Effect of alkyl chain length of alcohols on nematic uniaxial-to-biaxial phase transitions in a potassium laurate/alcohol/K2SO4/water lyotropic mixture

Lyotropic liquid crystalline quaternary mixtures of potassium laurate (KL), potassium sulphate (K2SO4)/alcohol (n-OH)/water, with the alcohols having different numbers of carbon atoms in the alkyl chain (n), from 1-octanol to 1-hexadecanol, were investigated by optical techniques (optical microscopy and laser conoscopy). The biaxial nematic phase domain is present in a window of values of n = n(KL) +/- 2, where n(KL) = 11 is the number of carbon atoms in the alkyl chain of KL. The biaxial phase domain became smaller and the uniaxial-to-biaxial phase transition temperatures shifted to relatively higher temperatures upon going from 1-nonanol to 1-tridecanol. Moreover, compared with other lyotropic mixtures these new mixtures present high birefringence values, which we expect to be related to the micellar shape anisotropy. Our results are interpreted assuming that alcohol molecules tend to segregate in the micelles in a way that depends on the relative value of n with respect to nKL. The larger the value of n, the more alcohol molecules tend to be located in the curved parts of the micelle, favoring the uniaxial nematic calamitic phase with respect to the biaxial and uniaxial discotic nematic phases.

Remarkable coordination behavior of alkyl isocyanides toward unsaturated vicinal frustrated P/B Lewis pairs

The conjugated frustrated phosphane/borane Lewis pairs formed by 1,1-carboboration of a substituted diphenylphosphino acetylene, undergo a synergistic 1,1-addition reaction to n-butyl isocyanide with formation of new B-C and P-C bonds to the former isonitrile carbon atom. Using tert-butyl isocyanide dynamic behaviour between the isocyanide-[B] adduct and the 1,1-addition product formation was observed in solution. The different modes of isocyanide binding to the FLPs in the solid state were characterized using X-ray crystal structure analyses and comprehensive 11B and 31P solid-state magicangle- spinning (MAS-) NMR experiments. The free FLP, the Lewis adduct at the borane group, and the cyclic product resulting from isocyanide addition to both reaction centers, can be differentiated via 11B and 31P isotropic chemical shifts, 11B nuclear electric quadrupole coupling constants, isotropic indirect 11B-31P spin-spin coupling constants, and 11B...31P internuclear distances measured by rotational echo double resonance.

Disulfide Biochemistry in 2-Cys Peroxiredoxin: Requirement of Glu50 and Arg146 for the Reduction of Yeast Tsa1 by Thioredoxin

2-Cys peroxiredoxin (Prx) enzymes are ubiquitously distributed peroxidases that make use of a peroxidatic cysteine (Cys(P)) to decompose hydroperoxides. A disulfide bond is generated as a consequence of the partial unfolding of the alpha-helix that contains Cys(P). Therefore, during its catalytic cycle, 2-Cys Prx alternates between two states, locally unfolded and fully folded. Tsa1 (thiol-specific antioxidant protein 1 from yeast) is by far the most abundant Cys-based peroxidase in Saccharomyces cerevisiae. In this work, we present the crystallographic structure at 2.8 angstrom resolution of Tsa1(C47S) in the decameric form [(alpha(2))(5)] with a DTT molecule bound to the active site, representing one of the few available reports of a 2-Cys Prx (AhpC-Prx1 subfamily) (AhpC, alkyl hydroperoxide reductase subunit C) structure that incorporates a ligand. The analysis of the Tsa1(C47S) structure indicated that G1u50 and Arg146 participate in the stabilization of the Cys(P) alpha-helix. As a consequence, we raised the hypothesis that G1u50 and Arg146 might be relevant to the Cys(P) reactivity. Therefore, Tsa1(E50A) and Tsa1(R146Q) mutants were generated and were still able to decompose hydrogen peroxide, presenting a second-order rate constant in the range of 10(6) M-1 S-1. Remarkably, although Tsa1(E50A) and Tsa1(R146Q) were efficiently reduced by the low-molecular-weight reductant DTT, these mutants displayed only marginal thioredoxin (Trx)-dependent peroxidase activity, indicating that G1u50 and Arg146 are important for the Tsa1-Trx interaction. These results may impact the comprehension of downstream events of signaling pathways that are triggered by the oxidation of critical Cys residues, such as Trx. (C) 2012 Elsevier Ltd. All rights reserved.

New insights on reaction pathway selectivity promoted by crown ether phase-transfer catalysis: Model ab initio calculations of nucleophilic fluorination

Crown ethers have the ability of solubilizing inorganic salts in apolar solvents and to promote chemical reactions by phase-transfer catalysis. However, details on how crown ethers catalyze ionic S(N)2 reactions and control selectivity are not well understood. In this work, we have used high level theoretical calculations to shed light on the details of phase-transfer catalysis mechanism of KF reaction with alkyl halides promoted by 18-crown-6. A complete analysis of the of the model reaction between KF(18-crown-6) and ethyl bromide reveals that the calculations can accurately predict the product ratio and the overall kinetics. Our results point out the importance of the K* ion and of the crown ether ring in determining product selectivity. While the K* ion favors the S(N)2 over the E2 anti pathway, the crown ether ring favors the S(N)2 over E2 syn route. The combination effects lead to a predicted 94% for the S(N)2 pathway in excellent agreement with the experimental value of 92%. A detailed analysis of the overall mechanism of the reaction under phase-transfer conditions also reveals that the KBr product generated in the nucleophilic fluorination acts as an inhibitor of the 18-crown-6 catalyst and it is responsible for the observed slow reaction rate. (C) 2012 Elsevier B.V. All rights reserved.

On the formation of carbonate adducts of fatty alcohols, sterols, and sugars in biological conditions

The formation and properties of carbonate adducts of some organic hydroxy compounds in aqueous medium were investigated. Fatty alcohols and sugars were chosen as representative classes of biological interest, and the medium was carbonated aqueous solution with pH ranging from 3.0 to 8.3. Capillary electrophoresis with two capacitively coupled contactless conductivity detectors (C4Ds) was used for quantitation and to obtain the mobility of the monoalkyl carbonates (MACs), which were used to determine the equilibrium and kinetic constants of the reaction as well as the diffusion coefficients. For increasing chain length of the alcohols, the equilibrium constant tends to the unit, which suggests that fatty alcohols can form the corresponding MACs. The formation of MACs for cyclohexanol and cyclopentanol also suggest the existence of similar species for sterols. Carbonate adducts of fructose, glucose, and sucrose were also detected, which suggests that these counterparts of the well-known phosphates can also occur in the cytosol. Our calculations suggest that one in 1000 to one in 10 000 molecules of these hydroxy compounds would be available as the corresponding MAC in such a medium. Experiments carried out at pH values less than 3.0 showed that there is a catalytic effect of hydronium on the interconversion of bicarbonate and a MAC. Taking into account the great number of hydroxy compounds similar to the ones investigated and that bicarbonate is ubiquitous in living cells, one can anticipate the existence of a whole new class of carbonate adducts of these metabolites.

4-Organochalcogenoyl-1H-1,2,3-triazoles: synthesis and functionalization by a nickel-catalyzed Negishi cross-coupling reaction

A general method for the synthesis of triazoles containing selenium and tellurium was accomplished via a CuCAAC reaction between organic azides and a terminal triple bond, generated by in situ deprotection of the silyl group. The reaction tolerates alkyl and arylazides, with alkyl and aryl substituents directly bonded to the chalcogen atom. The products were readily functionalized by a nickel-catalyzed Negishi cross-coupling reaction, furnishing the aryl-heteroaryl products at the 4-position in good yields. (C) 2012 Elsevier Ltd. All rights reserved.

Mechanism and Efficiency of Cell Death of Type II Photosensitizers: Effect of Zinc Chelation

A series of meso-substituted tetra-cationic porphyrins, which have methyl and octyl substituents, was studied in order to understand the effect of zinc chelation and photosensitizer subcellular localization in the mechanism of cell death. Zinc chelation does not change the photophysical properties of the photosensitizers (all molecules studied are type II photosensitizers) but affects considerably the interaction of the porphyrins with membranes, reducing mitochondrial accumulation. The total amount of intracellular reactive species induced by treating cells with photosensitizer and light is similar for zinc-chelated and free-base porphyrins that have the same alkyl substituent. Zinc-chelated porphyrins, which are poorly accumulated in mitochondria, show higher efficiency of cell death with features of apoptosis (higher MTT response compared with trypan blue staining, specific acridine orange/ethidium bromide staining, loss of mitochondrial transmembrane potential, stronger cytochrome c release and larger sub-G1 cell population), whereas nonchelated porphyrins, which are considerably more concentrated in mitochondria, triggered mainly necrotic cell death. We hypothesized that zinc-chelation protects the photoinduced properties of the porphyrins in the mitochondrial environment.

Effect of amino-terminated substrates onto surface properties of cellulose esters and their interaction with lectins

Films of cellulose acetate butyrate (CAB) and carboxymethylcellulose acetate butyrate (CMCAB) were deposited from ethyl acetate solutions onto bare silicon wafers (Si/SiO2) or amino-terminated surfaces (APS) by means of equilibrium adsorption. All surfaces were characterized by means of ellipsometry, atomic force microscopy (AFM) and contact angle measurements. The presence of amino groups on the support surface favored the adsorption of CAB and CMCAB, inducing the orientation almost polar groups to the surface and the exposition of alkyl group to the air. Such molecular orientation caused increase of the dispersive component of surface energy (gamma(d)(s)) and decrease of the polar component of surface energy (gamma(p)(s)) of cellulose esters in comparison to those values determined for films deposited onto bare Si/SiO2 wafers. Adsorption behavior of jacalin or concanavalin A onto CAB and CMCAB films was also investigated. The adsorbed amounts of lectins were more pronounced on cellulose esters with high (gamma(p)(s)) and total surface energy (gamma(t)(s)) values. (C) 2011 Elsevier B.V. All rights reserved.

Solvation in aqueous binary mixtures: consequences of the hydrophobic character of the ionic liquids and the solvatochromic probes

Information on the solvation in mixtures of water, W, and the ionic liquids, ILs, 1-allyl-3-R-imidazolium chlorides; R = methyl, 1-butyl, and 1-hexyl, has been obtained from the responses of the following solvatochromic probes: 2,6-dibromo-4-[(E)-2-(1-R-pyridinium-4-yl)ethenyl] phenolate, R = methyl, MePMBr2; 1-octyl, OcPMBr(2), and the corresponding quinolinium derivative, MeQMBr(2). A model developed for solvation in binary mixtures of W and molecular solvents has been extended to the present mixtures. Our objective is to assess the relevance to solvation of hydrogen-bonding and the hydrophobic character of the IL and the solvatochromic probe. Plots of the medium empirical polarity, E-T(probe) versus its composition revealed non-ideal behavior, attributed to preferential solvation by the IL and, more efficiently, by the IL-W hydrogen-bonded complex. The deviation from linearity increases as a function of increasing number of carbon atoms in the alkyl group of the IL, and is larger than that observed for solvation by W plus molecular solvents (1-propanol and 2-(1-butoxy)ethanol) that are more hydrophobic than the ILs investigated. This enhanced deviation is attributed to the more organized structure of the ILs proper, which persists in their aqueous solutions. MeQMBr(2) is more susceptible to solvent lipophilicity than OcPMBr(2), although the former probe is less lipophilic. This enhanced susceptibility agrees with the important effect of annelation on the contributions of the quinonoid and zwitterionic limiting structures to the ground and excited states of the probe, hence on its response to both medium composition and lipophilicity of the IL.

Kinetics and product distribution of p-nitrophenyl phosphate dianion solvolysis in ternary DMSO/alcohol/water mixtures are compatible with metaphosphate formation

The rate of solvolysis of p-nitrophenyl phosphate (PNPP) dianion in DMSO/water strongly decreases by increasing water concentration. Addition of linear alcohols (methanol, propanol, butanol, pentanol, and hexanol) at constant DMSO/water molar ratio produced an even sharper rate decrease. Alkyl phosphate formation, resulting from PNPP solvolysis in ternary DMSO/water/alcohol mixtures, increased with alcohol concentration and was essentially temperature independent. Methanol and hexanol were the poorest nucleophiles under all conditions. Activation energies and enthalpies for solvolysis in ternary mixtures were similar and entropies varied with alcohol concentration. Taken together these results can be best interpreted in terms of a dissociative mechanism with the intervention of metaphosphate. Copyright (C) 2011 John Wiley & Sons, Ltd.

High Viscosity of Imidazolium Ionic Liquids with the Hydrogen Sulfate Anion: A Raman Spectroscopy Study

Ionic liquids based on 1-alkyl-3-methylimidazolium cations and the hydrogen sulfate (or bisulfate) anion, HSO4-, are much more viscous than ionic liquids with alkyl sulfates, RSO4-. The structural origin of the high viscosity of HSO4- ionic liquids is unraveled from detailed comparison of the anion Raman bands in 1-ethyl-3-methylimidazolium hydrogen sulfate and 1-butyl-3-methylimidazolium hydrogen sulfate with available data for simple HSO(4)(-) salts in crystalline phase, molten phase, and aqueous solution. Two Raman bands at 1046 and 1010 cm(-1) have been assigned as symmetric stretching modes nu(s)(S = O) of HSO4-, the latter being characteristic of chains of hydrogen-bonded anions. The intensity of this component increases in the supercooled liquid phase. For comparison purposes, Raman spectra of 1-ethyl-3-methylimidazolium ethyl sulfate and 1-butyl-3-methylimidazolium methyl sulfate have been also obtained. There is no indication of difference in the strength of hydrogen bond interactions of imidazolium cations with HSO4- or RSO4- anions. Raman spectra at high pressures, up to 2.6 GPa, are also discussed. Raman spectroscopy provides evidence that hydrogen-bonded anions resulting in anion-anion interaction is the reason for the high viscosity of imidazolium ionic liquids with HSO4-. If the ionic liquid is exposed to moisture, these structures are disrupted upon absorption of water from the atmosphere.

Structural aspects of polyanion and hydrophobically modified polycation multilayers on hydrophilic or hydrophobic surfaces

Multilayer films of carboxymethylcellulose (CMC), a polyanion, and bromide salts of poly(4-vinylpyridine) quaternized with linear aliphatic chains of 2 (ethyl) and 5 (pentyl) carbon atoms, coded as QPVP-C2 and QPVP-C5, respectively, were fabricated by layer-by-layer (LbL) self-assembly onto Si/SiO2 wafers (hydrophilic substrate) or polystyrene, PS, films (hydrophobic substrate). The films were characterized by means of ex situ and in situ ellipsometry, atomic force microscopy (AFM), contact angle measurements and sum frequency generation vibrational spectroscopy (SFG). Antimicrobial tests were used to assess the exposure of pyridinium moieties to the aqueous medium. In situ ellipsometry indicated that for Si/SiO2 the chains were more expanded than the PS films and both substrates systems composed of QPVP-C5 were thicker than those with QPVP-C2. For dried layers, the alkyl side group size had a small effect on the thickness evolution, regardless of the substrate. At pH 2 the multilayers showed high resistance, evidencing that the build-up is driven not only by cooperative polymer-polymer ion pairing, but also by hydrophobic interactions between the alkyl side chains. The LbL films became irregular as the number of depositions increased. After the last deposition, the wettability of QPVP-C2 or QPVP-C5 terminated systems on the Si/SiO2 wafers and PS films were similar, except for QPVP-C2 on Si/SiO2 wafers. Unlike the morphology observed for LbL films on Si/SiO2 wafers, PS induced the formation of porous structures. SFG showed that in air the molecular orientation of pyridinium groups in multilayers with QPVP-C5 was stronger than in those containing QPVP-C2. The exposure of pyridinium moieties to the aqueous medium was more pronounced when the LbL were assembled on Si/SiO2 wafers.

Neutral Gold Complexes with Tridentate SNS Thiosemicarbazide Ligands

Na[AuCl4].2H(2)O reacts with tridentate thiosemicarbazide ligands, H(2)L1, derived from N-[N',N'-dialkylamino(thiocarbonyl)]benzimidoyl chloride and thiosemicarbazides under formation of air-stable, green [AuCl(L1)] complexes. The organic ligands coordinate in a planar SNS coordination mode. Small amounts of gold(I) complexes of the composition [AuCl(L3)] are formed as side-products, where L3 is an S-bonded 5-diethylamino-3-phenyl-1-thiocarbamoyl-1,2,4-triazole. The formation of the triazole L3 can be explained by the oxidation of H(2)L1 to an intermediate thiatriazine L2 by Au3+, followed by a desulfurization reaction with ring contraction. The chloro ligands in the [AuCl(L1)] complexes can readily be replaced by other monoanionic ligands such as SCN- or CN- giving [Au(SCN)(L1)] or [Au(CN)(L1)] complexes. The complexes described in this paper represent the first examples of fully characterized neutral Gold(III) thiosemicarbazone complexes. All the [AuCl(L1)] compounds present a remarkable cell growth inhibition against human MCF-7 breast cancer cells. However, systematic variation of the alkyl groups in the N(4)-position of the thiosemicarbazone building blocks as well as the replacement of the chloride by thiocyanate ligands do not considerably influence the biological activity. On the other hand, the reduction of Au-III to Au-I leads to a considerable decrease of the cytotoxicity.

Self-Assembly of Poly(4-vynil-N-alkylpyridinium) Bromide onto Silica: Effect of Side-Chain Length on Structural Aspects at a Molecular Level

Self-assembly of poly(4-vynil-N-alkyl)pyridinium bromide with alkyl side chains of 2, 5, 7, 10, or 16 carbons from ethanolic solutions onto flat silica surfaces was studied by means of ellipsometry, atomic force microscopy (AFM), contact angle measurements, and sum-frequency generation (SFG) vibrational spectroscopy in the CH3 and CH2 stretch region. Ab initio quantum-chemical calculations on the N-alkylpyridinium side-group with restricted Hartree-Fock (RHF) method and 6-311G (d,p) basis set were C one to estimate the charge distribution along the pyridinium ring and the alkyl side-chain. SFG results showed that longer side chains promote the disorientation of the alkyl groups at the surface, corroborating with the contact angle values. AFM images revealed film homogeneity, regardless the alkyl side group. However, after 24 h contact with water, ringlike structures appeared on the film surfaces, when the polycation alkyl side chain had 7 or less carbons, and as the alkyl chain increased to 10 or 16 carbons, the films dewetted because the hydrophobic interactions prevailed over the electrostatic interactions between the pyridinium charged groups and the negatively charged SiO2 surface. Under acid conditions (HCl 0.1 mol.L-1), the film mean thickness values decreased up to 50% of original values when the alkyl side chains were ethyl or pentyl groups due to ion-pair disruption, but for longer groups they remained unchanged. Quantum-chemical optimization and Mulliken electron population showed that (i) from C2 to C15 the positive charge at the headgroup (HG) decreased 0.025, while the charge at combined HG + alpha-CH2 increased 0.037; and (ii) for C6 or longer, the alkyl side group presents a tilt in the geometry, moving away from the plane. Such effects summed up over the whole polymer chain give support to suggest that when the side chains are longer than 7 carbons, the hydrophobic interaction decreases film stability and increases acid resistance.

Selectivity and Mechanisms Driven by Reaction Dynamics: The Case of the Gas-Phase OH-+CH3ONO2 Reaction

Well-established statistical approaches such as transition-state theory based on high-level calculated potential energy profiles are unable to account for the selectivity observed in the gas-phase OH- + CH3ONO2 reaction. This reaction can undergo bimolecular nucleophilic displacement at either the carbon center (S(N)2@C) or the nitrogen center (S(N)2@N) as well as a proton abstraction followed by dissociation (E(CO)2) pathway. Direct dynamics simulations yield an S(N)2:E(CO)2 product ratio in close agreement with experiment and show that the lack of reactivity at the nitrogen atom is due to the highly negative electrostatic potential generated by the oxygen atoms in the ONO2 group that scatters the incoming OH-. In addition to these dynamical effects, the nonstatistical behavior of these reactions is attributed to the absence of equilibrated reactant complexes and to the large number of recrossings, which might be present in several ion-molecule gas-phase reactions.