Properties of polysaccharides grafted on nanoparticles investigated by EPR

The in vivo fate of nanoparticles developed as drug delivery systems is influenced by the surface characteristics of the colloidal particles. In the present work, surface characteristics of a series of poly(isobutylcyanoacrylate) nanoparticles prepared by redox radical emulsion polymerization with polysaccharides of different molecular weight and nature were characterized by EPR. To this aim, a spin label was grafted on the polysaccharide chains after synthesis of the nanoparticles. The percentage of label showing fast movements was evaluated from EPR spectra which were analyzed according to the Kivelson theory. The results showed that mobility depended on temperature, type, and molecular weight of the polysaccharides. Differences between nanoparticles appeared with low-molecular-weight polysaccharides, while over a defined molecular weight which depended on the nature of the polysaccharide, the spin label behaved almost the same way in the different types of nanoparticles. Paradoxically, the percentage of fast moving label was the highest when linked to the shortest chitosan, which was the most rigid polysaccharide tested in this study. Thus, it was concluded that the apparent mobility of the polysaccharide evaluated by the EPR method depended on the capacity of the polysaccharide chains to fold making possible hydrophobic interactions between the label and the nanoparticle core. The transition between the unfolded-folded regiment depended on the molecular weight and on the nature of the polysaccharide. Results of this study may be useful to improve the understanding of the nanoparticle interactions with blood proteins and complement which in turn influence the in vivo fate of nanoparticles used as drug delivery systems.     

Understanding the factors controlling the photo-oxidation of natural DNA by enantiomerically pure intercalating ruthenium polypyridyl complexes through TA/TRIR studies with polydeoxynucleotides and mixed sequence oligodeoxynucleotides

Ruthenium polypyridyl complexes which can sensitise the photo-oxidation of nucleic acids and other biological molecules show potential for photo-therapeutic applications. In this article a combination of transient visible absorption (TrA) and time-resolved infra-red (TRIR) spectroscopy are used to compare the photo-oxidation of guanine by the enantiomers of [Ru(TAP)₂(dppz)]²⁺ in both polymeric {poly(dG-dC), poly(dA-dT) and natural DNA} and small mixed-sequence duplex-forming oligodeoxynucleotides. The products of electron transfer are readily monitored by the appearance of a characteristic TRIR band centred at ca. 1700 cm⁻¹ for the guanine radical cation and a band centered at ca. 515 nm in the TrA for the reduced ruthenium complex. It is found that efficient electron transfer requires that the complex be intercalated at a G-C base-pair containing site. Significantly, changes in the nucleobase vibrations of the TRIR spectra induced by the bound excited state before electron transfer takes place are used to identify preferred intercalation sites in mixed-sequence oligodeoxynucleotides and natural DNA. Interestingly, with natural DNA, while it is found that quenching is inefficient in the picosecond range, a slower electron transfer process occurs, which is not found with the mixed-sequence duplex-forming oligodeoxynucleotides studied.

Efficient electron transfer requires the complex to be intercalated at a G-C base-pair. Identification of preferred intercalation sites is achieved by TRIR monitoring of the nucleobase vibrations before electron transfer.     

Influence of an 8-trifluoroacetyl group on flavanol couplings

The effect of an electron attracting substituent in the Lewis acid catalyzed oligomerization of flavanols was investigated. The results showed that the presence of a COCF₃, at the 8 position of a (+)-catechin unit strongly influenced the attack of the 6 free nucleophile flavanol position by the electrophile generated from a 4-O-alkyloxy protected catechin unit. This was observed either with TiCl₄ or TMSOTf as Lewis acids in which the carbon-carbon bond formation was inhibited and the corresponding dimer was detected in small amount. On the contrary, the formation of a carbon-oxygen bond was observed and the corresponding C-4→O→C-3 ether linked procyanidin dimer was isolated in a good yield. In order to avoid the participation of the C-3 hydroxyl group in the dimerization reaction, the two flavanol units were forced into C-4→C-8 coupling by use of an internal link. The structural elucidation of the isolated compounds was achieved through MS and NMR spectroscopy.     

Is there a future for sequential chemical extraction?

Since their introduction in the late 1970s, sequential extraction procedures have experienced a rapid increase in use. They are now applied for a large number of potentially toxic elements in a wide range of sample types. This review uses evidence from the literature to consider the usefulness and limitations of sequential extraction and thereby to assess its future role in environmental chemical analysis. It is not the intention to provide a comprehensive survey of all applications of sequential extractions or to consider the merits and disadvantages of individual schemes. These aspects have been covered adequately in other, recent reviews. This review focuses in particular on various key issues surrounding sequential extractions such as nomenclature, methodologies, presentation of data and interpretation of data, and discusses typical applications from the recent literature for which sequential extraction can provide useful and meaningful information. Also covered are emerging developments such as accelerated procedures using ultrasound- or microwave energy-assisted extractions, dynamic extractions, the use of chemometrics, the combination of sequential extraction with isotope analysis, and the extension of the approach to non-traditional analytes such as arsenic, mercury, selenium and radionuclides.     

Competitive Complexation of Lanthanide Cations by Xylitol and Nitrate Anion in Aqueous Solution: A Microcalorimetric Investigation

The apparent equilibrium constants and enthalpies of complexation of Nd³⁺, Sm³⁺, Eu³⁺, and Gd³⁺ by xylitol in aqueous solutions containing NaNO₃ at an ionic strength of 2.0 mol-kg^–1 have been determined by microcalorimetry at 25°C. Since nitrate anion weakly complexes the lanthanide cations, these values are analyzed in terms of competition between xylitol and NO ₃ ^- The method leads to the apparent equilibrium constants and enthalpies of complexation of the lanthanide cations by NO ₃ ^- at this particular ionic strength. Despite the difficulties encountered in characterizing rather weak associations, the results are, whenever comparison is possible, in good agreement with those obtained by direct microcalorimetry. The advantage of this competition method is that it can be used when the enthalpic effects are too weak and insufficiently concentration dependent for direct microcalorimetric determination. In the present case, it allows us to thermodynamically characterize the formation of SmNO ₃ ²⁺ and EuNO ₃ ²⁺ , processes we have not been able to study directly.     

Flexible diphosphine ligands with overall charges of 0, +1, and +2: critical role of the electrostatics in favoring trans over cis coordination

The influence of the formal electrostatic interaction on the cis/trans coordination mode at a PdCl(2) center is investigated in a family of isostructural flexible diphosphine ligands Ph(2)P-X-C(6)H(4)-Y-PPh(2), where X and Y stand for neutral or cationic N,C-imidazolylene linkers. While the neutral and monocationic diphosphine spontaneously behave as classical cis-chelating ligands, only the dicationic diphosphine, where the electrostatic repulsion between the formal positive charges specifically takes place, is observed to behave as a trans-chelating ligand. The crucial role of electrostatics is analyzed on the basis of (31)P NMR data in solution and X-ray diffraction data in the crystal state. Comparative theoretical studies of the cis- and trans-chelated complexes, including EDA, static (31)P NMR, MESP, and AIM analyses, have been undertaken on the basis of DFT calculations in the gas phase or in the acetonitrile continuum. Whereas the cis-coordination mode is shown to be thermodynamically favored for the neutral ligand, the trans-coordination mode is found to be preferred for the dicationic homologue. The stereochemical preference is thus shown to be parallel to the expected effect of the formal electrostatic interaction. The results open perspectives for control of the cis- and trans-chelating behavior of flexible bidentate ligands by more or less reversible charge transfer at the periphery of the coordination sphere of a metallic center.     

Selective pressurized liquid extraction of estrogenic compounds in soil and analysis by gas chromatography-mass spectrometry

A selective pressurized liquid extraction (SPLE) method, followed by gas chromatography–mass spectrometry (GC–MS), for the simultaneous extraction and clean-up of estrone (E1), 17β-estradiol (E2), 17α-ethynylestradiol (EE2), estriol (E3) and bisphenol A (BPA) from soil samples is described. The on-line clean-up of soil by SPLE was achieved using different organic matter retainers, including silica, alumina and Florisil, the most effective being silica. Thus, different amounts of silica, in conjunction with different extraction solvents (acetone, ethyl acetate, isohexane and dichloromethane), either alone or in combination, were used to extract the target chemicals from spiked soil samples. It was shown that 3 g silica resulted in satisfactory rates of recovery of target compounds and acetone:dichloromethane (1:3, v/v) was efficient in extracting and eluting estrogenic compounds for SPLE. Variables affecting the SPLE efficiency, including temperature and pressure were studied; the optimum parameters were 60 °C and 1500 psi, respectively. The limits of detection (LODs) of the proposed method were 0.02–0.37 ng g⁻¹ for the different estrogenic chemicals studied. The outputs using the proposed method were linear over the range from 0.1 to 120 ng g⁻¹ for E1, E2, EE2, 0.2–120 ng g⁻¹ for E3, and 0.5–120 ng g⁻¹ for BPA. The optimized method was further verified by performing spiking experiments in natural soil matrices; good rates of recovery and reproducibility were achieved for all selected compounds and the method was successfully applied to soil samples from Northeast Scotland, for the determination of the target compounds.     

Modelling of synchrotron SAXS patterns of silicalite-1 zeolite during crystallization

Synchrotron small angle X-ray scattering (SAXS) was used to characterize silicalite-1 zeolite crystallization from TEOS/TPAOH/water clear sol. SAXS patterns were recorded over a broad range of length scales, enabling the simultaneous monitoring of nanoparticles and crystals occurring at various stages of the synthesis. A simple two-population model accurately described the patterns. Nanoparticles were modeled by polydisperse core-shell spheres and crystals by monodisperse oblate ellipsoids. These models were consistent with TEM images. The SAXS results, in conjunction with in situ light scattering, showed that nucleation of crystals occurred in a short period of time. Crystals were uniform in size and shape and became increasingly anisotropic during growth. In the presence of nanoparticles, crystal growth was fast. During crystal growth, the number of nanoparticles decreased gradually but their size was constant. These observations suggested that the nanoparticles were growth units in an aggregative crystal growth mechanism. Crystals grown in the presence of nanoparticles developed a faceted habit and intergrowths. In the final stages of growth, nanoparticles were depleted. Concurrently, the crystal growth rate decreased significantly.     

Towards the limit of atropochiral stability: H-MIOP, an N-heterocyclic carbene precursor and cationic analogue of the H-MOP ligand

The configurational stability of biaryl motifs is addressed for the 1-naphthyl-N-benzimidazolyl motif substituted by a single diphenylphosphinyl group at the 2-position. The atropoenantiomers of the N-methylated cation H-MIOP, a less sterically locked analogue of the neutral H-MOP ligand, were resolved by enantiospecific cleavage of the N(2)C-P bond of the resolved enantiomers of BIMIONAP. The latter were obtained by enantiospecific N-methylation of the previously resolved enantiomers of neutral BIMINAP. PdCl(2) complexes of the P,C-chelating N-heterocyclic carbene (NHC)-phosphine ligands derived from (R)- and (S)-H-MIOP were prepared by two enantiospecific routes: by N(2)C-P bond cleavage from the (R)- and (S)-BIMIONAP-PdCl(2) complexes, or by simultaneous coordination of the P and C atoms of the in situ generated free NHC-phosphine. The enantiomerization pathways of H-MOP, H-MIOP, and corresponding NHC-phosphine have been investigated at the B3PW91/6-31G(d,p) level of theory. The calculated enantiomerization barriers of H-MOP and H-MIOP in acetonitrile are equal to 176.0 and 146.4 kJ mol(-1), respectively, and are mainly determined by the distortion of the naphthalene and/or benzimidazole motifs in the transition state. Beyond the stability of their optical rotation at room temperature, the respective calculated Oki's racemization temperatures of 334 and 225 °C allowed us to consider both ligands as configurationally stable.