Atom lithography with two-dimensional optical masks

With a two-dimensional (2D) optical mask at =1083 nm, nanoscale patterns are created for the first time in an atom lithography process using metastable helium atoms. The internal energy of the atoms is used to locally damage a hydrophobic resist layer, which is removed in a wet etching process. Experiments have been performed with several polarizations for the optical mask, resulting in different intensity patterns, and corresponding nanoscale structures. The results for a linear polarized light field show an array of holes with a diameter of 260 nm, in agreement with a computed pattern. With a circularly polarized light field a line pattern is observed with a spacing of /2^0.5=766 nm. Simulations taking into account many possible experimental imperfections can not explain this pattern. PACS 32.80.Lg; 39.25.+k; 81.16.Nd     

Excited state molecular structures and reactions directly determined by ultrafast electron diffraction

In this communication, we report on the use of ultrafast electron diffraction to determine structural dynamics of excited states and reaction products of isolated aromatic carbonyls, acetophenone and benzaldehyde. For a 266 nm excitation, a bifurcation of pathways is structurally resolved, one leading to the formation of the triplet state (quinoid structure) and another to chemical products: for benzaldehyde the products are benzene and carbon monoxide (hydrogen migration and bond rupture) while those for acetophenone are the benzoyl and methyl radicals (bond rupture). The refined structures are compared with those predicted by theory. These dark structures and their radiationless transitions define the reduced energy landscape for complex reactions.     

An efficient one-pot synthesis of novel 4-aryl-1-methyloxindoles

An unprecedented synthetic approach to novel 4-aryl-1-methyloxindoles is described. The method involves the intramolecular palladium-catalyzed amidation of N-methyl-2,6-dibromophenylacetamide followed by an in situ Suzuki cross-coupling reaction with a (hetero)arylboronic acid in a one-pot reaction.     

Ion chromatographic determination of transition metals in irradiated nuclear reactor surveillance samples

The determination of transition metal ions in radioactive (+/-25 microCi/g) low-alloy steels (nuclear reactor surveillance samples) by ion chromatography (IC) is described. The analysis has been done directly without prior separation of the iron matrix. The eluted metal ions have been detected with a UV-visible spectrophotometric detector after post-column complexation with 4-(2-pyridylazo)resorcinol. The results are in a good agreement with the certified values for the standard reference material used. The method was applied to nuclear reactor surveillance samples for the determination of Cu, Mn, Co and Ni.     

Improving efficiency of large time-scale molecular dynamics simulations of hydrogen-rich systems

A systematic analysis is performed on the effectiveness of removing degrees of freedom from hydrogen atoms and/or increasing hydrogen masses to increase the efficiency of molecular dynamics simulations of hydrogen-rich systems such as proteins in water. In proteins, high-frequency bond-angle vibrations involving hydrogen atoms limit the time step to 3 fs, which is already a factor of 1.5 beyond the commonly used time step of 2 fs. Removing these degrees of freedom from the system by constructing hydrogen atoms as dummy atoms, allows the time step to be increased to 7 fs, a factor of 3.5 compared with 2 fs. Additionally, a gain in simulation stability can be achieved by increasing the masses of hydrogen atoms with remaining degrees of freedom from 1 to 4 u. Increasing hydrogen mass without removing the high-frequency degrees of freedom allows the time step to be increased only to 4 fs, a factor of two, compared with 2 fs. The net gain in efficiency of sampling configurational space may be up to 15% lower than expected from the increase in time step due to the increase in viscosity and decrease in diffusion constant. In principle, introducing dummy atoms and increasing hydrogen mass do not influence thermodynamical properties of the system and dynamical properties are shown to be influenced only to a moderate degree. Comparing the maximum time step attainable with these methods (7 fs) to the time step of 2 fs that is routinely used in simulation, and taking into account the increase in viscosity and decrease in diffusion constant, we can say that a net gain in simulation efficiency of a factor of 3 to 3.5 can be achieved. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 786–798, 1999     

Identification of putative proteins involved in granule biogenesis of tick salivary glands.

Ticks secrete bioactive components during feeding that assist them in gaining a blood meal. Compounds secreted are stored in granules until a stimulus induces secretion during feeding. Biogenesis of tick secretory granules has not been investigated before. An adequate understanding of granule biogenesis could advance our understanding of tick salivary gland biology and could aid in the rational design of tick control methods. Putative tick salivary gland proteins 1-4 (TSGP1-4) involved in granule biogenesis were identified in this study based on their abundance in salivary gland extracts and granule preparations and their ability to aggregate under conditions of slight acidity and high calcium concentration. TSGP2 and TSGP3 have been identified as previously described toxic and nontoxic homologues, respectively, while toxicity was also associated with TSGP4.