Simultaneous determination of phenolic compounds and triterpenic acids in oregano growing wild in Greece by 31P NMR spectroscopy

³¹P nuclear magnetic resonance (NMR) spectroscopy was used to detect and quantify simultaneously a large number of phenolic compounds and the two triterpenic acids, ursolic acid and oleanolic acid, extracted from two oregano species Origanum onites and Origanum vulgare ssp. Hirtum using two different organic solvents ethanol and ethyl acetate. This analytical method is based on the derivatization of the hydroxyl and carboxyl groups of these compounds with the phosphorous reagent 2‐chloro‐4,4,5,5‐tetramethyl‐1,3,2‐dioxa phospholane and the identification of the phosphitylated compounds on the basis of the ³¹P chemical shifts. Unambiguous assignment of the ³¹P NMR chemical shifts of the dihydroxy‐ and polyhydroxy‐phenols in oregano species as well as those of the triterpenic acids was achieved upon comparison with the chemical shifts of model compounds assigned by using two‐dimensional NMR techniques. Furthermore, the integration of the appropriate signals of the hydroxyl derivatives in the corresponding ³¹P NMR spectra and the use of the phosphitylated cyclohexanol as an internal standard allowed the quantification of these compounds. The validity of this technique for quantitative measurements was thoroughly examined. Copyright © 2012 John Wiley & Sons, Ltd.     

Critical Comparison of Two Models Describing Segmental Dynamics of Polymer Chains in Dilute Solutions

This study attempts to find connection between the Dejean de la Batie‐Laupretre‐Monnerie bimodal correlation function for polymer chain motions in dilute solutions and the model‐free approach developed by Lipari and Szabo. Numerical fits of the Dejean de la Batie‐Laupretre‐Monnerie model to the model‐free approach in both the time and frequency domains allow the determination of specific conditions, under which the motional parameters (correlation times and amplitudes of internal motions) of one model can be converted to the parameters of the other model. This conversion is very useful in cases where NOESY experiments in conjunction with computational methods, incorporating the model‐free approach, are used for the determination of the three dimensional structures of synthetic and biological macromolecules.     

Polymerization of oriented monomers. IX. Further studies on the polymerization and vesicular properties of allyl and diallyl based monomeric and polymerized quaternary ammonium salts

The polymerization of the previously reported allyl and diallyl quaternary ammonium salts, which from vesicles in water, to their polymerized analogs was further investigated. The structure of the polymer derived from the diallyl derivative, for which some controversy existed, was elucidated by ¹³C-NMR spectroscopy. Mixed vesicles, based on the monoallyl monomer and incorporating certain vesicle forming quaternary ammonium salts, cholesterol, or cholesteryl acrylate were also formed. These mixed vesicles were characterized primarily as far as their stability is concerned. It was found that the incorporation of the additives does not enhance the stability of the mixed vesicles compared to that exhibited by monoallyl simple vesicles. In addition, polymerization of mixed monoallyl-cholesteryl acrylate vesicles results in the destruction of their structure. On the contrary, simple polymerized vesicles, resulting from the diallyl monomer, showed excellent long-term stability. Specifically, samples prepared two years ago are still dispersed and, very possibly, they will remain so for a longer period since they are not showing signs of precipitation.     

Mesomorphic-like character of long-chain amine salts formed by template neutralization on polyacids

Several examples of monomeric and polymerized surfactants exhibiting liquid-crystalline behaviour have recently been reported. In the present investigation a novel process for the formation of liquid crystals is introduced, according to which certain functionalized polymers induce mesomorphism when they react with appropriate amines. For instance, neutralization of polyacrylic or polymaleic acid with long-chain amines forms alkylammonium salt surfactants which exhibit mesomorphic-like character. In this type of experiment, however, the known lamellar structure of alkylammonium salts was modified by this so-called template neutralization. The carboxylate groups, due to their location on the polymeric chain, appear to exert a topochemical effect on alkylammonium moieties which in turn affect the mesomorphic behaviour compared with that of common surfactants. The mesomorphic behaviour of these materials was established by optical microscopy and differential scanning calorimetry.     

Polymerization of oriented monomers. II. Polymerization of 4-vinylpyridine perchlorate in micellar aggregates

Polymerization of 4-vinylpyridinium perchlorate (I) is studied under isotropic and anisotropic conditions. Anisotropy or orientation of I may be achieved by micelle formation in concentrated aqueous or acid solutions, i.e., above the monomer's critical micelle concentration (CMC). Some properties of the isotropically and anisotropically obtained polymers are comparatively studied with the emphasis placed on the elucidation of their structure by ¹³C-NMR spectroscopy. By the same spectroscopic technique the microstructure of 1,2-polymer is further investigated in conjunction with the proposed polymerization model. It is concluded that polymer structure is affected by the organization of the monomers into micelles whereas their microstructure is not.     

Polymerization of oriented monomers. VIII. Polymerization of allyl and diallyl vesicle-forming quaternary ammonium salts

The synthesis of two novel allyl and diallyl vesicle-forming quaternary ammonium salts is reported. The topochemical polymerization of these monomeric vesicles to their polymerized counterparts was performed by γ-ray irradiation and the observed differentiation in polymerization rates of allyl and diallyl monomers was attributed to different polymerization mechanisms. It was further established that only polymerized vesicles which result from diallyl monomer retain the structure of the monomeric vesicles and exhibit simultaneously higher stability.     

A powerful force-based approach for the limit analysis of three-dimensional frames

For the estimation of the strength of a structure, one could avoid detailed elastoplastic analysis and resort, instead, to direct limit analysis methods that are formulated within linear programming. This work describes the application of the force method to the limit analysis of three-dimensional frames. For the limit analysis of a framed structure, the force method, being an equilibrium-based approach, is better suited than the displacement method and results, generally, to faster solutions. Nevertheless, the latter has been used mostly, since it has a better potential for automation. The difficulty for the direct computerization of the force method is to automatically pick up the structure’s redundant forces. Graph theory concepts may be used to accomplish this task, and a numerical procedure was proposed for the optimal plastic design of plane frames. An analogous approach is developed herein for the limit analysis of space frames which is computationally more cumbersome than the limit analysis of plane frames. The proposed procedure results in hypersparse matrices, and in conjunction with the kinematic upper bound linear program which is solved by a sparse solver, the proposed method appears computationally very efficient. It is also proved that it is much more effective than any displacement-based formulation. The robustness and efficiency of the approach are testified by numerical examples for grillages and multi-storey frames that are included.     

Conformational analysis of ochratoxin a by NMR spectroscopy and computational molecular modeling

Two-dimensional NMR spectroscopy has been used for a complete assignment of the proton and carbon-13 spectra of the metabolite from Aspergillus ochraceus, ochratoxin A. In addition, phase-sensitive nuclear Overhauser effect spectrometry experiments and computational molecular modeling (MM2 and MMFF force field programs) have been employed to examine the conformational properties of ochratoxin A in chloroform solutions. Particular attention has been given to intramolecular hydrogen-bonding formation involving the phenolic group on dihydroisocoumarin, which may be responsible for the toxic mechanism of ochratoxin A.     

Molecular relaxation processes in very dilute systems: A 13C NMR study of alcohols in alkanes

¹³C T₁ relaxation times have been determined for n-butanol in C₆D₁₂ in the concentration range 0.001 ? _x ? 1 (using ¹³C labelled alcohol) and for t-butanol in the range 0.01 ? _x ? 1. In the former case, this has allowed us to probe molecular mobility down to the region of monomeric alcohols. Comparison with previous results from measurements of ¹H chemical shifts, dielectric relaxation, and the picosecond dynamics of electron solvation allows us to build up a detailed picture of molecular clustering and liquid structure in alkane—alcohol mixtures. For n-butanol, the local liquid structure is established by x = 0.2 while t-butanol does not appear to form aggregates larger than trimers until x = 0.5.