13-Hydroxyacenaphato[1,2-b]quinolizinium bromide as a new flouorescence indicator

13-Hydroxyacenaphtho[1,2-b]quinolizinium bromide (13-HQBr)was selected as a fluorescence indicator to determine basic compounds in non-aqueous media. This compound possesses an acidic phenolic hydroxyl group. It presents varying absorption (ROH, 408, 430 nm; RO(-) 456, 478 nm) and excitation spectra (ROH, 425 nm; RO, 471 nm) depending on the pH of the media, but the same emission fluorescence spectrum (ROH = RO(-), 526 nm) at different pH in buffered aqueous solutions. However, in acidic non-aqueous media (acetic, formic and trifluoroacetic acids), it can be observed that the fluorescence emission spectra differ for the ionized (lambda(em) = 530 nm) and non-ionized (lambda(em) = 440, 470 nm) forms. The fluorescence intensity at the characteristic peaks depends on the acid-base equilibria in the ground and excited states. Therefore, this property could be used to evaluate the concentration of basic compounds, showing a good linearity range between fluorescence intensity and basic sample concentration.     

Texture changes inside smectic-C droplets in azobenzene Langmuir monolayers

Preparation of Langmuir monolayers of a mixture of trans- and cis-isomers of an azobenzene derivative, 4-[4-[(4-octylphenyl)azo]phenoxy]butanoic acid, results in the segregation of birefringent trans-isomer domains embedded in an isotropic medium of cis-isomers. Brewster angle microscopy observations allow us to identify different textures inside the domains depending on surface pressure, temperature, and domain size. The evolution of the monolayer in the dark, from initial droplets formed after spreading to a stable stripe texture, is described. The dynamics of domain coalescence and some morphological transitions induced by temperature and surface pressure changes are also discussed. A simple theoretical model is included to supplement some of these experimental observations.     

Bestimmung der Aktivitätskoeffizienten im chromatographischen Modell „binärer Lösungen“

An explanation is given for the two possible ways of determination of the activity coefficients of solutes, when applying a new thermodynamic model of adsorption and partition chromatography, i. e. the binary solutions model.

     

Quantitative analysis of caffeic acid phenethyl ester in crude propolis by liquid chromatography-electrospray ionization mass spectrometry

Caffeic acid phenethyl ester (CAPE), an active component of propolis from honeybee hives, is known to have antimitogenic, anticarcinogenic, antinflammatory, and immunomodulatory properties. The paper describes a rapid and simple liquid chromatography-electrospray ionisation mass spectrometry method for qualitative and quantitative determination of CAPE. The chromatographic separation was performed with a Luna RP-C18 column using a water-acetonitrile linear gradient. The method was linear over a 0.125-80 ng/mL range (LOD = 62.5 pg/mL). The method was applied for the quantitation of caffeic acid phenethyl ester in crude propolis samples, which were analysed directly after extraction with ethyl acetate solution.     

Structural Study of C-undecylcalix[4]resorcinarene Solvate with Dioxane

The crystal structure of the molecular complex of C-undecylcalix[4]resorcinarene with dioxane has been determined by X-ray analysis. The asymmetric unit contains one host and four guest molecules. The calix[4]resorcinarene moiety adopts a bowl conformation with C_4v symmetry. Four undecyl chains are axially oriented. Calix molecules are packed in a bowl-to-bowl fashion with alternating hydrophilic and hydrophobic layers. One of the hydrophilic dioxane molecules is located at the rim of the calix moiety and is hydrogen bonded to the other one. There is no interaction to attract, or direct the dioxane molecule into the interior of the cavity. There is an exo complex formed. The dioxane molecules – located in the hydrophobic part – are highly disordered.     

Spectroscopic properties of the photoproducts of pyridoxal-5′-P irradiation: Catalytic site recognition of ribonuclease A

Photoproducts of pyridoxal-5-P, i.e., 4-pyridoxic-5-P and bis-pyridoxal-5-P, have been studied by spectroscopic methods. The spectroscopic properties of bis-pyridoxal-5-P (bis-PLP) resemble those of pyridoxal-5-P (PLP) under similar experimental conditions. The coupling of methylen hydrogens to the phosphorus atom has been shown by NMR spectroscopy. The singlet in the³¹P-NMR spectra and the triplet in¹H-undecoupled experiments confirm the presence of the phosphate group in the 5 position of the structure of the vitamin. The effect of pH and solvent composition on the relative distribution of species of bis-pyridoxine-5-P (bis-PNP) has been investigated by absorption and fluorescence spectroscopy. The acid-base dissociation of the phosphate group is easily detected by emission spectroscopy. Bis-PNP and bis-PLP bind to the enzyme RNase A and they behave as competitive inhibitors with respect to the substrate cytidine-2-3-cyclic phosphate. The natural forms of vitamin B₆, pyridoxine, and pyridoxine-5-P have no effect on the catalytic activity of the protein. Experimental evidence derived from fluorescence and inhibition experiments is consistent with the hypothesis that bis-PNP recognizes the catalytic site of RNase A.