Nonpolar Substrates Research Articles

Active Subtilisin-Like Protease from a Hyperthermophilic Archaeon in a Form with a Putative Prosequence

The gene encoding subtilisin-like protease T. kodakaraensis subtilisin was cloned from a hyperthermophilic archaeon Thermococcus kodakaraensis KOD1. T. kodakaraensis subtilisin is a member of the subtilisin family and composed of 422 amino acid residues with a molecular weight of 43,783. It consists of a putative presequence, prosequence, and catalytic domain. Like bacterial subtilisins, T. kodakaraensis subtilisin was overproduced in Escherichia coli in a form with a putative prosequence in inclusion bodies, solubilized in the presence of 8 M urea, and refolded and converted to an active molecule. However, unlike bacterial subtilisins, in which the prosequence was removed from the catalytic domain by autoprocessing upon refolding, T. kodakaraensis subtilisin was refolded in a form with a putative prosequence. This refolded protein of recombinant T. kodakaraensis subtilisin which is composed of 398 amino acid residues (Gly(-82) to Gly(316)), was purified to give a single band on a sodium dodecyl sulfate (SDS)-polyacrylamide gel and characterized for biochemical and enzymatic properties. The good agreement of the molecular weights estimated by SDS-polyacrylamide gel electrophoresis (44,000) and gel filtration (40,000) suggests that T. kodakaraensis subtilisin exists in a monomeric form. T. kodakaraensis subtilisin hydrolyzed the synthetic substrate N-succinyl-Ala-Ala-Pro-Phe-p-nitroanilide only in the presence of the Ca(2+) ion with an optimal pH and temperature of pH 9.5 and 80 degrees C. Like bacterial subtilisins, it showed a broad substrate specificity, with a preference for aromatic or large nonpolar P1 substrate residues. However, it was much more stable than bacterial subtilisins against heat inactivation and lost activity with half-lives of >60 min at 80 degrees C, 20 min at 90 degrees C, and 7 min at 100 degrees C.

Optical Properties of Cubic Gallium Nitride on SiC/Si Pseudo-Substrates

The cubic phase of III-nitrides has received much less attention than the hexagonal structure, because of difficulties inherent to the growth of this meta-stable phase. However, the possibility to avoid the effects of piezoelectric fields in cubic phase GaN grown on non-polar substrates gives new interest for this material. In this work, we present optical properties (photoluminescence and photoreflectance) of cubic GaN layers grown by MBE on c-SiC/Si pseudo-substrates at room and low temperature. Photoluminescence properties of doped (n- and p-type) c-GaN layers and of AlGaN layers are also presented.

Thermal Probe Measurements of the Glass Transition Temperature for Ultrathin Polymer Films as a Function of Thickness

The glass transition temperature of polystyrene and poly(methyl methacrylate) films on polar and nonpolar substrates was measured as a function of thickness using a thermal probe in contact with a polymer film. Using a technique called local thermal analysis, heat loss into the film was monitored as the temperature of the probe was ramped from ambient temperature to temperatures as high as 200 °C. The glass transition temperature was determined from a change in slope in the heat loss versus temperature plot. The Tg of polystyrene on silicon oxide decreased by as much as 25 °C below the bulk value for films 13 nm thick. The same trend in the glass transition temperature was observed for polystyrene films on silicon oxide treated with hexamethyldisilizane (HMDS). The Tg of poly(methyl methacrylate) on silicon oxide increased by up to 7 °C above the bulk value for films 18 nm thick. For poly(methyl methacrylate) on silicon oxide treated with HMDS, the Tg decreased by 10 °C below the bulk value for films 21 n...

Structural dipoles at interfaces between polar II-VI semiconductors CdSand CdTe and non-polar layered transition metal dichalcogenide semiconductorsMoTe2 and WSe2

The electronic structure of heterojunctions between II-VIsemiconductors (CdS, CdTe) and layered transition metal dichalcogenidesemiconductors (MoTe2, WSe2) were investigated by soft x-rayphotoelectron spectroscopy. The interfaces were formed sequentially bymolecular beam epitaxy (MBE) of the II-VI semiconductors on van der Waals(0001)-surfaces of layered compound single crystals. The II-VI semiconductorsgrow with their polar hexagonal axis ([111] for CdTe, [0001] for CdS)parallel to the non-polar substrate [0001]-axis, without evidence for latticedistortions as prooved by TEM and LEED. Band offsets were determined from softx-ray photoelectron spectra. Large substrate independent interface dipoles of1.2 eV for CdS and 0.8 eV for CdTe were determined. The deviation ofexperimental band offsets from the electron affinity rule prediction isdescribed by a simple model which takes the polarity of the grown interfacesinto account.

Determination of the Polarities of ZnO Thin Films on Polar and Nonpolar Substrates Using Scanning Nonlinear Dielectric Microscopy

Using scanning nonlinear dielectric microscopy (SNDM) which we developed, we determined the polarities of ZnO thin films on various substrates including polar materials. SNDM is a useful tool for investigating the anisotropy of polar materials by measuring the nonlinear dielectric constant, whose sign changes in accordance with the polarity of the specimen. In conventional methods based on detecting the piezoelectric and pyroelectric responses, it is very difficult to determine the polarities of thin films, particularly in the case of laying these films on the polar substrates, because the detected signals from thin films are very small and those from the substrates are large. Our SNDM method, however, enables us to determine the polarities of thin films on polar substrates easily. Using SNDM, the polarities of ZnO thin films on several kinds of polar and nonpolar substrates were determined. We also determined experimentally that ZnO thin films grew with a sign opposite to the substrate polarity and it was suggested that pyroelectric effects mainly governed the polarity of ZnO films.

Influence of generation number on the dewetting of hypergraft poly(2-ethyl-2-oxazoline) films

We investigate the influence of polymer branching (generation number G) on the dewetting of hypergraft poly(2-ethyl-2-oxazoline) (PEOX) polymer films from model non-polar synthetic polymer and inorganic substrates. The present study contrasts the cases of a zeroth generation G0 hypergraft, which is a comb polymer, and a G2 hypergraft, which resembles a nearly spherical 'microgel' particle. The early stage of dewetting is found to be similar in the G0 and G2 films and is largely independent of substrate (entangled polystyrene film or acid-cleaned silica wafer). However, the late stage of dewetting in the G2 films differs significantly from the GO films because of an inhibition of hole coalescence in the intermediate stage of film dewetting. The holes in the G2 hyperbranched films continue to grow in size until they impinge on each other to form a foam-like structure with a uniform 'cell' size. The boundaries of these cells break up and the vertices of the former cellular network retract to form a relatively uniform droplet configuration. In contrast, hole coalescence in the GO films occurs readily, leading to a polydisperse hole size distribution, and the fluid borders of the holes break up to form the non-uniform 'Voronoi' droplet configuration normally associated with film dewetting. Similar trends have been observed in the dewetting of linear polymer films where entangled polymers behave similarly to the branched (G2) polymers of the present study. This effect is attributed to the viscoelasticity of the G2 hyperbranched polymer fluid.

Adhesion at partially restructured polymer surfaces

The controlled modification of the surface structure of styrene-4-vinyl pyridene (SVP) and of ethylene-acrylic acid (EAA) copolymers was achieved by the timed exposure of the polymers to non-orienting (n-alkane) and orienting (water) fluids. The degree of surface modification was monitored by periodic measurements of non-dispersive surface energies from static contact angle data. Assemblies of rigid PVC/SVP, of poly(methyl methacrylate) (PMMA)/EAA and of polypropylene (PP)/EAA were prepared with the SVP and EAA in various stages of restructuring. Bond characteristics of these assemblies were evaluated from single lap-shear measurements. Bond strengths responded strongly to the orientational states of the copolymer adhesives. In PVC/SVP and PMMA/EAA, significant increases in bond strength were observed when orientational changes raised the nondispersive contribution, γ nd, to the surface energies of these copolymers. Surprisingly, full increments in bond strengths were realized at only about 25–40% of the total increment in γ nd. Surface restructuring had little effect on adhesion with the non-polar PP substrate. Residual bond strengths, following controlled aging, varied directly with the initial bond strength. The residual bond strengths may be used to specify ideal restructuring events in the adhesives which would render the assemblies fully resistant to the chosen aging process. Acid/base interaction concepts have been used to rationalize the observations. These suggest that only relatively few polar elements need to migrate into the surface region of the adhesives to maximize initial bond strength, but larger numbers are needed to ensure adequate resistance to aging.

Growth, optical, and electron transport studies across isotype n-GaAs/n-Ge heterojunctions

A diode structure consisting of a polar epilayer on a nonpolar substrate grown by metalorganic vapor phase epitaxy often faces problems of antiphase domain formation in the polar semiconductor and cross diffusion across the heterointerface. Ge outdiffusion into GaAs epilayers was studied by low temperature photoluminescence spectroscopy after etching the film from the surface. The absence of p-n junction formation inside the Ge substrate from interdiffusion of Ga and As has been studied by current-voltage characteristics using mesa diodes. These observations were confirmed by electrochemical capacitance voltage polaron profiler and secondary ion mass spectroscopy techniques. To understand the material quality and current conduction mechanism across the GaAs/Ge heterojunction, I-V characteristics of Si-doped n-GaAs/n-Ge isotype heterojunctions using Au Schottky diodes have been studied for different doping densities. A plethora of growth conditions appear in the literature concerning the attempt to grow antiphase domain (APD)-free GaAs on Ge. In the present case, even though the growth temperature regime is close to reported values, the main difference in minimizing APD formation may arise from the growth rates (∼3 μm/h) and the V/III ratio (∼88).

Activation of Aromatic C−H Bonds by (dmpe)Pt(Me)X (X = Me, O2CCF3, OTf) Systems

A series of complexes (dmpe)Pt(X)2 (X = Me, Cl, O2CCF3, OTf) have been prepared and their thermal stabilities examined. In contrast to (dmpe)Pt(Me)(Cl), which is stable up to 150 °C in benzene solution, thermolysis of (dmpe)Pt(Me)(O2CCF3) in benzene at 125 °C results in methane loss and the formation of (dmpe)Pt(Ph)(O2CCF3) as the major product, along with small amounts of the disproportionation products (dmpe)Pt(O2CCF3)2 and (dmpe)Pt(Ph)2 (8−10%). Kinetic studies in C6H6 and C6D6 indicate that the disappearance of (dmpe)Pt(Me)(O2CCF3) is first-order, with an overall kinetic isotope effect of 3.3(2). Methyl/aryl metathesis rates are not inhibited by added TBA+O2CCF3-. Thermolysis of (dmpe)Pt(Me)2 in benzene at 180 °C for 18 h gave a mixture of unreacted (dmpe)Pt(Me)2 (15%), (dmpe)Pt(Me)(Ph) (51%), and (dmpe)Pt(Ph)2 (34%). Thermolysis of (dmpe)Pt(Me)(O2CCF3) in either 1,2- or 1,4-difluorobenzene at 125 °C afforded 2:1:1 statistical mixtures of (dmpe)Pt(Ar)(O2CCF3), (dmpe)Pt(Ar)2, and (dmpe)Pt(O2CCF3)2 (Ar = 2,3-C6H3F2 or 2,5-C6H3F2, respectively). Thermolysis of (dmpe)Pt(Me)(O2CCF3) in toluene affords a mixture of ortho, meta, and para tolyl products, (dmpe)Pt(C6H4Me)(O2CCF3). The metathesis rate for polar 1,2-difluorobenzene is not significantly different than that observed for the nonpolar aromatic substrates. Surprisingly, the aryl/aryl exchange rates for (dmpe)Pt(Ph)(O2CCF3) in C6D6 and C7D8 are 10−20 times faster than observed methyl/aryl exchange rates. These data may be accommodated by a mechanism involving a rate-limiting addition of aryl C−H bonds directly to a 4-coordinate (dmpe)Pt(II) or a 3-coordinate (η1-dmpe)Pt(II) center.

Comparative studies of Si-doped n-type MOVPE GaAs on Ge and GaAs substrates

Comparative studies of silicon (Si) incorporation in GaAs on both polar GaAs and nonpolar Ge substrates by low temperature photoluminescence (LTPL) spectroscopy were carried out. The PL spectrum shifts towards higher energy with growth temperature, arsine (AsH 3) and trimethylgallium (TMGa) mole fractions on Ge substrates; whereas the PL spectrum shifts towards higher energy with growth temperature and shifts to lower energy with AsH 3 and TMGa mole fractions on GaAs substrates. The shift in PL peak energy towards the higher energy is due to the increase in electron concentration The full width at half maximum (FWHM) increases with increasing growth temperature, AsH 3 and TMGa mole fractions on Ge substrates. But the FWHM increases with increasing growth temperature and decreases with increasing AsH 3 and TMGa mole fractions on GaAs substrates. A vacancy control model may explain the PL peak shift towards higher energy with increasing AsH 3 mole fraction on Ge substrates and with increasing TMGa mole fraction on GaAs substrates. The experimental results of the studies of the effect of TMGa mole fraction variation on zinc (Zn)-doped GaAs on both GaAs and Ge substrates were presented for better understanding of the growth process.

Photoluminescence studies on Si-doped GaAs/Ge

Photoluminescence (PL) spectroscopy has been used to study the silicon incorporation in polar GaAs on nonpolar Ge substrates. Shifts of PL spectra towards higher energy with growth temperature, trimethylgallium (TMGa) and arsine (AsH3) mole fractions were observed. The full width at half maximum increases with increasing growth temperature, AsH3 and TMGa mole fractions. The peak at 1.49 eV has been attributed to band-to-acceptor transition involving residual carbon. The PL peak energy shifts towards higher energy with increasing growth temperature due to the increase in electron concentration. A vacancy control model may explain the PL shift towards higher energy with increasing AsH3 mole fraction. The PL peak shifts towards higher energy with increasing TMGa mole fraction. The experimental results about the growth temperature, trimethylgallium, and arsine mole fractions on silicon-doped GaAs on GaAs were presented for comparison. The outdiffusion of Ge into the GaAs epitaxial layer was hardly to be seen from the secondary ion mass spectroscopy result.

The P-glycoprotein efflux pump: how does it transport drugs?

Pgp is an atypical translocating ATPase, with low affinity for ATP and high constitutive ATPase activity. Pgp also has an unusually broad specificity for hydrophobic substrates, including many chemotherapeutic drugs. Transport studies in reconstituted systems indicate that drug transport requires ATP hydrolysis and is active, generating a drug concentration gradient. Binding of drugs and ATP to Pgp induces conformational changes in the protein, and the drug binding site is conformationally coupled to the NBDs. Evidence accumulated to date suggests that the transporter interacts directly with nonpolar substrates within the membrane environment, and may act as a drug flippase, moving drugs from the inner to the outer leaflet of the bilayer. Chemosensitizers that block the action of Pgp are proposed to act as alternative substrates, but their high rate of spontaneous flip-flop across the membrane results in futile cycling of the transporter.

Acid-Activated Organoclays: Preparation, Characterization and Catalytic Activity of Acid-Treated Tetraalkylammonium-Exchanged Smectites

The catalytic activity of acid-treated clays for reactions involving polar substrates is optimized when the acidity of the surface and the swelling ability of the catalyst are at a maximum. In contrast the activity for reactions using nonpolar substrates is best when the surface area is maximized and the catalyst presents an essentially hydrophobic surface which serves to attract the nonpolar reagents. We have investigated the catalytic ability of acid-activated organoclays (AAOCs) which should provide reasonable levels of acidity, hydrophobicity, and swelling ability for use with nonpolar reagents. A range of organoclays containing tetramethylammonium, dodecyltrimethylammonium, or octadecyltrimethylammonium cations at the 25, 50, or 100% exchange level were prepared and subjected to selected acid-leaching procedures at either 20 or 95 degrees C. The activity of these AAOCs for the conversion of a-pinene to camphene was investigated. The conditions used for acid leaching seldom removed extensive amounts of organocation, and the yields (40% conversion to camphene) compared favorably with those reported for pillared clays. Acid-leached tetramethylammonium clays were the most active with yields four times higher than those for the corresponding parent clay. Acid-leached dodecyltrimethylammonium and octadecyltrimethylammonium clays were only active when the organocations occupied 25% of the exchange sites.

Initial growth processes of Ag on polar and non-polar semiconductor substrates

Initial growth processes of Ag on both InSb(111)A and α-Sn(111) substrates at room temperature have been investigated by using reflection high-energy electron diffraction and Auger electron spectroscopy. The results show that the growth features are quite different for each system: Ag grows in the Stranski-Krastanov mode on InSb(111)A, while for Ag/α-Sn(111) the majority of the deposited Ag atoms are consumed in forming an Ag-Sn alloy. Discrete variational-Xα calculations showed that an onset of such growth modes is closely related to interfacial chemical bonding features for both systems.

Exceptionally stable salt bridges in cytochrome P450cam have functional roles.

A long-standing puzzle in structure-function studies of cytochrome P450cam is how the substrate, camphor, reaches the buried active site. The crystal structure shows no channel from the surface to the active site large enough for substrate to pass through. Recent experiments indicate that access of the rather nonpolar substrate to the active site is controlled by electrostatic interactions and may involve rupture of the two salt links to Asp251 [Deprez, E., Gerber, N. C., Di Primo, C., Douzou, P., Sligar, S. G., & Hui Bon Hoa, G. (1994) Biochemistry 33, 14464-14468]. Consequently, we have computed the electrostatic strength of 53 ionic pairs, including 32 salt links, in cytochrome P450cam by numerical solution of the finite-difference linearized Poisson-Boltzmann equation. The calculated electrostatic free energies, delta Gtot, of the salt links range from -9 to +6 kcal/mol with approximately 60% of the salt links being energetically favorable and 40% being unfavorable with respect to mutation to their uncharged, nonpolar isosteres. Strikingly, of the four most stable salt links in the protein (delta Gtot < -6 kcal/mol), two involve the propionate groups of the heme and the other two involve Asp251. In the modeled D251N mutant, for which electrostatic effects on substrate binding are diminished, the latter two salt links lose their stability (delta Gtot > -2.4 kcal/mol). Thus it appears that cytochrome P450cam has evolved four unusually strong salt bridges, stabilized by surrounding charged and polar groups in the protein, to keep its heme cofactor in place and to regulate substrate binding.

ZnSe epitaxial growth on Si(100) and Ge(100) by H-radical assisted MOCVD

Epitaxial ZnSe is a polar material grown on non-polar substrates such as Si(100) and Ge(100) by metal organic chemical vapor deposition methods. Epitaxial growth of ZnSe on non-polar materials is more difficult than on polar materials, for example, on GaAs. For the growth of ZnSe on non-polar substrates, a polarization of substrate surface at the initial step is of importance. The FWHM of XRD peak obtained for ZnSe(100) grown on Ge is wider than that obtained for ZnSe(100) grown on GaAs, in spite of both Ge and GaAs having nearly the same lattice constant. Further, the surface morphology of ZnSe films grown on Ge and GaAs substrates is observed to be different. It is considered that the surface polarization of Ge substrate at the initial stage is not uniform. This results in a lower number of growth sites on Ge substrate compared with that on GaAs substrate.

Allenylidenes: their multifaceted chemistry at rhodium

Rhodium allenylidenes, which represent a class of ‘flat’ metallabutatrienes, are useful tools for the generation of metal-bound species that are hardly accessible (sometimes not accessible at all) by other synthetic routes. In contrast to various octahedral allenylidene metal complexes, e.g. of Cr 0 , Mn I or Ru II , the square-planar rhodium compounds trans- [RhX(CCCRR′)(PPr i 3 ) 2 ] are attacked by both nucleophiles and electrophiles. Moreover, non-polar substrates such as H 2 or Cl 2 react cleanly with the title complexes to give products in which the allenylidene unit is preserved as part of a newly formed ligand. Attempts to prepare a square-planar cationic metallaheptahexaene, trans-[Rh(CCCRR′) 2 (PP r i 3 ) 2 ] + , unexpectedly led to the formation of two isomeric hexapentaenerhodium(I) complexes of which the isomer having the polyene coordinated in an unsymmetrical fashion is the thermodynamically more stable. C–C coupling reactions also occur on treatment of trans-[RhCl(CCCPh 2 )(PPr i 3 ) 2 ] with vinyl Grignard reagents or phenylacetylene leading to highly unsaturated RhC 5 and RhC 5 P frameworks from which vinylallenes and phosphacumuleneylides are obtained.

Effect of wax on the adhesion strength between acrylic urethane film and chromated steel sheet, and the friction coefficient of the film

The effect of wax in an acrylic urethane (AU) film on the adhesion strength between AU film and steel sheet treated with Zn-Ni and chromium was investigated by varying the amount of wax using a pull-off test. The adhesion strength decreased as the amount of wax increased. Since the wax in the AU film can react with the reactive functional groups of imine ester (hardener), the cohesive strength of the AU film can be reduced. The distribution of wax in the AU film on a polar and a nonpolar substrate, whose contact angles with water were 30° and 75°, respectively, was examined using contact angle measurements and X-ray photoelectron spectroscopy (XPS). The distribution of wax in the AU film was greatly affected by the degree of polarity of the substrate. In the case of the nonpolar substrate, the wax in the AU film migrated towards the film/nonpolar substrate interface to form a weak boundary layer, where the failure occurred. In the case of the polar substrate, the amount of wax at the film/polar substrate interface was much smaller than that at the film/nonpolar substrate interface and increased linearly with increasing wax content in the AU film. The friction coefficient of the AU film against chromated steel sheet was also measured with varying amounts of imine-ester and additives such as wax and colloidal silica. The friction coefficient appears to be influenced only by the amount of wax.

Lipase-catalysed enantioselective hydrolysis of bicyclo[3.2.0]heptanol esters in supercritical carbon dioxide

The esters derived from 2- exo-bromo-3- endo-hydroxybicyclo[3.2.0]heptan-6-one were treated with Lipolase under hydrolytic conditions in supercritical carbon dioxide (SCCO 2). All the esters tested gave access to optically active material (e.e. 50–90%). The esters with an additional keto or hydroxy group were hydrolysed to 35–40% conversion while nonpolar lipophilic substrates gave poor conversions (3–5%) under the same conditions.

Effect of glutathione, glutathione sulphonate and S-hexylglutathione on the conformational stability of class pi glutathione S-transferase

The glutathione S-transferases (GST) are a supergene family of phase II detoxification enzymes which catalyse the S-conjugation between glutathione and an electrophilic substrate. The active site can be divided into two adjacent functional regions, a highly specific G-site for binding the physiological substrate glutathione and a nonspecific H-site for binding nonpolar electrophilic substrates. Equilibrium and kinetic unfolding experiments employing tryptophan fluorescence and enzyme activity measurements were performed to study the effect of ligand binding to the G-site on the unfolding and stability of the porcine class pi glutathione S-transferase against urea. The presence of glutathione caused a shift in the equilibrium-unfolding curves towards lower urea concentrations and enhanced the first-order rate constant for unfolding suggesting a destabilisation of the pGSTP1-1 structure against urea. The presence of either glutathione sulphonate or S-hexylglutathione, however, produced the opposite effect in that their binding to the G-site appeared to exert a stabilising effect against urea. The binding of these glutathione analogues also reduced significantly the degree of cooperativity of unfolding indicating a possible change in the protein's unfolding pathway.