinium-Anthracenes by Solvents of n-Donor Properties The fluorescence of azinium-anthracenes is quenched by solvents of n-donor properties. A model of static quenching by electron transfer is developed analogous to the dynamic process. The rates of quenching correlate with the donor properties of the solvent (ionization potential) and the acceptor properties of the fluorophor (reduction potential). In the same way as in the case of dynamic quenching this static process is characterized by a high free enthalpy of activation. The existence of a thermally induced step in the process of the electron transfer affects the temperature dependence of the fluorescence quantum yields. The analysis of this temperature dependence on the basis of an Arrhenius equation shows a linear relationship.     

Solvent—solute interaction. Spectrophotometric and polarographic characteristic of copper—II. Acetylacetonate in organic solvents

Bis-acetylacetonato-copper (II) was spectrophotometrically studied in 13 organic solvents. Correlation between the molar absorptivity of d→d donicity of solvents has been observed. This phenomenon is discussed in terms of donor-acceptor concept for solvent-solute interaction. In the presence of Li⁺ions more complex equilibria exist in solutions. The possible structures of adducts are proposed.     

Spectrofluorometric Analysis of Aromatic Compounds: Review of Applicability of Nitromethane as a Selective Fluorescence Quenching Agent for Identification of Alternant vs. Nonalternant Polycyclic Aromatic Hydrocarbons

Applicability of nitromethane as a selective fluorescence quenching agent for discriminating between alternant versus nonalternant polycyclic aromatic compounds is critically reviewed. Experimental data for 170 different polycyclic aromatic hydrocarbons (PAHs) and polycyclic aromatic nitrogen hetero-atoms (PANHs) dissolved in nonelectrolyte solvents and in aqueous micellar solvent media are presented. Specific items examined include the basic quenching mechanism and determination of rate constants, chemical and instrumental artifacts associated with accurate determination of fluorescence emission intensities, and the effect that solvent polarity, surfactant headgroup charge and electron donating and electron withdrawing substituents have in regards to fluorescence emission quenching by nitromethane.     

Extraction of biologically active compounds from Sideritis ssp. L.

In this study extraction of polyphenols and flavonoids from cultivated hybrid Sideritis scardica × Sideritis syriaca, known for its rich content of phenolics and flavonoids with antioxidant activity, was investigated. Extractions have been done by ethanol and water-ethanol, respectively. High equilibrium values of the extracted species were obtained—17.55 mg/(g solid) total phenolics and 5.7 mg/(g solid) total flavonoids with ethanol as solvent. The influence of the solvent on the total yield and the content of biologically active compounds were studied. Maximum polyphenolics and flavonoids extraction was observed for water-ethanol solvent ratio 20/80. Increase of the content of ethanol in the solvents led to lower total yield of extracts but higher percentage of polyphenolics. The extraction kinetics showed that 90% of the phenolic compounds were extracted during the first 2.5 h. The experimental kinetics was described by a constant effective diffusion coefficient D_e = 1.5 × 10⁻¹² m²/s in the solid, accounting for the actual particle size distribution.     

2－芳基苯并噁唑的荧光猝灭和光诱导电子转移研究

本工作对两种不同的２－芳基苯并　唑化合物溶液荧光被四氯化碳所猝灭的机理进行了详细研究，通过多种途径研究表明该猝灭过程具有光诱导电子转移性质，工作还利用此电子转移所形成的活泼自由基来引发烯类单体的聚合，得到了有一定聚合度的聚甲基丙烯酸甲酯。     

Highly selective CH2Cl2 fluorescent sensor based on Cd(II) metal-organic framework

Hydrothermal reaction of Cd^2 + ions and 5-(4-carboxybenzyloxy)isoph-thalic acid (H₃L) afforded a new 3D metal-organic framework (Cd-MOF), namely [Cd(HL)(2,2-bipy)]_n·1.25nH₂O. The luminescent properties of Cd-MOF dispersed in different solvents have been investigated systematically, demonstrating unique selectivity for the detection of CH₂Cl₂ via a fluorescence quenching mechanism. The luminescent intensity of Cd-MOF was selectively quenched by Ni^2 + ions.     

Dynamical adsorption and temperature-programmed desorption of VOCs (toluene, butyl acetate and butanol) on activated carbons

The elimination of solvent vapors from paint shop atmospheres and their recovery for possible re-use is a current environmental requirement. We have studied the dynamical adsorption at 23 °C of typical car paint solvents, i.e. toluene, butylacetate and butanol, on two microporous activated carbons, and the thermal regeneration of these carbons with hot air at 150 °C. A sequence of seven adsorption-desorption cycles with a mixture of these solvents left the carbons with some textural changes but the adsorption capacity remained virtually unaffected. Heavy volatile compounds, proceeding from the possibly unstabilized binder, are eliminated in the course of the first adsorption-desorption runs. Differential scanning calorimetry applied to the desorption in argon of pure solvents showed that the desorption enthalpies ΔH_des are close to, or slightly larger than, the evaporation enthalpies ΔH_vap except for butanol, which exhibited a particularly high ΔH_des value. Temperature-programmed desorption experiments, obtained with carbon samples heated in a helium flow containing oxygen traces, evidenced the desorption of numerous oxidation products. This finding may have consequences for hot air regeneration processes.     

Extended and threefold helical conformations in gels of isotactic polystyrene

Summary Gels of isotactic polystyrene (iPS) were prepared, using solvents of different molecular geometries, by rapid quenching of the solutions formed at high temperatures. The extended conformation was observed in gels prepared with bulky, cyclic hydrocarbon solvents, whereas the aromatic solvents promoted the threefold helical structure. The gels containing the threefold helix were more elastic compared to those with the extended conformation. The effect of the solvent geometry is explained qualitatively.     

Löschung der Fluoreszenz aromatischer Urethane durch Ketone

Fluorescence Quenching of N-Arylurethanes by Ketones Fluorescence and photo-Fries reaction of N-aryl-urethanes can be quenched by diffusion-controlled energy transfer to biacetyl, acetone and other aliphatic ketones. Quenching of the photo reaction and fluorescence obeys the Stern- Volmer Equation. The efficiency of quenching may by influenced by solvents and substituents changing the S₁ energies of donors and acceptors. The identical rate constants determined by independent methods demonstrate that quenching of urethane fluorescence and photoreaction by aliphatic ketones occurs completely by singlet energy transfer.     

Photophysics of polyaniline: Sequence-length distribution dependence of photoluminescence quenching as studied by fluorescence measurements and Monte Carlo simulations

The dependence of the fluorescence quenching of electropolymerized poly(aniline-co-m-chloroaniline) with polymer composition has been investigated. Fluorescence emission in polyaniline is quenched when the polymer is oxidized (brought to emeraldine state); the copolymers exhibit decreasing quenching as chloroaniline contents increases. Quenching shows a strong decrease in the presence of 0.1% m-chloroaniline monomers in the feed. The presence of dichloroaniline units in the copolymer was confirmed by XPS measurements and a terpolymerization reaction scheme was developed, obtaining the kinetic parameters. By Monte Carlo simulation the sequence length distributions for different compositions were obtained and compared; it was found that quenching, for low aniline contents, requires aniline sequences of at least three units. The strong decrease in quenching at low m-chloroaniline contents is attributed to a double effect: breaking of conjugation in the emeraldine form by the presence of the chlorinated unit, and a disruption of the close chain packing in the crystalline domains, preventing state delocalization and thus efficient quenching.     

Photocatalytic decomposition of trichlorophenol by zinc(II) phthalocyanine derivatives in aerated organic solvents

Trichlorophenol was photodecomposed by irradiation with visible light using zinc(II) phthalocyanine derivatives as a photosensitizer in aerated organic solvents. An oxidative quenching pathway via a generation of hyperoxide was proposed.     

Effect of solvents on inhibition of collagen-bound proteases by chlorhexidine

Chlorhexidine (CHX) incorporation during dentine bonding with dental adhesives enhances bond durability. Choosing an appropriate solvent for CHX to enhance resin–dentine bond durability is important. This study was conducted to examine the effect of solvents on the inhibition of dentine collagen-bound proteases by chlorhexidine. The inhibitory effects of CHX (0.2% and 2%) incorporated in three different solvents (water/ethanol/acetone) on the inhibition of dentine matrix-bound proteases were investigated by assessing the change in dry mass of demineralized dentine beams and the release of solubilized collagen peptides after 30 days. Solvents without CHX were used as controls. The beams treated with 2% CHX exhibited significantly lesser loss of dry mass and hydroxyproline release when incorporated in ethanol than in water (p<0.05). No significant difference in the inhibitory effect on dentine matrix-bound proteases was found between 0.2% and 2% CHX (p>0.05). The incorporation of 2% chlorhexidine in ethanol showed greater inhibitory effect on dentine matrix-bound proteases than in water.     

A comparative study on solvents for separation of tert-amyl ethyl ether and ethanol mixtures. New experimental data for 1-ethyl-3-methyl imidazolium ethyl sulfate ionic liquid

Ionic liquids have potential applications on separation technologies, as solvents for liquid extraction, gas absorbers and others. In this work, the ability of 1-ethyl-3-methyl imidazolium ethyl sulfate, [emim][EtSO₄], to carry out the separation of tert-amyl ethyl ether and ethanol mixtures has been studied. New experimental LLE data for the ternary system are reported. Data correlation was done by means of the NRTL equation with satisfactory results (rmsd=0.429%). Extraction process was studied by means of solute distribution ratio and selectivity of the ionic liquid. Results were compared to other solvents from the literature. Solute distribution ratio for [emim][EtSO₄] was acceptable (higher than 1) and selectivity was better than for other solvents. Then, extraction processes were also simulated using conventional software and obtained results are shown for comparison. Best TAEE recovery was obtained with [emim][EtSO₄] and water.     

Evaluation of solvents for carbon-13 n.m.r. analysis of solvent-refined coals. sym-Triazine a new carbon-13 n.m.r. solvent

Solvent-refined lignite (SRL) and bituminous coals (SRC) are only partially soluble in good standard carbon-13 n.m.r. solvents. A survey of other possible solvents for SRLs and SRCs showed that sym-triazine at 86 °C solvated SRL as well as pyridine, but did not absorb in the regions of interest (5–60 and 90–160 ppm relative to TMS). Model hydrocarbons, phenols and an alcohol in sym-triazine gave aromaticities (f_a) accurate within the standard deviation (0.01 to 0.001) when proper FT carbon-13 techniques were used. An alkene and an aliphatic amine reacted with sym-triazine and gave inaccurate values of f_a.     

Relaxationsprozesse in optisch angeregten EDA-Komplexen des TCNE

Relaxation Processes in Optically Excited EDA Complexes of TCNE The quenching of fluorescence of aromatic π-donors by TCNE served as an qualitative criterion for the participation of excited states of EDA complexes in deactivation processes in solution. Due to the energetic situation of the states taking part in deactivation the quenching process in the chosen TCNE-donor complexes is controlled by diffusion. In unpolar solvents and in the presence of oxygen phosphorescence of donors and ionic dissociation of EDA complexes was not be observed. While at 25°C the deactivation of the EDA excited states occurs mainly radiationless, the appropriate CT fluorescence emissions at – 196°C are evident.