Reductive activation of nitrate reductases

Protein film voltammetry of Paracoccus pantotrophus respiratory nitrate reductase (NarGH) and Synechococcus elongatus assimilatory nitrate reductase (NarB) shows that reductive activation of these enzymes may be required before steady state catalysis is observed. For NarGH complementary spectroscopic studies suggest a structural context for the activation. Catalytic protein film voltammetry at a range of temperatures has allowed quantitation of the activation energies for nitrate reduction. For NarGH with an operating potential of ca. 0.05 V the activation energy of ca. 35 kJ mol-1 is over twice that measured for NarB whose operating potential is ca. -0.35 V.     

Single nanocrystal arrays on patterned poly(ethylene glycol) copolymer microstructures using selective wetting and drying

Single nanocrystal arrays were fabricated on sub-microwells of poly(ethylene glycol) (PEG) copolymer using selective wetting on the hydrophilic regions of the exposed substrate surface and subsequent drying. Templates were produced by molding a thin film of a PEG-based random copolymer on hydrophilic substrates such as glass or silicon dioxide. The polymeric microstructures provide a topographical barrier around the well, which makes it possible to create nanocrystal arrays with controlled geometrical features. The size of the nanocrystal was found to decrease with decreasing well size and also decrease with decreasing topological height. A simple empirical equation was derived to predict the size of the crystal as a function of the pattern size and height, which is in good agreement with the experimental data.     

The arrangement of first- and second-shell water molecules in trivalent aluminum complexes: results from density functional theory and structural crystallography

The structural and energetic features of a variety of gas-phase aluminum ion hydrates containing up to 18 water molecules have been studied computationally using density functional theory. Comparisons are made with experimental data from neutron diffraction studies of aluminum-containing crystal structures listed in the Cambridge Structural Database. Computational studies indicate that the hexahydrated structure Al[H(2)O](6)(3+) (with symmetry T(h)()), in which all six water molecules are located in the innermost coordination shell, is lower in energy than that of Al[H(2)O](5)(3+).[H(2)O], where only five water molecules are in the inner shell and one water molecule is in the second shell. The analogous complex with four water molecules in the inner shell and two in the outer shell undergoes spontaneous proton transfer during the optimization to give [Al[H(2)O](2)[OH](2)](+).[H(3)O(+)](2), which is lower in energy than Al[H(2)O](6)(3+); this finding of H(3)O(+) is consistent with the acidity of concentrated Al(3+) solutions. Since, however, Al[H(2)O](6)(3+) is detected in solutions of Al(3+), additional water molecules are presumed to stabilize the hexa-aquo Al(3+) cation. Three models of a trivalent aluminum ion complex surrounded by a total of 18 water molecules arranged in a first shell containing 6 water molecules and a second shell of 12 water molecules are discussed. We find that a model with S(6) symmetry for which the Al[H(2)O](6)(3+) unit remains essentially octahedral and participates in an integrated hydrogen bonded network with the 12 outer-shell water molecules is lowest in energy. Interactions between the 12 second-shell water molecules and the trivalent aluminum ion in Al[H(2)O](6)(3+) do not appear to be sufficiently strong to orient the dipole moments of these second-shell water molecules toward the Al(3+) ion.     

Motor-protein "roundabouts": microtubules moving on kinesin-coated tracks through engineered networks

Nanotechnology promises to enhance the functionality and sensitivity of miniaturized analytical systems. For example, nanoscale transport systems, which are driven by molecular motors, permit the controlled movement of select cargo along predetermined paths. Such shuttle systems may enhance the detection efficiency of an analytical system or facilitate the controlled assembly of sophisticated nanostructures if transport can be coordinated through complex track networks. This study determines the feasibility of complex track networks using kinesin motor proteins to actively transport microtubule shuttles along micropatterned surfaces. In particular, we describe the performance of three basic structural motifs: (1) crossing junctions, (2) directional sorters, and (3) concentrators. We also designed track networks that successfully sort and collect microtubule shuttles, pointing the way towards lab-on-a-chip devices powered by active transport instead of pressure-driven or electroosmotic flow.     

Electronic detection of the enzymatic degradation of starch

[reaction: see text] The enzymatic degradation of starch can be monitored electronically using single-walled carbon nanotubes (SWNTs) as semiconducting probes in field-effect transistors (FETs). Incubation of these devices in aqueous buffer solutions of amyloglucosidase (AMG) results in the removal of the starch from both the silicon surfaces and the side walls of the SWNTs in the FETs, as evidenced by direct imaging and electronic measurements.     

Self-assembling structures of long-chain phenyl glucoside influenced by the introduction of double bonds

Four long-chain phenyl glucoside amphiphiles possessing a saturated or unsaturated long alkyl chain group as the self-assembling unit of a highly organized molecular architecture were synthesized. Their self-assembling properties were investigated by EF-TEM, SEM, CD, FT-IR, and XRD. Compound 2 possessing one double bond in the lipophilic portion showed twisted helical fibers, which formed a bilayered structure with a 3.59 nm period, while compound 3 showed the helical ribbons and left-handed nanotubular structures with 150-200 nm inner diameters and ca. 20 nm of wall. Very interestingly, compound 4 possessing three double bonds showed a nanotubular structure with ca. 70 nm of inner diameter through a helical ribbon, which formed a loose bilayered structure with 4.62 nm. These results indicate that self-assembling properties strongly depend on the number of cis double bonds.     

Mechanistic study of thermal behavior and combustion performance of epoxy resins: I homopolymerized TGDDM

Tetraglycidyl 4,4′-diaminodiphenylmethane (TGDDM) undergoes homopolymerization on heating. Intramolecular reactions which compete with crosslinking favor the formation of cyclic structures with increasing thermal and fire resistance of the resin, whereas physical mechanical properties tend to decrease. The mechanism of thermal decomposition of TGDDM is studied by thermogravimetry, differential scanning calorimetry and thermal volatilization analysis with characterization of volatiles evolved and residue left. Thermal degradation of poly-(TGDDM) starts at 260°C with elimination of water from secondary alcoholic groups which is a typical pathway for epoxy resin degradation. Resulting unsaturations weaken bonds in the β-position and provoke the first chain breaking at allyl–amine and allyl–either bonds. With increasing temperature, saturated alkyl–ether bonds and alkyl carbon–carbon bonds are broken first, followed by the most stable alkyl–aryl bonds at T>365°C. The combustion performance of TGDDM is discussed on the basis of the thermal degradation behavior.