Supramolecular helical mesomorphic polymers. Chiral induction through H-bonding

The work described here concerns a challenge of general interest in supramolecular chemistry: the achievement of chiral helical organizations with controlled structures. This work provides a strategy to obtain supramolecular polymers in which a chiral helical conformation has been induced by a noncovalent association, that is, through hydrogen bonding. Polycatenar 2,4,6-triarylamino-1,3,5-triazines, which organize into columnar mesophases and are susceptible to H-bonding interactions, were chosen as a starting point to build up the chiral supramolecular structure. The stacking of these mesogens has been forced to wind in a helical way by means of H-bond association with (R)-3-methyladipic acid, within the mesophase. The optically active columnar organization has been studied in depth by optical microscopy, differential scanning calorimetry (DSC), X-ray diffraction, and circular dichroism. Formation of stable complexes between the triazine units and (R)-3-methyladipic acid has also been investigated by means of NMR diffusion-ordered spectroscopy (DOSY) experiments in chloroform.     

Statistical thermodynamics models for polyatomic adsorbates: application to adsorption of n-paraffins in 5A zeolite

Experimental adsorption isotherms of five n-paraffins (ethane, propane, butane, pentane, and hexane) in 5A zeolite were described by means of a statistical thermodynamics model for linear adsorbates (MLA) developed by Ramirez-Pastor et al. (1999) and compared with the well-known multisite Langmuir model (MSL) of Nitta et al. (1984). The experimental data, obtained by different authors in a wide range of temperatures and pressures, were correlated by using an algorithm of multiple fitting. Two main conclusions were drawn from the analysis of experimental data: (i) for small molecules (ethane, propane), MLA is the more accurate model, validating the hypothesis of the linear rigid character of the adsorbate and reinforcing previous results obtained from the analysis of computational experiments developed for dimers and linear trimers; (ii) for large molecules (n-butane, n-pentane, n-hexane), the better performance of the MSL model suggests that the admolecules adsorb in a nonlinear structure. The isosteric heat of adsorption dependence on the number of carbons obtained from our study, ranging between 23.84 kJ/mol for ethane and 59.26 kJ/mol for hexane, showed a very good agreement with previous results reported in the literature, confirming the consistency of our analysis.     

Asymmetric synthesis of [7]helicene-like molecules

[reaction: see text] A new approach to nonracemic [7]helicene-like molecules has been developed. Stereoselective Co(I)-mediated [2 + 2 + 2] cycloisomerization of aromatic triynes containing an asymmetric carbon atom produces [7]helicene-like scaffolds in diastereomeric ratios up to 100:0. This central-to-helical chirality transfer can be controlled by the absolute configuration at the asymmetric center and by the presence of carbon substituents.     

Capillary electrophoretic studies of acid-base properties of sanguinarine and chelerythrine alkaloids

Capillary zone electrophoresis with UV detection was used for determination of dissociation constants of alkaloids sanguinarine and chelerythrine. Despite the limited solubility of the uncharged forms of the alkaloids resulting in insufficient analytical signal at higher pH the reliable dissociation constants were obtained when acidified samples containing low amount of the alkaloid were injected into the capillary. The precipitation of the alkaloid in the capillary induced by injecting sample of low pH into the background electrolyte of higher pH does not affect the mobility of the alkaloid if its concentration injected exceeds the solubility only to a small extent. Dissociation constants (pK(R+)) of sanguinarine and chelerythrine calculated to 8.3 +/- 0.1 and 9.2 +/- 0.1, respectively, are relevant to Good buffers of ionic strength of 30 mM.     

Stationary-phase effects in gradient high-performance liquid chromatography

The type of the stationary phase for reversed-phase liquid chromatography significantly affects the sample polarity range that can be covered using gradients of organic solvents in water. The polarity range available for gradient separations of samples containing compounds differing in the lipophilic parts of the molecules can be characterized by "gradient lipophilic capacity", Pl, based on the retention of standard compounds with a repeat lipophilic structural unit, such as a methylene group. The gradient lipophilic capacity is also suitable to characterize the separation possibilities of the columns in non-aqueous reversed-phase gradient elution of strongly non-polar compounds, such as triacylglycerols. In the same way, the suitability of various columns for reversed-phase gradient separations of oligomers can be characterized by "gradient oligomer capacity", as demonstrated in the example of oligo(ethylene glycols). To enable a comparison of the properties of stationary phases independent of column efficiency and dimensions, the gradient lipophilic capacity or the gradient oligomer capacity should be normalized for a "standard" column plate number, gradient range and volume (in column hold-up volume units). The gradient lipophilic capacity or the gradient oligomer capacity and the number of compounds that can be resolved during a gradient run decrease as the initial concentration of the strong solvent in the mobile phase increases and (or) the gradient time decreases. These quantities can be used to select a suitable column and to adjust the optimum gradient profile (the initial composition of the mobile phase and the gradient steepness) with respect to the time of analysis and the number of oligomers or other compounds with regular repeat structural groups that can be resolved during the gradient run.     

A modified HPLC method for the determination of aspartic acid racemization in collagen from human dentin and its comparison with GC

The D/L ratio of aspartic acid enantiomers in proteins of low turnover is generally accepted as a reliable procedure for age determination. In our study, twelve samples of eyetooth dentin were analyzed for age determination. The pure insoluble collagen isolated from eyetooth dentin was obtained by an EDTA demineralization process. Free amino acids obtained after collagen hydrolysis were converted into o-phthaldialdehyde-N-acetyl-L-cysteine (OPA-NAC) derivatives for HPLC analysis under modified conditions and into trifluoroacetic acid isopropyl esters for GC analysis, respectively. The modified HPLC procedure used phosphate buffer and acidified sample matrix prior to injection which resulted in suppression of peak tailing of both diastereomers, thus allowing achievement of both good selectivity and good resolution. To ensure the high accuracy of the developed method the other parameters, i.e. specificity, precision, linearity, LOD, and LOQ, were also determined. Nine collagen samples covering the age range of 18 to 84 years were used for the determination of coefficient of racemization (KR) and calculation of parameters for age estimation. The regression equations for the data set analyzed were as follows: KR= 0.0005 x age + 0.0262 (R2 = 0.9639) for HPLC, and KR= 0.0006 x age + 0.0319 (R2 = 0.9374) for GC, respectively.     

Determination of moniliformin in maize by ion chromatography

An ion chromatographic method for the determination of the mycotoxin moniliformin in maize was developed. The method contains a fast and simple clean-up procedure, which allows high volume sample injection. No further pre-concentration step is required. The limit of quantification was estimated to be 0.12 mg/kg moniliformin in maize. In contrast to the two previously published ion chromatographic methods, the moniliformin peak is base line separated and occurs in a region of a smooth base line in the ion chromatogram. The newly developed method showed a recovery through the whole analytical procedure of 97+/-3.5 %, whereas most previously described analytical methods had recoveries in the 70% range.     

Efficiency of decomposition procedures for the determination of some elements in soils by atomic spectroscopic methods

Nine decomposition procedures for soil samples, such as via acid mixtures and fusion in open systems and microwave and autoclave dissolution (in closed systems under elevated temperature and pressure) were assessed using certified soil reference materials. The determination of various elements in solutions was performed by FAAS and ICP-OES and the results were compared with certified values and with direct current arc OES analysis. Received: 3 March 1997 / Revised: 30 May 1997 / Accepted: 3 June 1997