Effect of Mobile Phase pH* on Chromatographic Behaviour in Chiral Ligand-Exchange Thin-Layer Chromatography (CLETLC) of Amino Acid Enantiomers

The enantiomers of some amino acids have been separated on commercial chiral TLC plates in reversed-phase mode. The effect of the pH* of the aqueous-organic mobile phase on the retention and mobility of the enantiomers and on selectivity was investigated. It was shown that for most of the amino acids investigated the highest enantioselectivity was obtained at pH* 3–4 or 6–7. The drift and disturbance of the baseline on the chromatograms were also much smaller at pH* 3–4 and 6–7.     

The synthesis and the solvent and substituent effect on the spectroscopic characteristic of 3-ethyl-2-(p-substitued styryl)benzothiazolium iodides

The photophysical and photochemical properties of p-substitued 2-styryl-ethylbenzothiazolium iodides, possessing different electron-withdrawing or electron-donating groups are described. The dyes were prepared by the condensation of 3-ethyl-2-methylbenzothiazole salts with p-substituted benzaldehydes. The synthesis of suitable substrates is presented as well. We describe here the absorption, emission spectra and the luminescence quantum yield of hemicyanine dyes (SH) measured in 11 different organic solvents of varying polarity. Molecular structure of the synthesized dyes was established by (1)H NMR, electronic absorption and fluorescence spectrometry. The spectral data confirmed that all the compounds exist in E-configuration of their styryl residues. The planar molecular conformation is typical for the compounds with five-membered side aromatic moieties (for example benzothiazole). The compounds possessing N-alkyl substituent in phenyl ring, in contrast to the compounds with other substituents, exhibit low fluorescence quantum yield in THF solution. This indicates that for N-alkyl derivatives the non-radiative processes are much more effective than the radiative ones. The electronic absorption and fluorescence emission spectra of tested dyes demonstrate high sensitivity to the nature of substituent introduced into the aromatic ring.     

Luminescent organoboron quinolate polymers

The synthesis of well-defined luminescent organoboron polymers via a novel three-step procedure starting from silylated polystyrene is reported. Highly selective borylation of poly(4-trimethylsilylstyrene) (PS-Si), followed by replacement of the bromine substituents in poly(4-dibromoborylstyrene) (PS-BBr) with substituted thienyl groups (R = H, 3-hexyl, 5-hexyl), and final introduction of the 8-hydroxyquinolato moiety yields a series of new organoboron quinolate polymers in 67-83% isolated yield. The hexyl-substituted polymers are highly soluble and solution-processable yielding thin films that efficiently emit light at 513-514 nm upon excitation at 395 nm.     

A many-body diagrammatic theory of rotation–vibration spectra

A many-body diagrammatic perturbation theory of rotation–vibration spectra is elaborated. The present approach is based on two many-body techniques, namely on the second quantization formalism (a rotating–vibrating molecule is formally treated here as a system of interacting vibrons, obeying the Bose–Einstein statistics) and the many-body diagrammatic theory of a model Hamiltonian, initially suggested in the microscopic theory of nuclei and in the last decade very frequently exploited in the accounting for the correlation effects in many electron systems. In the framework of this theory, the rotation–vibration energies are determined as the eigenvalues of a finite-dimensional model eigenproblem.     

On the separation of nonadditive symmetric mixtures in nanoscopic slitlike pores: A simple model for racemic fluids

A grand canonical ensemble Monte Carlo simulation method is used to study the adsorption of nonadditive symmetric mixtures of Lennard-Jones spherical particles in nanoscopic slitlike pores. The walls of the pore are assumed to be formed by the parallel (100) planes of the model face centered cubic crystal of adjustable corrugation potential. It is demonstrated that depending on the nonadditivity effects in the mixture and the pore width the condensed phases formed inside the pore may have different structures. In particular, it is shown that the mixture may separate into layers containing only one component each and the stacking may depend on the pore width and properties of the mixture.     

pH gradient high-performance liquid chromatography: theory and applications

pH gradient high-performance liquid chromatography (HPLC) is a method of reversed-phase high-performance liquid chromatography suitable for ionogenic substances. It consists in programmed increase during the chromatographic process of the eluting strength of eluent with respect to the analytes separated. On the analogy of the conventional organic modifier gradient reversed-phase HPLC, in the pH gradient approach the eluting strength of the mobile phase increases due to its changing pH: increasing in case of acids or decreasing in case of bases. At the same time the content of organic modifier remains constant. A theory of the pH gradient HPLC has been elaborated. The resulting mathematical model is easily manageable. Its ability to predict changes in retention and separation of analytes following the changes in chromatographic conditions is demonstrated. The pH gradient method is uniquely suitable to determine pKa values of analytes. An equation is presented allowing to calculate pKa values basing on appropriate retention data. The effects on pKa are discussed of the concentration of methanol in the mobile phase. The RP HPLC-derived pKa data correlate to the reference pKa values (w(w)pKa) but are not identical. That may be explained by the effects on the chromatographically determined pKa of the specific interactions of analytes with stationary phases. The proposed pH gradient RP HPLC procedure offers a fast and convenient means to get comparable acidity parameters for larger series of compounds, like drug candidates, also when the analytes are available only in minute amounts and/or as complex mixtures.     

Nitrate ion association with Nd3+

The overall stability constantsK ₁ andK ₂ of NdNO ₃ ²⁺ and Nd(NO₃) ₂ ⁺ complexes were determined (K ₁=1.77;K ₂=1.28) using an extraction method with tri-n-butyl phosphate as the extractant. The ratio ₁/₂ of the stepwise stability constants is discussed. It was established that the Nd(NO₃) ₂ ⁺ complex was an outer-sphere ion pair.

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Die Assoziation von Nitrat-Ion mit Nd³⁺

Zusammenfassung Mittels einer Extraktionsmethode wurden die StabilitätskonstantenK ₁ undK ₂ von NdNO ₃ ²⁺ - und Nd(NO₃)⁺-Komplexen bestimmt (K ₁=1.77;K ₂=1.28; Tri-n-butylphosphat als Extraktionsmittel). Das Verhältnis ₁/₂ der stufenweisen Stabilitätskonstanten wird diskutiert. Es stellte sich heraus, daß der Nd(NO₃)⁺-Komplex als ein Outer-Shere-Ionenpaar vorliegt.

     

Effect of molecular weight on the exponential growth and morphology of hyaluronan/chitosan multilayers: a surface plasmon resonance spectroscopy and atomic force microscopy investigation

The layer-by-layer growth of multilayer assemblies of two polysaccharides, the polyanion hyaluronan (HA) and the polycation chitosan (CH), was investigated using atomic force microscopy (AFM) and surface plasmon resonance (SPR) spectroscopy, with primary emphasis on the effect of the polysaccharide molecular weights on the film thickness and surface morphology. The HA/CH multilayers exhibit an exponential increase of the optical film thickness with the number of deposited bilayers. We show that the multilayer thickness at a given stage depends on the size of both CH, the diffusing polyelectrolyte, and HA, the non-diffusing species. Assemblies (12 bilayers) of high molecular weight polysaccharides (HA, 360,000; CH, 160,000) were twice as thick (approximately 900 nm vs approximately 450 nm) as those obtained with low molecular weight polymers (HA, 30,000; CH, 31,000), as assessed by AFM scratch tests. The exponential growth rate is the same for the high and low molecular weight pairs; the larger film thicknesses observed by SPR and by AFM arising from an earlier onset of the steep exponential growth phase in the case of the high molecular weight pair. In all cases, isolated islets form during the deposition of the first CH layer onto the underlying HA. Upon further film growth, individual islets coalesce into larger vermiculate features. The transition from distinct islands to vermiculate structures depends on the molecular weights of the polysaccharides and the lower molecular weight construct presents larger worm-like surface domains than the high molecular weight pair.     

The kinetics of tritium-hydrogen exchange in xanthosine,theobromine and monomethylated derivatives of xanthine

Tritium exchange at C-8 of xanthosine, theobromine, 1-, 3- and 7-methylxanthine in water has been studied. The rates of detritiation of these compounds have been determined over a pH range at constant temperature. Several mechanisms of exchange involving various ionic forms of substrate operating at different pH have been suggested.     

A study of the composition and properties of the ion-association complex of Rhodamine B with silicomolybdic acid, with a view to its analytical application

The composition of the ion-association complex of Rhodamine B with silicomolybdate has been examined by Job's method and spectrophotometric titration. The ratio of Rhodamine B to silicon in the complex is 4 : 1. The same ratio is obtained by analysis of the crystalline complex. The composition of the complex is identical in aqueous medium, an organic solvent (ethanol) and in the crystalline state (C(28)H(30)N(2)O(3))(4)SiMo(12)O(40)). The complex is stable in the organic solvent and has its absorption maximum at 555 nm and a molar absorptivity of 5 x 10(5) 1.mole(-1)cm(-1). The complex can be advantageously used for the determination of silicon.