Investigation of the influence of spacer arm on the structural evolution of affinity ligands supported on agarose

The influence of the spacer arm on the interaction between agarose and a supported ligand was investigated through molecular dynamics for a combination of several spacers. The spacers differ for degree of hydrophobicity, length, and chemical composition, which was varied through insertion of thio, ether, and CH(2) groups. Agarose was modeled through a modified Glycam force field, whose parameters were determined through ab initio calculations. The structural model of agarose used for the calculations was obtained through MD studies of the conformational evolution of several agarose single and double helixes. The simulations showed that a modification of the spacer properties could determine a change of the stable structure of the ligand with respect to the support. In particular, if the spacer is hydrophilic and rigid, the favored structure is with extended spacer and solvated ligand. Either increasing the spacer length, and thus its flexibility, or decreasing its solvation free energy, which corresponds to diminishing its affinity for water, rapidly leads to a conformational change in which the ligand adsorbs on agarose. Interestingly, we found that if the spacer is long and hydrophilic, a third metastable structure, in which the spacer is sandwiched between the ligand and agarose, is possible. Simulations of several ligands adsorbed on neighboring sites on agarose showed that if the support is not held fixed through restraints, the interaction force between vicinal ligands is sufficient to determine a major conformational change of the system.     

Atomistic simulation of structure and dynamics of columnar phases of hexabenzocoronene derivatives

Using atomistic molecular dynamics simulations we study solid and liquid crystalline columnar discotic phases formed by alkyl-substituted hexabenzocoronene mesogens. Correlations between the molecular structure, packing, and dynamical properties of these materials are established.     

Quantum‐chemical study of the singlet oxygen emission

Intensity distribution in rotational lines of the 0–0 band of the a¹Δ_g → X³Σ transition in the oxygen molecule at λ = 1270 nm is studied by quadratic response (QR) method in a framework of multiconfiguration self‐consisted field (MCSCF) theory. The distance dependence of the transition magnetic moment and the (0–0)/(0–1) bands intensity ratio are calculated. A short review of previous theory of the red and infrared atmospheric oxygen bands and of their enhancement by collisions is presented to analyze and compare the new results. Enhancement of these bands in O₂ collisions with Li₂ and N₂ molecules is calculated by QR method. Diamagnetic species simulate solvent molecules of different optical polarizability. Specific influence of collisions on vibronic bands is stressed. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

New Synthetic Method for Bicyclic Compounds Through a Sequence of Ene-Type Chlorination,Palladium Catalyzed Dehydrochlorination,and Intramolecular Diels-Alder Reaction

A sequence of ene-type chlorination, palladium catalyzed dehydrochlorination, and the intramolecular Diels-Alder reaction afforded a new method for conversion of prenyl derivatives into bicyclic compounds.     

Synthesis of Diels–Alder adducts of the quinolizidine alkaloids N-methylcytisine, (−)-leontidine,and (−)-thermopsine with N-phenylmaleimide

The Diels–Alder adducts of the quinolizidine alkaloids N-methylcytisine, (?)-leontidine, and (?)-thermopsine with N-phenylmaleimide have been synthesized. The structures and absolute configurations of the new asymmetric centers of the products were determined by NMR spectroscopy experiments, QC-calculations, and X-ray data.     

Optimization and validation of organochlorine compounds in adipose tissue by SPE‐gas chromatography

Scientific evidence has shown an association between organochlorine compounds (OCC) exposure and human health hazards. Concerning this, OCC detection in human adipose samples has to be considered a public health priority. This study evaluated the efficacy of various solid‐phase extraction (SPE) and cleanup methods for OCC determination in human adipose tissue. Octadecylsilyl endcapped (C₁₈‐E), benzenesulfonic acid modified silica cation exchanger (SA), poly(styrene‐divinylbenzene (EN) and EN/RP₁₈ SPE sorbents were evaluated. The relative sample cleanup provided by these SPE columns was evaluated using gas chromatography with electron capture detection (GC–ECD). The C₁₈‐E columns with strong homogenization were found to provide the most effective cleanup, removing the greatest amount of interfering substance, and simultaneously ensuring good analyte recoveries higher than 70%. Recoveries > 70% with standard deviations (SD) < 15% were obtained for all compounds under the selected conditions. Method detection limits were in the 0.003–0.009 mg/kg range. The positive samples were confirmed by gas chromatography coupled with tandem mass spectrometry (GC‐MS/MS). The highest percentage found of the OCC in real samples corresponded to HCB, o,p′‐DDT and methoxychlor, which were detected in 80 and 95% of samples analyzed respectively. Copyright © 2012 John Wiley & Sons, Ltd.     

1H NMR Study on the Isomerization of 2,4,6-Triphenyl-4H-Seleno(Thio)Pyrans

It is shown using ¹H NMR spectroscopy that 2,4,6-triphenyl-4H-seleno(thio)pyrans can be isomerized into 2H-isomers by the action of lithium perchlorate or lithium chloride in DMF or only by DMF. The reaction time is found to depend on the Lewis acidity of used salt. It is supposed that isomerization in presence of Lewis acids proceeds as endocyclic 1,3-proton transfer or as a series of two 1,2-hydride shifts.     

Molecular Recognition of Carbonyl Compounds by Uranyl-salophen Based Neutral Receptors Driven by Van Der Waals Forces

The complexation of the salophen-uranyl metallocleft 2 and of its half-cleft analogue 3 with enones and other carbonyl compounds was assessed in chloroform by UV-Vis titration and, occasionally, by FT-IR measurements. Complexes with receptors 2 and 3 are in all cases more stable than those with the control unsubstituted uranyl-salophen 1 , showing that in addition to the primary binding force provided by coordination of the carbonyl oxygen to the uranium, a significant driving force for complexation, typically in the range of 2-3 kcal/mol, results from van der Waals interactions of the guest with the aromatic walls. Replacement of the phenyl group in 3 with larger aromatic residues to give 4 and 5 , led to enhanced complex stabilities, due to more extended contact surfaces between host and guest.