Ultrafast to slow orientational dynamics of a homeotropically aligned nematic liquid crystal

The orientational dynamics of a homeotropically aligned nematic liquid crystal, 4'-pentyl-4-biphenylcarbonitrile (5-CB), is studied over more than six decades of time (500 fs to 2 mus) using optical heterodyne detected optical Kerr effect experiments. In contrast to the dynamics of nematogens in the isotropic phase, the data do not decay as a highly temperature-dependent exponential on the longest time scale, but rather, a temperature-independent power law spanning more than two decades of time, the final power law, is observed. On short time scales (approximately 3 ps to approximately 1 ns) another power law, the intermediate power law, is observed that is temperature dependent. The power law exponent of the correlation function associated with the intermediate power law displays a linear dependence on the change in the nematic order parameter with temperature. Between the intermediate power law and the final power law, there is a crossover region that displays an inflection point. The temperature-dependent orientational dynamics in the nematic phase are shown to be very different than those observed in the isotropic phase.     

DTA/DSC study of nanocrystallization in oxyfluoride glasses

Crystallization of glasses of the Na₂O-Al₂O₃-SiO₂-LaF₃ system was studied by DTA/DSC, XRD and TEM methods. It has been found that the nanocrystallization of LaF₃ in the NAS-based glass strongly depends on ratio of Na₂O/(SiO₂+Al₂O₃) and amount of doped LaF₃. Analysis of the local atomic interactions in the structure of oxyfluoride glasses has been used to explain the course of the crystallization. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis and Reactivity of (Pentafluorophenyl)platinate(II) Complexes with Bridging 1,8-Naphthyridine (napy) and X Ligands (X = C(6)F(5), OH, Cl, Br, I, SPh). Crystal Structure of [NBu(4)][Pt(2)(&mgr;-napy)(&mgr;-OH)(C(6)F(5))(4)].CHCl(3)

By reaction of [NBu(4)](2)[Pt(2)(&mgr;-C(6)F(5))(2)(C(6)F(5))(4)] with 1,8-naphthyridine (napy), [NBu(4)][Pt(C(6)F(5))(3)(napy)] (1) is obtained. This compound reacts with cis-[Pt(C(6)F(5))(2)(THF)(2)] to give the dinuclear derivative [NBu(4)][Pt(2)(&mgr;-napy)(&mgr;-C(6)F(5))(C(6)F(5))(4)] (2). The reaction of several HX species with 2 results in the substitution of the bridging C(6)F(5) by other ligands (X) such as OH (3), Cl (4), Br (5), I (6), and SPh (7), maintaining in all cases the naphthyridine bridging ligand. The structure of 3 was determined by single-crystal X-ray diffraction. The compound crystallizes in the monoclinic system, space group P2(1)/n, with a = 12.022(2) ?, b = 16.677(3) ?, c = 27.154(5) ?, beta = 98.58(3) degrees, V = 5383.2(16) ?(3), and Z = 4. The structure was refined to residuals of R = 0.0488 and R(w) = 0.0547. The complex consists of two square-planar platinum(II) fragments sharing a naphthyridine and OH bridging ligands, which are in cis positions. The short Pt-Pt distance [3.008(1) ?] seems to be a consequence of the bridging ligands.     

Development and validation of a high-performance liquid chromatographic method for the analysis of antioxidative phenolic compounds in fennel using a narrow bore reversed phase C18 column

A simple high-performance liquid chromatography (HPLC) method for the separation and quantitative determination of the main antioxidant phenolic compounds from bitter fennel, Foeniculum vulgare, was developed. The use of a narrow bore reversed phase (RP) C₁₈ column and an acidic mobile phase enabled ten compounds (caffeoylquinic acids, dicaffeoylquinic acids, flavonoids and rosmarinic acid) to be separated within a 40 min time analysis. The method was validated to demonstrate its selectivity, linearity, precision, accuracy and robustness. In addition, some parameters were studied to optimize the complete extraction of the phenolic compounds. The method was applied to the evaluation of three different fennel materials: distilled and non-distilled aerial parts, as well as defatted fruits. Distilled fennel was found to contain a higher proportion of antioxidant phenolic compounds than the non-distilled plan material.     

Spectrophotometric determination of aluminium and nickel

A simple spectrophotometric method, based on the complexes with xylenol orange (XO) and EDTA, is presented for the rapid determination of aluminium and nickel, respectively, in synthetic samples of hydrotalcite. The method only requires the solubilization in sulphuric acid of the inorganic material before the ligand addition. Under optimum conditions, the complexes Al-XO and Ni-EDTA showed maximum absorption at 554 nm and 380 nm, respectively. The method obeyed Beer's law in the concentration range 0.14-1.8 microg mL(-1) of aluminium, and 30-2730 microg mL(-1) of nickel. Molar absorptivities were 2.45 x 10(4) and 14.85 L mol(-1) cm(-1) while Sandell's sensitivities were 1.1 x 10(-3) and 3.9 microg cm(-2) for aluminium and nickel, respectively. The standard addition technique was used and the recoveries obtained revealed that the proposed procedure shows good accuracy.     

RNA catalysis in model protocell vesicles

We are engaged in a long-term effort to synthesize chemical systems capable of Darwinian evolution, based on the encapsulation of self-replicating nucleic acids in self-replicating membrane vesicles. Here, we address the issue of the compatibility of these two replicating systems. Fatty acids form vesicles that are able to grow and divide, but vesicles composed solely of fatty acids are incompatible with the folding and activity of most ribozymes, because low concentrations of divalent cations (e.g., Mg(2+)) cause fatty acids to precipitate. Furthermore, vesicles that grow and divide must be permeable to the cations and substrates required for internal metabolism. We used a mixture of myristoleic acid and its glycerol monoester to construct vesicles that were Mg(2+)-tolerant and found that Mg(2+) cations can permeate the membrane and equilibrate within a few minutes. In vesicles encapsulating a hammerhead ribozyme, the addition of external Mg(2+) led to the activation and self-cleavage of the ribozyme molecules. Vesicles composed of these amphiphiles grew spontaneously through osmotically driven competition between vesicles, and further modification of the membrane composition allowed growth following mixed micelle addition. Our results show that membranes made from simple amphiphiles can form vesicles that are stable enough to retain encapsulated RNAs in the presence of divalent cations, yet dynamic enough to grow spontaneously and allow the passage of Mg(2+) and mononucleotides without specific macromolecular transporters. This combination of stability and dynamics is critical for building model protocells in the laboratory and may have been important for early cellular evolution.     

New 1,2,3‐triazolo[4,5‐d]1,2,4‐triazolo[3,4‐b]pyridazine derivatives II

New tricyclic 1,2,3‐triazolo‐1,2,4‐triazolo‐pyridazine derivatives, bearing a methyl substituent on the 1,2,3‐triazole ring, were prepared as potential biological agents. N‐Methylation of dimethyl 1,2,3‐triazole‐4,5‐dicarboxylate allowed synthesis of the isomeric 1‐methyl‐4,7‐dihydroxy and 2‐methyl‐4,7‐dihydroxy triazolo‐pyridazines 4a and 4b which, by a chlorination reaction, gave the corresponding 1‐methyl‐4‐chloro‐( 6a ), 1‐methyl‐7‐chloro‐ ( 6b ) and 2‐methyl‐4‐chloro‐ ( 9 ) substituted 1,2,3‐triazolo‐pyridazines. The nucle‐ophilic substitution with hydrazine hydrate and the suitable cyclization to form the 1,2,4‐triazole ring, provided the expected tricyclic isomeric derivatives 8a, 8b and 11 respectively. The p‐methoxybenzyl substituent, introduced as a leaving group to obtain either v‐triazolo‐pyridazine or v‐triazolo‐s‐triazolo‐pyri‐dazine derivatives unsubstituted on the 1,2,3‐triazole ring, appeared inadequate. Some compounds underwent binding assays toward the adenosine A₁and A_2A receptors.     

PHOTOALLERGIC RESPONSES TO CHEMICALS

Abstract— –Photochemical and immunologic knowledge about photoallergy to chemicals is briefly summarized. Studies in in vitro systems have demonstrated that photoallergic compounds can covalently bond to proteins through a photochemical reaction. The immunologic nature of the photoallergic response is based mainly on clinical observations, induction of photoallergy in man and in guinea-pigs and on results of in vitro immunologic tests.
Studies of the photoreactions of the photoallergic compound, 3,3',4',5-tetrachlorosalicylanilide (TCSA), with proteins are discussed. TCSA noncovalently bonds to human serum albumin prior to irradiation. Prior interaction is essential for formation of a photoaddition product indicating that a short-lived reactive species derived from TCSA is involved in the photoaddition and limiting the number of skin proteins which can participate in antigen formation. By fragmentation of the TCSA-albumin photoadduct with CNBr, it was determined that TCSA can bond to at least three sites on the albumin molecule. TCSA also can sensitize the photooxidation of histidine in albumin.     

Ionic‐Liquid‐Based Polymer Electrolytes for Battery Applications

The advent of solid‐state polymer electrolytes for application in lithium batteries took place more than four decades ago when the ability of polyethylene oxide (PEO) to dissolve suitable lithium salts was demonstrated. Since then, many modifications of this basic system have been proposed and tested, involving the addition of conventional, carbonate‐based electrolytes, low molecular weight polymers, ceramic fillers, and others. This Review focuses on ternary polymer electrolytes, that is, ion‐conducting systems consisting of a polymer incorporating two salts, one bearing the lithium cation and the other introducing additional anions capable of plasticizing the polymer chains. Assessing the state of the research field of solid‐state, ternary polymer electrolytes, while giving background on the whole field of polymer electrolytes, this Review is expected to stimulate new thoughts and ideas on the challenges and opportunities of lithium‐metal batteries.     

Glucose–Nucleobase Pseudo Base Pairs: Biomolecular Interactions within DNA

Noncovalent forces rule the interactions between biomolecules. Inspired by a biomolecular interaction found in aminoglycoside–RNA recognition, glucose‐nucleobase pairs have been examined. Deoxyoligonucleotides with a 6‐deoxyglucose insertion are able to hybridize with their complementary strand, thus exhibiting a preference for purine nucleobases. Although the resulting double helices are less stable than natural ones, they present only minor local distortions. 6‐Deoxyglucose stays fully integrated in the double helix and its OH groups form two hydrogen bonds with the opposing guanine. This 6‐deoxyglucose‐guanine pair closely resembles a purine‐pyrimidine geometry. Quantum chemical calculations indicate that glucose‐purine pairs are as stable as a natural T‐A pair.