Solvent-dependent host–guest complexation of two homologous merocyanines by a water-soluble calix[8]arene: Spectroscopic analysis and structural calculations

The sulfonated calixarene I₈C₁₂ acts as a host for homologous merocyanines Mc1 and Mc2 in organic solvents, exhibiting neither selectivity towards the guest dyes nor solvent dependence of the complexation equilibria. In water, on the contrary, only the lower homologue, Mc1, is solubilized in the presence of the calixarene. A combination of UV–visible and fluorescence spectroscopic and photophysical analysis and MD structural simulation of the calixarene-dye complexes was employed to account for the observations, and suggests that a radical change in the complexation mode occurs upon moving from an organic to an aqueous environment.     

Ethylene polymerization by novel Ziegler–Natta‐type catalysts obtained in situ by the oxidative addition of 8‐hydroxyquinoline‐based ligands to bis(1,5‐cyclooctadiene)nickel(0) and methylaluminoxane

Novel catalytic systems, prepared in situ by the oxidative addition of 8‐hydroxyquinoline ligands to bis(1,5‐cyclooctadiene)nickel(0) and activated by methylaluminoxane, were studied in ethylene polymerization. When 8‐hydroxyquinoline was employed, only oligomeric products were obtained. On the contrary, 5,7‐dinitro‐8‐hydroxyquinoline gave linear polyethylene (PE), but with a modest activity. For the catalyst based on 5‐nitro‐8‐hydroxyquinoline, the productivity was largely dependent on the content of free trimethylaluminum (TMA) present in the commercial aluminoxane. The progressive optimization of the TMA/oligomeric methylaluminoxane ratio increased the productivity, which reached 700 kg of PE/(mol of Ni × h), by an order of magnitude. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 200–206, 2006     

Selective synthesis of isobutanol by means of the Guerbet reaction: Part 2. Reaction of methanol/ethanol and methanol/ethanol/n-propanol mixtures over copper based/MeONa catalytic systems

The synthesis of isobutanol via the Guerbet condensation between methanol and ethanol was studied by using sodium methoxide (MeONa) as soluble basic component and copper-based catalysts as heterogeneous dehydrogenating/hydrogenating metal species. The effect of the nature of the catalyst and the relative amount of its individual components with respect to the reacting alcohols as well as of temperature on productivity and selectivity of the process was investigated. The collected data indicated that the copper chromite/MeONa was more active than Cu-Raney/MeONa system. The reaction was shown to proceed with the formation only of n-propanol and isobutanol. Ethanol conversion up to 61% with selectivity to isobutanol up to 98.4% was obtained. The same catalytic systems were also employed in the reaction of the methanol/ethanol/n-propanol ternary mixture. Again copper chromite/MeONa resulted more active than the Cu-Raney/MeONa system. Finally, experiments were carried out on methanol/n-propanol mixtures in the presence of the copper chromite/MeONa catalytic system by recycling both the recovered solid copper component and the liquid reaction mixture for evidencing eventual copper leaching by MeONa. On the basis of the obtained results it was concluded that in the Guerbet reaction copper chromite works as heterogeneous catalyst.     

Ab initio computation of force constants: Part VI. Applications of the force relaxation method for geometry optimization

Techniques to improve the computational efficiency of the force relaxation method are discussed. Force constants for fragments in previously computed smaller molecules can be transferred to construct a guess force constant matrix. Additional force constants that may be needed can be computed by a procedure which uses only one additional force calculation per diagonal force constant required. A scaling technique to improve convergence on the optimized geometry is discussed.     

S-Adenosyl methionine/S-adenosyl-L-homocysteine ratio determination by capillary electrophoresis employed as a monitoring tool for the antiviral effectiveness of adenosine analogs

S-Adenosyl-L-homocysteine hydrolase (SAHh) inhibitors have long been used as broad-range antivirals and have been recently evaluated as an experimental therapy of filovirus infections. In response to the need for a rapid laboratory testing method that could assess antiviral potency in vivo, our group developed a capillary electrophoresis (CE) method for the determination of the S-adenosyl-L-homocysteine (SAH) to S-adenosyl-L-methionine (SAM) ratio. After chloroacetaldehyde derivatization, SAH and SAM were detected using laser-induced fluorescence detection with a HeCd laser. Separation and quantitation of both SAH and SAM in human plasma were achieved in less than 1 min. The proposed method is rapid and reliable, and could be easily applied to routine monitoring of clinical and preclinical trials subjects.     

Methyl and silyl mesolytic dissociations in the radical cations and radical anions of but-1-ene,allylsilane, hexa-1,3-diene,and penta-2,4-dienylsilane. CAS-MCSCF and coupled cluster theoretical study

Methyl or silyl dissociation in the CH(2)=CHCH(2)-XH(3) (a-XH(3)(*)(+)) and CH(2)=CHCH=CHCH(2)-XH(3) (p-XH(3)(*) (+)) radical cations (X = C, Si) yields a(+) or p(+) and XH(3)(*). Similarly, the radical anions a-CH(3)(*) (-) and p-CH(3)(*) (-) give the pi-delocalized anion and CH(3)(*) preferentially. In contrast, a-SiH(3)(*) (-) and p-SiH(3)(*-) prefer to dissociate into the pi-delocalized radical and silide. All reactions are endoergic: by 43-50 kcal mol(-)(1) in the radical cations, and easier to some extent in the radical anions, that require 29-33 (X = C) and 13-14 kcal mol(-)(1) (X = Si). The fragmentation energy profiles do not present significant barriers for the backward process in the case of the radical cations. All radical anions exhibit an energy maximum along the dissociation pathway, but the barrier is lower than the dissociation limit. Fragmentation is "activated" more in the anions than in the cations with respect to homolysis in the corresponding neutrals (that requires 72-81 kcal mol(-)(1)). Wave function analysis indicates that the C-X bond cleavage in the hydrocarbon radical ions, although formally comparable to a homolytic process, is at variance with this model, due to the spin recoupling of one of the two C-X bond electrons with the originally unpaired electron. This is basically true also for the silyl-substituted radical anions, in which the initial more delocalized charge distribution might suggest some heterolytic character of the bond cleavage.     

Purpurin-18 in combination with light leads to apoptosis or necrosis in HL60 leukemia cells

Photodynamic therapy (PDT), a cancer treatment using a photosensitizer and visible light, has been shown to induce apoptosis or necrosis. We report here that Purpurin-18 (Pu18) in combination with light induces rapid apoptotic cell death in the human leukemia cell line (HL60) at low doses and necrosis at higher concentrations. Cells treated with Pu18 and light under apoptotic conditions exhibited DNA laddering and an increase in both cellular content of subdiploid DNA and externalization of phosphatidylserine (PS), indicating DNA fragmentation and loss of membrane phospholipid asymmetry. In the absence of light activation, Pu18 at nanomolar concentrations had no detectable cytotoxic effect. Caspase-3 activity was increased even after 1 h from treatment with low doses of Pu18 and light. The PS exposure and nuclear features of apoptosis were prevented by treatment of cells before illumination with caspase inhibitors benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (Z-VAD-FMK) and benzyloxycarbonyl-Asp-Glu-Val-Asp-fluoromethylketone (Z-DEVD-FMK). Conversely, the caspase-1 inhibitor, acetyl-Tyr-Val-Ala-Asp-aldehyde (Ac-YVAD-CHO) failed to suppress the apoptosis. No protective effect of the three caspase inhibitors was observed when the cells were exposed to necrotic concentrations of Pu18 and light. Our results show that caspase-3, but not caspase-1, is involved in the signaling of apoptotic events in PDT with Pu18-induced apoptosis of HL60 cells. Moreover, both the time course of PS exposure and the effect of caspase inhibitors on it indicate that it is regulated in the same manner as DNA fragmentation.     

Enhancement of luminescence efficiency of f–f transitions from Tb due to energy transfer from Ce in Al2O3 crystalline ceramic powders prepared by low temperature direct combustion synthesis

In this Letter, we report the structural and luminescent properties of Tb:Ce:Al₂O₃ crystalline ceramic powders prepared through combustion synthesis at low temperature (280 °C). The presence of Ce³⁺ (1.0 mol%) in the sample resulted in an enhancement of Tb³⁺ (1.0 mol%) overall emission intensity by a factor of 50. The analysis of the luminescence dynamics for the ⁵D₄ → ⁷F₆ transition (545 nm) of Tb³⁺ demonstrated that the mechanism responsible for the large enhancement of luminescence observed is efficient energy transfer from Ce³⁺ to Tb³⁺.