Intramolecular diels-alder/1,3-dipolar cycloaddition cascade of 1,3,4-oxadiazoles

Full details of a systematic exploration of the intramolecular [4 + 2]/[3 + 2] cycloaddition cascade of 1,3,4-oxadiazoles are disclosed in which the scope and utility of the reaction are defined.     

Two-dimensional self-assembly of linear molecular rods at the liquid/solid interface

We report on the synthesis and scanning tunneling microscopy (STM) studies of a series of linear molecular rods (1-5) comprising different numbers and/or spatial arrangements of perfluorinated benzene and benzene subunits interlinked with diacetylenes in the para position and decorated with or without terminal dodecyl chains. The molecules organize themselves into well-ordered 2D crystal structures at the liquid/solid interface through intermolecular and molecule-substrate interactions. Whereas the molecules substituted by dodecyl chains form the lamellar structures with alternating rigid core rows and alkyl chain rows, the unsubstituted ones change the orientation of the rigid backbones with respect to the lamellar axis. The molecular arrangement is not influenced by fluoro substituents on any phenyl ring of the backbone, which suggests that the interactions between the π-conjugated backbones are dominated by close packing rather than by the dipole moments of the rods or fluorine-based intermolecular interactions.     

Development of a robust technique for sampling volatile metal(loid)s in wetlands

The formation of volatile organic and inorganic metals and metalloids in aquatic environments is a known, but not very intensively investigated, process. Several techniques have been developed over the past 10 years to determine these trace components. These techniques are of limited use in wetland environments, where samples have to be taken from the soil-water interface, and require an immediate sample analysis due to thermodynamic instabilities of the volatile metal(loid)s. This paper presents an innovative sampling technique for total concentrations of volatile metal(loid)s in wetlands, based on an in situ gas-water separation via a porous PTFE membrane and stabilising the volatile metal(loid)s in a liquid sorbent (NaOCl solution). Samples may thus be collected even at remote sites, where longer storage times have to be accounted for. The sampling system was tested by means of a laboratory facility simulating the generation of arsine and dimethyl arsine under abiotic conditions as well as under field conditions. Results for sampling efficiency, reproducibility, and long-term storage are presented. Application of the sampling system in the field is shown.     

Total synthesis of (+/-)-perophoramidine

The first total synthesis of racemic perophoramidine is described. The key step features the highly stereoselective introduction of the vicinial quaternary centers via base-promoted carbon-carbon bond formation between a 3-alkylindole and a 3-bromo-3-alkylindolin-2-one. This transformation presumably proceeds through a conjugate addition or Diels-Alder cycloaddition of the 3-alkylindole with a 3-alkylindol-2-one intermediate.     

Accurate mass measurement and tandem mass spectrometry of intact globin chains identify the low proportion variant hemoglobin Lepore-Boston-Washington from the blood of a heterozygote

Within a mixture of proteins, minor polymorphic components are difficult to identify using a conventional proteomic approach. Their identification generally requires multi-dimensional separation steps, before or after proteolytic cleavage, followed by sequence analysis of the proteolytic products. In this study, we investigated the potential of tandem mass spectrometry for protein characterization by identifying the delta-beta hybrid human hemoglobin variant Lepore-Boston-Washington using electrospray ionization tandem mass spectrometry. Hemoglobin Lepore-Boston-Washington occurs mainly in heterozygotes, where it comprises approximately 10% of the total non-alpha-chains, the dominant non-alpha-chain being the normal beta (approximately 90%). Furthermore, Hemoglobin Lepore-Boston-Washington has an average molecular mass (15,865.23 Da) that is only 2 Da lower than that of the normal beta-chain (15,867.24 Da). Consequently, it cannot be resolved from the normal beta-chain by mass spectrometry. Here we show how Hemoglobin Lepore-Boston-Washington was identified directly from the diluted blood of a heterozygote by analyzing the product ions from the Lepore-Boston-Washington and normal beta-chain ions without prior separation of the individual chains. This study shows the potential of the tandem mass spectrometry for identifying a minor component in an unseparated mixture of proteins.     

Stereoelectronic effects in negatively and positively (Protonated) charged species. Ab initio studies of the gauche effect in 1,4-dioxa systems

A computational study ab initio of the conformational dependence of proton affinities of 2-methoxyethoxide (MEO), dimethoxyethane (DME), and 1,4-dioxane has been carried out at the MP2/6-31+G level of theory. The results were discussed in comparison with reference systems, from simple alkoxides and ethers to anomeric moieties, in open, cyclic, and bicyclic molecules. The COCCOC species are stronger bases than the COCOC (anomeric) ones and approach regular ethers in their strength. The gauche forms in MEO and DME are altogether stronger bases than the anti forms, and anti (equatorial) protonation is preferred over gauche (axial), unless ditopic protonation is possible, like in aga-DME or cis-tetraoxadecalin. The gauche effect plays a significant role in the formation, relative stability, and reactivity of the charged species.     

Confined supramolecular nanostructures of mesogen-bearing amphiphiles

Stable surface nanostructures with different morphology have been successfully constructed by modifying the chemical structure of synthetic amphiphiles; by introducing mesogenic groups into bolaform amphiphiles, stable spaghetti-like or stripe-like nanostructures can be obtained; it is believed that such a kind of surface structure could be used for templating synthesis and assembly.     

Monte Carlo versus molecular dynamics simulations in heterogeneous systems: an application to the n-pentane liquid-vapor interface

The Monte Carlo (MC) and molecular dynamics (MD) methodologies are now well established for computing equilibrium properties in homogeneous fluids. This is not yet the case for the direct simulation of two-phase systems, which exhibit nonuniformity of the density distribution across the interface. We have performed direct MC and MD simulations of the liquid-gas interface of n-pentane using a standard force-field model. We obtained density and pressure components profiles along the direction normal to the interface that can be very different, depending on the truncation and long range correction strategies. We discuss the influence on predicted properties of different potential truncation schemes implemented in both MC and MD simulations. We show that the MD and MC profiles can be made in agreement by using a Lennard-Jones potential truncated via a polynomial function that makes the first and second derivatives of the potential continuous at the cutoff distance. In this case however, the predicted thermodynamic properties (phase envelope, surface tension) deviate from experiments, because of the changes made in the potential. A further readjustment of the potential parameters is needed if one wants to use this method. We conclude that a straightforward use of bulk phase force fields in MD simulations may lead to some physical inconsistencies when computing interfacial properties.     

Structure Elucidation and Activity of Kolossin A,the D‐/L‐Pentadecapeptide Product of a Giant Nonribosomal Peptide Synthetase

The largest continuous bacterial nonribosomal peptide synthetase discovered so far is described. It consists of 15 consecutive modules arising from an uninterrupted, fully functional gene in the entomopathogenic bacterium Photorhabdus luminescens. The identification of its cryptic biosynthesis product was achieved by using a combination of genome analysis, promoter exchange, isotopic labeling experiments, and total synthesis of a focused collection of peptide candidates. Although it belongs to the growing class of D ‐/ L ‐peptide natural products, the encoded metabolite kolossin A was found to be largely devoid of antibiotic activity and is likely involved in interspecies communication. A stereoisomer of this peculiar natural product displayed high activity against Trypanosoma brucei rhodesiense, a recalcitrant parasite that causes the deadly disease African sleeping sickness.