One- and two-photon ionization of DNA single and double helices studied by laser flash photolysis at 266 nm

The ionization of the DNA single and double helices (dA)20, (dT)20, (dAdT)10(dAdT)10 and (dA)20(dT)20, induced by nanosecond pulses at 266 nm, is studied by time-resolved absorption spectroscopy. The variation of the hydrated electron concentration with the absorbed laser intensity shows that, in addition to two-photon ionization, one-photon ionization takes place for (dAdT)10(dAdT)10, (dA)20(dT)20 and (dA)20 but not for (dT)20. The spectra of all adenine-containing oligomers at the microsecond time-scale correspond to the adenine deprotonated radical formed in concentrations comparable to that of the hydrated electron. The quantum yield for one-photon ionization of the oligomers (ca. 10(-3)) is higher by at least 1 order of magnitude than that of dAMP, showing clearly that organization of the bases in single and double helices leads to an important lowering of the ionization potential. The propensity of (dAdT)10(dAdT)10, containing alternating adenine-thymine sequences, to undergo one-photon ionization is lower than that of (dA)20(dT)20 and (dA)20, containing adenine runs. Pairing of the (dA)20 with the complementary strand leads to a decrease of quantum yield for one photon ionization by about a factor of 2.     

Synthesis of novel heterocyclic structures via reaction of isocyanides with S-trans-enones

The reaction of enone 1, bearing an internal nucleophilic moiety, i.e., furan or pyrrole (X = O, NR'), with isocyanides is presented. The formation of products resulting from the reaction of the zwitterionic intermediate 2 with a second equivalent of isocyanide prior to cyclization to give 3, as well as the direct formation of 4 from 2, is described.     

Sr4PbPt4O11, the first platinum oxide containing Pt2 ions

We report the synthesis and crystal structure of the new compound Sr₄PbPt₄O₁₁, containing platinum in highly unusual square pyramidal coordination. The crystals were obtained in molten lead oxide. The structure was solved by X-ray single crystal diffraction techniques on a twinned sample, the final R factors are R=0.0260 and wR=0.0262. The symmetry is triclinic, space group P1¯, with , , , α=90.421(3)°, β=89.773(8)°, γ=90.140(9)° and Z=2. The structure is built from dumbell-shaped Pt₂O₉ entities formed by a dinuclear metal-metal bonded Pt₂⁶⁺ ion with asymmetric environments of the two Pt atoms, classical PtO₄ square plane and unusual PtO₅ square pyramid. Successive Pt₂O₉ entities deduced from 90° rotations are connected through the oxygens of the PtO₄ basal squares to form [Pt₄O₁₀⁸⁻]_∞ columns further connected through Pb²⁺ and Sr²⁺ ions. Raman spectroscopy confirmed the peculiar platinum coordination environment.     

Probing proteins on functionalized silicon surfaces using matrix-assisted laser desorption/ionization mass spectrometry

Flat H-terminated Si(111) substrates modified with alkyl monolayers terminated with hydrophobic and hydrophilic functional groups were prepared using known surface functionalization methods and characterized by FTIR, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The surfaces were then used for the study of non-specific binding of proteins from complex mixtures (using standard mixture of proteins with average molecular weight approximately 6-66 kDa) by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). Protein adsorption on these surfaces (following on-probe fractionation of the mixture) was found to be dependent on the nature of surface functional groups, and nature and pH of rinsing solutions used. The results obtained in this work demonstrate that simple silicon-based surface modifications can be effective for direct analysis of complex mixtures by MALDI-MS. Preliminary results obtained using similarly functionalized porous silicon substrates proved that such substrates are (due to their increased surface areas) better performing than flat silicon.     

Synthesis and characterization of nanoapatites organofunctionalized with aminotriphosphonate agents

Organofunctionalized apatite nanoparticles were prepared using a one step process involving dissolution/precipitation of natural phosphate rock and covalent grafting of nitrilotris(methylene)triphosphonate (NTP). The synthesized materials were characterized by Brunauer–Emmett–Teller (BET) surface measurement, thermogravimetry, inductively coupled plasma emission spectroscopy (ICP–ES), elemental analysis, multinuclear solid state cross-polarization/magic angle spinning (CP/MAS) and single-pulse NMR spectroscopy, transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDXA). After grafting BET measurements yielded particle specific surface areas ranging from 88 to 193 m² g^?1 depending on the grafted phosphonate. The results show that the surfaces of the nanoapatite particles can be covered with functional groups bound through a variable number of R–P–O–Ca bonds to render them organoapatites.     

Variable temperature STM study of Co deposition on a dodecanethiol self assembled monolayer

The present scanning tunneling microscopy study reports on the growth processes of Co vapor-deposited on a dodecanethiol (DDT) self-assembled monolayer (SAM)/Au(111). We observe strongly modified surface and depth diffusions of Co adatoms depending on the growth temperature. Co deposited at 300 K shows an extremely incomplete regime of condensation on the organic layer. Besides, Co penetrates the DDT monolayer and resides at the DDT/Au(111) interface as 2D clusters. This phenomenon takes place through defects in the SAM which are transient channels. In contrast, Co deposited at 50 K shows a complete condensation and nucleates on defects of the SAM layer as 3D islands sitting most likely on top of the DDTs. These results are of interest in the growing field of organic spintronics where the quality of the organic/ferromagnetic interface is a key issue.     

Amidase activity and thermal stability of human thrombin

Previous studies of amidase activity of human α-thrombin have yielded variable results and the decrease of this activity as a function of time and temperature has never been quantified. As this protease is an efficient tool in biochemistry and biotechnology thanks to its extreme selectivity, amidase activity and stability of thrombin were investigated with the synthetic substrate Tos-Gly-Pro-Arg-pNa. Enzyme activity as a function of temperature showed an optimum peak at 45‡C. The pH dependence of the activity showed a maximum around 9.5. The addition of NaCl promoted an increase of the activity. Stability of thrombin decreased rapidly when increasing the temperature from 25–45‡C and when diluting the enyzme. The presence of glycerol and ethylene glycol promoted a small increase of thrombin half life, whereas polyethylene glycol had a more pronounced positive effect even at very low concentrations.     

A new branching strategy for time constrained routing problems with application to backhauling

In this paper, we explore a new branching strategy for branch-and-bound approaches based on column generation for the vehicle routing problems with time windows. This strategy involves branching on resource variables (time or capacity) rather than on network flow variables. We also examine criteria for selecting network nodes for branching. To test the effectiveness of the branching strategy, we conduct computational experiments on time window constrained vehicle routing problems where backhauling is permitted only after all the shipments to clients have been made. The branching method proved very effective. In cases where time was the more binding constraint, time-based branching succeeded in decreasing the number of nodes explored by two thirds and the total computation time by more than half when compared to flow-based branching. The computational results also show that the overall algorithm was successful in optimally solving problems with up to 100 customers. It produced an average cost decrease of almost 7% when backhauling was permitted as compared to the cost involved when the client and the distributor routes were distinct.