Molecular modeling of polymer-clay nanocomposite precursors: Lysine in montmorillonite interlayers

The layered structure of clays with interlayer cations leads to unique chemical and mechanical properties, which have been capitalized on in the field of polymer/layered silicate nanocomposites. Hydrophilic silica surfaces can become organophilic with the inclusion of alkylammonium cations, which improve the wetting characteristics of the polymer matrix. In fact, the molecular level interactions of amino acids, either natural or non-natural, with clay surfaces are at the heart of fields of study as diverse as nanocomposites fabrication, drug delivery, bio-remediation of soils and catalysis of biological polymers, to name a few. The ubiquity of these systems and the potential uses to which they could be put suggests the necessity of a deeper understanding of the interplay of bonds, conformations, and configurations between the molecules and the hosts. The interactions of the amino acid lysine with sodium montmorillonite were studied using theoretical molecular modeling methods. The interlayer spacing of montmorillonite was increased by incorporating water molecules and allowing the system to evolve with molecular mechanics. Care was taken to retain the sodium cations in the interlayer. The initial amino acid conformation was obtained surrounding the molecule with numerous discrete water molecules and minimizing the system at the semi empirical level. The optimized amino acid was then placed in the interlayer space in a series of initial positions. Molecular mechanics calculations were performed and the final positions were analyzed. The results tended to indicate the preponderance of configurations which included surface-sodium-amino acid complexes with a variety of spatial arrangements. These results were compared with molecular dynamics calculations of similar systems from the literature.     

(Liquid + liquid) equilibria of (water + linalool + limonene) ternary system at T = (298.15, 308.15, and 318.15) K

(Liquid + liquid) equilibrium (LLE) data for {water (1) + linalool (2) + limonene (3)} ternary system at T = (298.15, 308.15, and 318.15 ± 0.05) K are reported. The organic chemicals were quantified by gas chromatography using a flame ionisation detector while water was quantified using a thermal conductivity detector. The effect of the temperature on (liquid + liquid) equilibrium is determined and discussed. Experimental data for the ternary mixture are compared with values calculated by the NRTL and UNIQUAC equations, and predicted by means of the UNIFAC group contribution method. It is found that the UNIQUAC and NRTL models provide a good correlation of the solubility curve at these three temperatures, while comparing the calculated values with the experimental ones, the best fit is obtained with the NRTL model. Finally, the UNIFAC model provides poor results, since it predicts a greater heterogeneous region than experimentally observed.     

Efficient catalyst-free bi- and triaroylation of aromatic rings in a single step

The exceptional leaving group ability of the trimethylstannyl group in electrophilic aromatic substitutions makes possible the synthesis, in a single step, of bi- and triaroylarenes through the catalyst-free, regioselective reaction of bi- and tristannylarenes with different aroyl halides in o-dichlorobenzene as solvent. Specific di- and triketones are obtained in good to excellent yields (45-83%).     

Quantitative Nanoscale Secondary Ion Mass Spectrometry (NanoSIMS) Imaging of Individual Vesicles to Investigate the Relation between Fraction of Chemical Release and Vesicle Size

We used correlative transmission electron microscopy (TEM) and nanoscale secondary ion mass spectrometry (NanoSIMS) imaging to quantify the contents of subvesicular compartments, and to measure the partial release fraction of ¹³C-dopamine in cellular nanovesicles as a function of size. Three modes of exocytosis comprise full release, kiss-and-run, and partial release. The latter has been subject to scientific debate, despite a growing amount of supporting literature. We tailored culturing procedures to alter vesicle size and definitively show no size correlation with the fraction of partial release. In NanoSIMS images, vesicle content was indicated by the presence of isotopic dopamine, while vesicles which underwent partial release were identified by the presence of an ¹²⁷I-labelled drug, to which they were exposed during exocytosis allowing entry into the open vesicle prior to its closing again. Demonstration of similar partial release fractions indicates that this mode of exocytosis is predominant across a wide range of vesicle sizes.     

Spectrofluorimetric determination of benzoimidazolic pesticides: effect of p-sulfonatocalix[6]arene and cyclodextrins

The effect of the addition of a macrocyclic host (H) such as p-sulfonatocalix[6]arene (C6S), native and modified cyclodextrins (CDs), on the fluorescence of benzoimidazolic fungicides (P), like Benomyl (BY) and Carbendazim (CZ), has been studied. The fluorescence of BY in water at pH 1.000 and 25.0 degrees C was increased in the presence of C6S, alphaCD and hydroxypropyl-beta-CD (HPCD). The association constants determined by fluorescence enhancement showed weak interactions (K(A) approximately 10(1) to 10(2) M(-1)) between the fungicide with both CDs, whereas they were stronger with C6S (K(A) approximately 10(5) M(-1)). Molecular recognition of BY for C6S was mainly attributed to electrostatic interactions, and for CDs to the hydrophobic effect and hydrogen bond formation. On the other hand, the fluorescent behaviour of CZ in the presence of C6S at pH 6.994 was interpreted as the formation of two complexes with 1:1 (P:H) and 1:2 (P:H(2)) stoichiometry, the latter being less fluorescent than the free analyte. Relative fluorescence quantum yield ratios between the complexed and free BY (phi(P:H)/phi(P)) were 2.00+/-0.05, 1.40+/-0.03 and 2.8+/-0.4 for C6S, alphaCD and HPCD, respectively. The analytical parameters improved in the presence of C6S and CDs. The best limit of detection (L(D), ng mL(-1)) was 17.4+/-0.8 with HPCD. The proposed method with C6S and HPCD was successfully applied to fortified samples of tap water and orange flesh extract with good recoveries (91-106%) and R.S.D. (< or = 2%) by triplicate analysis. The method is rapid, direct and simple and needs no previous degradation or derivatization reaction.     

Exploiting the Multidentate Nature of Chiral Disulfonimides in a Multicomponent Reaction for the Asymmetric Synthesis of Pyrrolo[1,2‐a]indoles: A Remarkable Case of Enantioinversion

A new multicomponent coupling reaction for the enantioselective synthesis of pyrrolo[1,2‐a]indoles under the catalysis of a chiral disulfonimide is described. The high specificity of the reaction is a consequence of the multidentate character of the Brønsted acid catalyst. Insights from DFT calculations helped explain the unexpected high enantioselectivity observed with the simplest 3,3′‐unsubstituted binaphthyl catalyst as a result of transition‐state stabilization by a network of cooperative noncovalent interactions. The remarkable enantioinversion resulting from the simple introduction of substituents at 3‐ and 3′‐positions, the first reported example of this phenomenon in the context of binaphthalene‐derived Brønsted acid catalysis, was instead attributed to destabilizing steric interactions.     

Effects of Visible‐Light Irradiation of Protoporphyrin IX on the Self‐Assembly of Tubulin Heterodimers

The formation and the effects of laser irradiation of the complex formed by protoporphyrin IX (PPIX) and tubulin was investigated. We have used tubulin as a model protein to investigate whether docked photoactive ligands can affect the structure and function of polypeptides upon exposure to visible light. We observed that laser irradiation in the Soret band prompts bleaching of the PPIX, which is accompanied by a sharp decrease in the intensity and average fluorescence lifetime of the protein (dominated by the four tryptophan residues of the tubulin monomer). The kinetics indicate non‐trivial effects and suggest that the photosensitization of the PPIX bound to tubulin prompts structural alterations of the protein. These modifications were also observed through changes in the energy transfer between Trp residues and PPIX. The results suggest that laser irradiation produces localized partial unfolding of tubulin and that the changes prompt modification of the formation of microtubules in vitro. Measurements of singlet oxygen formation were inconclusive in determining whether the changes are prompted by reactive oxygen species or other excited state mechanisms.     

2-Benzyl-2-methyl-2H-benzimidazole 1,3-dioxide derivatives: Spectroscopic and theoretical study

The spectroscopic behavior of 2-benzyl-2-methyl-2H-benzimidazole 1,3-dioxide derivatives in solution was studied in terms of electronic and nuclear magnetic resonance ((1)H and (13)C NMR) techniques. The experimental spectra were compared to the theoretical ones, obtained at DFT level, proving that the compounds adopt in solution a bird-like conformational distribution. Also, theoretically this conformational distribution resulted the most stable in gas phase. Infrared spectroscopy was used to study solid state behavior identifying experimentally the N-O stretching near to 1380, 1365 and 1225 cm(-1) and the vibrational benzimidazole skeleton near to 1610 and 1590 cm(-1). The vibrational spectrum was satisfactorily described by DFT calculations funding the N-O stretching as a coupled vibration near to 1470, 1350 and 1285 cm(-1). The fragmentation that takes place in mass spectrometry was assigned for all of the new derivatives.     

Synthesis, crystal structure and magnetic properties of novel heterobimetallic malonate-bridged MIIReIV complexes (M = Mn, Fe, Co and Ni)

Five novel ReIV-MII bimetallic complexes of formula [ReCl4(mu-mal)M(dmphen)2].MeCN [M = Co (1), Fe (2) and Ni (3)], [ReCl4(mu-mal)Ni(dmphen)(MeCN)2(H2O)].(MeCN)0.5(H2O)0.5 (4), and [ReCl4(mu-mal)Mn(dmphen)(H2O)2].dmphen.MeCN.H2O (5) (mal = malonate dianion, dmphen = 2,9-dimethyl-1,10-phenanthroline) have been synthesized, and the structures of 1, 2, 4, and 5 determined by single-crystal X-ray diffraction. The structures of 1 and 2 consist of neutral [ReCl4(mu-mal)M(dmphen)2] dinuclear units where the metal ions are linked through a malonate ligand which adopts simultaneously the bidentate (at ReIV) and monodentate (at MII) coordination modes. The bridging carboxylate-malonate group in them exhibits the anti-syn conformation. The rhenium atom is six-coordinated with four chloro atoms and two carboxylate-oxygens from a bidentate malonate group in a distorted octahedral environment. The M atom is five-coordinated being surrounded by four nitrogen atoms of two bidentate dmphen ligands and one oxygen atom of the malonato ligand. There are also ReIV(mu-mal)NiII dinuclear units in 4 with the same type of bridge, but the nickel atom is six-coordinated with one bidentate dmphen, two molecules of acetonitrile and one water molecule as peripheral ligands. Compound 5 is a neutral chain compound with regular alternating rhenium(IV) and manganese(II) ions. The [ReCl4(mal)]2- units in each chain act as bismonodenate ligands through two carboxylate-oxygen atoms toward [Mn(dmphen)(H2O)2]2+ entities. Variable-temperature magnetic measurements of 1-5 in the temperature range 2.0-300 K show the occurrence of weak antiferromagnetic interactions which are rationalized on the basis of the structural knowledge and simple orbital considerations. Very noticeable is compound 5, a ferrimagnetic chain with regular alternating ReIV and MnII cations.