New routes to manganese higher-nuclearity topologies: synthesis of the cluster [Mn8(mu4-O)4(phpz)8(thf)4

Reaction of Mn(ClO4)2.6H2O with 3(5)-methyl-5(3)-(2-hydroxyphenyl)pyrazole (H2phpz) affords a highly asymmetric octanuclear manganese(III) cluster resulting from the different bridging coordination modes of the ligand H2phpz.     

Kinetics of proton release after flash photolysis of 1-(2-nitrophenyl)ethyl sulfate (caged sulfate) in aqueous solution

The kinetics of proton release after laser photolysis of 1-(2-nitrophenyl)ethyl sulfate (caged sulfate) have been characterized by time-resolved absorbance and photoacoustic methods. The absorbance at approximately 400 nm is observed to rise with a biphasic behavior in which a prompt component (formation of the nitronic acid) is followed by a slower (tau approximately 63 +/- 6 ns) phase (deprotonation of the nitronic acid). The decay of this intermediate occurs with a lifetime which is affected by the pH of the solution and the laser pulse energy. In buffered aqueous solution at pH 7, 20 degrees C the aci-nitro decay rate is 18 +/- 4 s(-1). Protons are released to the solution with rate (1.58 +/- 0.09) x 10(7) s(-1) at neutral pH from the nitronic acid intermediate. From the numerical analysis of the protonation kinetics of suitable pH indicators, we could estimate the pK(a) of the nitronic acid as 3.69 +/- 0.05. At acidic pH, a substantial fraction of the aci-nitro intermediate is in the protonated form and this leads to a biphasic release of protons, with the slower phase being characterized by an apparent rate constant strongly dependent on the pH. The strongly acidic character of the final photoproduct (sulfate ion) means that there is negligible buffering of photoreleased protons.     

Polycyclic aromatic hydrocarbons in the atmosphere: monitoring, sources, sinks and fate. I: Monitoring and sources

This is the first of a series of two papers intended to review the state-of-the-art knowledge on atmospheric PAHs, concerning their monitoring, sources and transformation processes in the atmosphere. The monitoring section briefly introduces this class of compounds, mainly focusing on the 16 PAHs indicated by the US-EPA as priority pollutants. These compounds undergo partitioning between the gas phase and particulate, which has to be considered in the choice of the sampling methodology. Furthermore, sampling artifacts may arise from further phase transfers inside the sampling device. After sampling, extraction, clean up and detection/quantification procedures will follow. They are closely related since the choice of the extraction technique will heavily condition the clean-up step, and both procedures will place demands on the performance of the detection technique (usually GC-MS or HPLC). This is particularly true in the case of complex samples such as those arising from atmospheric sampling. The sources of atmospheric PAHs are then discussed with a particular focus on receptor models, which can allow the apportionment of PAH sources based on concentration data that can be routinely obtained by pollution control networks.     

Cinchona alkaloids in the asymmetric synthesis of 2-(phenylsulfanyl)aziridines

Enantioselective aza-Michael initiated ring closure (MIRC) additions of ethyl nosyloxycarbamate to 2-(phenylsulfanyl)-2-cycloalkenones catalysed by Cinchona alkaloids were studied. The results suggest that the enantioselectivity obtained is influenced by the structure of the catalyst and the different amination conditions. Substrate ring size also plays an important role.     

Capillary electrophoresis studies on the aggregation process of beta-amyloid 1-42 and 1-40 peptides

The possibility to monitor, in solution, the steps of beta-amyloid (Abeta) nucleation and therefore to describe this dynamic process by using capillary electrophoresis and under optimized experimental conditions is described. Striking differences in the electrophoretic patterns of Abeta 1-42 and Abeta 1-40 over time are here shown, and different aggregation states are elucidated, which reflect the very diverse oligomerization behavior of two very similar peptides. The isolation of one aggregated species of high molecular weight by ultracentrifugation allowed us to assess its role as toxic oligomer. The perturbation of the existing equilibrium among the identified species by the addition of small molecules can in principle interfere with the aggregation process of the peptides and ultimately prevent the plaque formation in vitro.     

Unusual Friedel-Crafts reactions,IX. One-step ortho-acylation of phenols with α,β-unsaturated acyl chlorides. Synthesis of 2'-hydroxychalcones and sorbicillin analogues

The reaction of bromomagnesium phenolates 1 in toluene with α, β-unsaturated acyl chlorides 2 at room temperature provides a straightforward highly selective synthesis of α,β-unsaturated 2-hydroxyarylketones 3. 2'-Hydroxychalcones, ortho-sorbylphenols including sorbicillin 3g, and ortho-propiolylphenols were usefully synthesized by this way. This reaction provides a further example of the synthetic versatility of the chelation-controlled approach in the elaboration of phenol derivatives.     

PEG-supported pyridylthioesters for racemization-free amide synthesis: a reagent that allows simultaneous product formation and removal from the polymer

Pyridylthioesters anchored to a modified poly(ethylene glycol) of M_w 5000 Da have been prepared in high yields. The thioesters were employed as a convenient starting material for the liquid-phase synthesis of various enantiomerically pure amides. This new methodology allowed to perform simultaneously the reaction with the poly(ethylene glycol)-supported reagent and the traceless removal of the final product from the polymer support in a single step. The products were obtained in high yield and purity.     

Evidence for two geminate rebinding states following laser photolysis of R state hemoglobin encapsulated in wet silica gels

In this letter we report the first experimental evidence for CO rebinding to human hemoglobin from multiple geminate states. The analysis of the rebinding kinetics using a maximum entropy method allowed the identification of two distinct rebinding states within the protein matrix, which become populated under conditions of increased viscosity in a silica gel at high glycerol concentration. Our findings suggest the presence of at least two distinct docking sites for the photolyzed ligand. Assuming a minimal four-state model, we estimate the microscopic rates and the activation energies for the elementary processes.     

Mode of binding of camptothecins to double helix oligonucleotides

We report an NMR study on the interaction of topotecan (Tpt) and other camptothecins (Cpts) with several double helix and single strand oligonucleotides. The results obtained by (31)P NMR spectroscopy, nuclear Overhauser experiments (NOE) and molecular dynamics (MD) simulations show that Cpt drugs do not intercalate into the double helix, as suggested by many authors. Phosphorus NMR spectra indicated that no deformation occurs at any level of the phosphodiester backbone, while 2D NOESY experiments allowed the detection of several contacts between the aromatic protons of Cpts and those of the double helix. Models of the drug/oligonucleotide complexes, built on the basis of NOE data, show that the drug is located at the end of the double helix, by stacking the A and B rings with the guanine or cytidine of the terminal CG base pairs, with a preference for the 3[prime or minute]-terminal end sites. Cpts interact with double strand, as well as with single strand oligomers, as can be seen from the NMR shift variation observed on the drug protons; but this shielding effect cannot be an evidence of intercalation, as it is largely due to external non-specific interactions of the positively charged drug with the negatively charged ionic surface of the oligonucleotide. The molecular weight of one of the complexes was obtained from the correlation time value. The conformational behaviour of the DNA fragment d(CGTACG)(2) was studied by MD simulations on a ns time scale in the presence of water molecules and Na(+) ions. Different models were examined and the deformations induced on the phosphodiester backbone by molecules that are known to intercalate, were monitored by MD simulations.