Effect of various surfactants on stability and thermophysical properties of nanofluids

Journal of Thermal Analysis and Calorimetry - The effect of Fe3O4 nanoparticles and carbon nanotubes (CNTs) on the viscosity of a nanofluid is experimentally investigated from 278 to 313 K...     

Influence of nanotopography on phospholipid bilayer formation on silicon dioxide

We have investigated the effect of well-defined nanoscale topography on the 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipid vesicle adsorption and supported phospholipid bilayer (SPB) formation on SiO2 surfaces using a quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM). Unilamellar lipid vesicles with two different sizes, 30 and 100 nm, were adsorbed on pitted surfaces with two different pit diameters, 110 and 190 nm, as produced by colloidal lithography, and the behavior was compared to results obtained on flat surfaces. In all cases, complete bilayer formation was observed after a critical coverage of adsorbed vesicles had been reached. However, the kinetics of the vesicle-to-bilayer transformation, including the critical coverage, was significantly altered by surface topography for both vesicle sizes. Surface topography hampered the overall bilayer formation kinetics for the smaller vesicles, but promoted SPB formation for the larger vesicles. Depending on vesicle size, we propose two modifications of the precursor-mediated vesicle-to-bilayer transformation mechanism used to describe supported lipid bilayer formation on the corresponding flat surface. Our results may have important implications for various lipid-membrane-based applications using rough or topographically structured surfaces.     

Glutathione transferase: new model for glutathione activation

Glutathione transferases are enzymes of the cellular detoxification system that metabolize a vast spectrum of xenobiotic and endobiotic toxic compounds. They are homodimers or heterodimers and each monomer has an active center composed of a G-site in which glutathione (GSH) binds and an H-site for the electrophilic substrate. When GSH binds to the G-site, the pKa value of its thiol group drops by 2.5 units; this promotes its deprotonation and, therefore, produces a strong nucleophilic thiolate that is able to react with the electrophilic substrate. The mechanism behind the deprotonation of the thiol group is still unknown. Some studies point to the fact that the GSH glutamyl alpha-carboxylate group is essential for GSH activation, whereas others indicate the importance of the active-center water molecules. On the basis of QM/MM calculations, we propose a mechanism of GSH activation in which a water molecule, acting as a bridge, is able to assist in the transfer of the proton from the GSH thiol group to the GSH glutamyl alpha-carboxylate group, after an initial GSH conformational rearrangement. We calculated the potential of mean force of this GSH structural rearrangement that would be necessary for the approach of both groups and we then performed a QM/MM ONIOM scan of water-assisted proton transfer. The overall free-energy barrier for the process is consistent with experimental studies of the enzyme kinetics.     

Kinetic characterization of an extremely slow DNA binding equilibrium

We here exploit the recently reported thermodynamic preference for poly(dAdT)(2) over mixed-sequence calf thymus (ct) DNA of two binuclear ruthenium complexes, DeltaDelta-[mu-bidppz(bipy)4Ru2](4+) (B) and DeltaDelta-[mu-bidppz(phen)(4)Ru(2)](4+) (P), that bind to DNA by threading intercalation, to determine their intrinsic dissociation rates. After adding poly(dAdT)(2) as a sequestering agent to B or P bound to ct-DNA, the observed rate of change in luminescence upon binding to the polynucleotide reflects the rate of dissociation from the mixed sequence. The activation parameters for the threading and dissociation rate constants allow us for the first time to characterize the thermodynamics of the exceedingly slow threading intercalation equilibrium of B and P with ct-DNA. The equilibrium is found to be endothermic by 33 and 76 kJ/mol, respectively, and the largest part of the enthalpy difference between the complexes originates from the forward threading step. At physiological temperature (37 degrees C) B and P have dissociation half-lives of 18 and 38 h, respectively. This is to our knowledge the slowest dissociating noncovalently bound DNA-drug reported. SDS sequestration is the traditional method for determination of rate constants for cationic drugs dissociating from DNA. However, the rates may be severely overestimated for slowly dissociating molecules due to unwanted catalysis by the SDS monomers and micelles. Having determined the intrinsic dissociation rates with poly(dAdT)(2) as sequestering agent, we find that the catalytic effect of SDS on the dissociation rate may be up to a factor of 60, and that the catalysis is entropy driven. A simple kinetic model for the SDS concentration dependence of the apparent dissociation rate suggests an intermediate that involves both micelles and DNA-threaded complex.     

Analysis of pesticide residues in fruit and vegetables with ethyl acetate extraction using gas and liquid chromatography with tandem mass spectrometric detection

A multiresidue method based on extraction with ethyl acetate has been used at the Swedish National Food Administration since 1989 to monitor pesticide residues in fruit and vegetables. The method has been continuously adjusted, resulting in simple and quick analyses of pesticide residues. To recover basic pesticides, the addition of an alkali is necessary. The addition of sodium hydrogen carbonate has been shown to recover all pesticides effectively without any degradation. The liquid chromatography (LC) with tandem mass spectrometry (MS/MS) technique has made it possible to analyse more polar pesticides and to replace many single methods. The latest development in the multiresidue method, comprising the use of gas chromatography (GC) with MS/MS, has further improved the analysis by replacing the conventional GC detectors. The need for cleanup has been reduced or eliminated entirely. Consequently, the method has been simplified in a way that makes it possible to recover all included analytes in many different matrices in one single extraction and to detect them either with GC-MS/MS or with LC-MS/MS.     

DHA-induced changes of supported lipid membrane morphology

Docosahexaenoic acid (DHA) is a polyunsaturated long fatty acid known to have fundamental effects on cell membrane function. Here, the effect of DHA on phosphocholine-supported lipid bilayers was measured using the quartz crystal microbalance with dissipation monitoring (QCM-D) technique. Above a concentration of 60 muM (i.e., near the critical micelle concentration), DHA had drastic effects on the viscoelastic properties of the supported membranes, suggesting a more complex process and structure than simple insertion of molecules in the bilayer. Fluorescence microscopy revealed the spontaneous formation of elongated out-growths from the bilayers, which were remarkable for their length ( approximately 100 mum) and extensive coverage of the surface. These results demonstrate the applicability of QCM-D as a method to screen for conditions where membrane remodeling occurs but also that complementary techniques are required to describe in more detail the changes in viscoelastic properties of the membrane. These results are highly relevant for the present rapid development in the field of model lipid membranes aiming toward increased knowledge about processes occurring at biological surfaces.     

A new mechanism of benzophenone photoreduction in photoinitiated crosslinking of polyethylene and its model compounds

The photolytic products and a new photoreduction mechanism of benzophenone (BP) as a photoinitiator in the photocrosslinking of polyethylene (PE) and its model compounds (MD) have been studied by means of fluorescence, ESR, ¹³C and ¹H NMR spectroscopy. The fluorescence spectra from the PE and MD systems demonstrate that the main photoreduction product of BP (PPB) is benzpinacol formed by the recombination of two diphenylhydroxymethyl (K^•) radical intermediates. The ESR spectrum obtained from the UV irradiation of the MD/BP system gives positive evidence of K^• radicals. Two new PPB products: an isomer of benzpinacol with quinoid structure, 1‐phenyl‐hydroxymethylene‐4‐diphenyl‐hydroxymethyl‐2,5‐cyclohexa‐diene and three kinds of α‐alkylbenzhydrols have been detected and identified for the first time by ¹³C and ¹H NMR spectroscopy from the MD systems. The latter could be formed by the reactions of K^• radicals with alkyl radicals produced by hydrogen abstraction of the excited triplet state ³(BP)* from polyethylene or its model compounds. These results provide new experimental evidence for elucidating the photoreduction mechanism of BP in the photoinitiated crosslinking of polyethylene. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 999–1005, 2000     

Amphiphilic graft copolymers - preparation and interfacial properties

Amphiphilic graft copolymers containing poly(ethylene oxide) (PEO) grafts have been prepared by various methods, for example, by coupling of reactive hydrophobic backbone polymers with end-functionalised PEO, by macromonomer copolymerisation, and by anionic graft polymerisation of EO onto polymer backbones carrying functional groups as initiator precursors. The graft copolymers are amphiphilic and were shown to accumulate at surfaces and interfaces in solution and in the solid state. Amphiphilic starch derivatives were prepared by reaction of amylose and starch with aliphatic α-epoxides.     

Isolation of Escherichia coli inner membranes by metal affinity two-phase partitioning

As reduction of sample complexity is a central issue in membrane proteomic research, the need for new pre-fractionation methods is significant. Here we present a method for fast and efficient enrichment of Escherichia coli inner membranes expressing a His-tagged integral membrane L-fucose-proton symporter (FucP). An enriched inner membrane fraction was obtained from a crude membrane mixture using affinity two-phase partitioning in combination with nickel-nitrilotriacetic acid (Ni-NTA) immobilized on agarose beads. Due to interaction between the beads and FucP, inner membranes were selectively partitioned to the bottom phase of a polymer/polymer aqueous two-phase system consisting of poly(ethylene glycol) (PEG) and dextran. The partitioning of membranes was monitored by assaying the activity of an inner membrane marker protein and measuring the total protein content in both phases. The enrichment of inner membrane proteins in the dextran phase was also investigated by proteomic methodology, including sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), trypsin digestion and liquid chromatography in combination with tandem mass spectrometry (LC-MS/MS). Using a high level of significance (99.95%) in the subsequent database search, 36 proteins assigned to the inner membrane were identified in the bottom phase, compared to 29 when using the standard sucrose gradient centrifugation method for inner membrane isolation. Furthermore, metal affinity two-phase partitioning was up to 10 times faster than sucrose gradient centrifugation. The separation conditions in these model experiments provide a basis for the selective isolation of E. coli membranes expressing His-tagged proteins and can therefore facilitate research on such membrane proteomes.     

Preparation and characterization of terraced surfaces of low-index faces of anatase, rutile, and brookite

Simple polishing and relatively low temperature annealing procedures for preparing atomically flat terraced surfaces of various single-crystal TiO2 polymorphs are described. Anatase (101), anatase (001), rutile (100), rutile (110), and brookite (111) surfaces could all be prepared with a terraced surface structure as revealed in AFM images. The rutile (100) and (110) and anatase (101) surfaces were also shown to produce acceptable LEED patterns immediately upon insertion into a UHV system without the usual sputter and anneal cycles.