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PubMed1994

Structure and function of the catalytic site mutant Asp 99 Asn of phospholipase A2: absence of the conserved structural water.

To probe the role of the Asp-99 ... His-48 pair in phospholipase A2 (PLA2) catalysis, the X-ray structure and kinetic characterization of the mutant Asp-99-->Asn-99 (D99N) of bovine pancreatic PLA2 was undertaken. Crystals of D99N belong to the trigonal space group P3(1)21 and were isomorphous to the wild type (WT) (Noel JP et al., 1991, Biochemistry 30:11801-11811). The 1.9-A X-ray structure of the mutant showed that the carbonyl group of Asn-99 side chain is hydrogen bonded to His-48 in the same way as that of Asp-99 in the WT, thus retaining the tautomeric form of His-48 and the function of the enzyme. The NH2 group of Asn-99 points away from His-48. In contrast, in the D102N mutant of the protease enzyme trypsin, the NH2 group of Asn-102 is hydrogen bonded to His-57 resulting in the inactive tautomeric form and hence the loss of enzymatic activity. Although the geometry of the catalytic triad in the PLA2 mutant remains the same as in the WT, we were surprised that the conserved structural water, linking the catalytic site with the ammonium group of Ala-1 of the interfacial site, was ejected by the proximity of the NH2 group of Asn-99. The NH2 group now forms a direct hydrogen bond with the carbonyl group of Ala-1.

Animals↗

PubMed1996

Influence of the carrier on the intrinsic rate of dissolution of diazepam in interactive mixtures.

The use of interactive mixtures of drugs adhering to the surface of carriers can promote drug dissolution. The mechanism of dissolution of such mixtures has been studied using the rotating-disc method under conditions eliminating secondary influences such as carrier surface characteristics and drug particle aggregation. Levich plots were used to characterize the dissolution behaviour. Diazepam-compactrol interactive mixtures had initial dissolution rates similar to that of pure diazepam owing to the deposition of a continuous layer of diazepam on the disc surface from the interactive mixture. Linear Levich plots were produced at all drug loadings and the presence of compactrol in the disc slightly enhanced dissolution rates. Dissolution rates for diazepam-emcompress interactive mixtures were lower than those of pure diazepam. The Levich plots for these systems were non-linear with increasing negative curvature as the diazepam loading decreased. The rate of dissolution of diazepam in the lactose interactive mixture was markedly higher than that of pure diazepam, but high diazepam loadings in the lactose mixtures inhibited diazepam dissolution. Rapid carrier dissolution caused surface retraction of the disc, enhancing the dissolution rate. The Levich plots showed an upward curvature due to turbulence. Linear Levich plots for diazepam and other benzodiazepines and for diazepam-compactrol interactive mixtures showed that their dissolution in pH 5 phosphate buffer was diffusion-controlled. The Levich plots for diazepam-emcompress interactive mixtures were indicative of some interfacial control during dissolution, but the hypothesis of common ion precipitation of dissolved carrier, calcium phosphate, onto the disc surface did not fully explain this effect.

Diazepam↗

PubMed1999

Conformation of a Cdc42/Rac interactive binding peptide in complex with Cdc42 and analysis of the binding interface.

Most of the putative effectors for the Rho-family small GTPases Cdc42 and Rac share a common sequence motif referred to as the Cdc42/Rac interactive binding (CRIB) motif. This sequence, with a consensus of I-S-x-P-(x)2-4-F-x-H-x-x-H-V-G [Burbelo, P. D., et al. (1995) J. Biol. Chem. 270, 29071-29074], has been shown to be essential for the functional interactions between these effector proteins and Cdc42. We have characterized the interactions of a 22-residue CRIB peptide derived from human PAK2 [PAK2(71-92)] with Cdc42 using proton and heteronuclear NMR spectroscopy. This CRIB peptide binds to GTP-gammaS-loaded Cdc42 in a saturable manner, with an apparent Kd of 0.6 microM, as determined by fluorescence titration using sNBD-labeled Cdc42. Interaction of the 22-residue peptide PAK2(71-92) with GTP-gammaS-loaded Cdc42 causes resonance perturbations in the 1H-15N HSQC spectrum of Cdc42 that are similar to those observed for a longer (46-amino acid) CRIB-containing protein fragment [Guo, W., et al. (1998) Biochemistry 37, 14030-14037]. Proton NMR studies of PAK2(71-92) demonstrate structuring of PAK2(71-92) in the presence of GTP-gammaS-loaded Cdc42, through the observation of many nonsequential transferred NOEs. Structure calculations based on the observed transferred NOEs show that the central portion of the Cdc42-bound CRIB peptide assumes a loop conformation in which the side chains of consensus residues Phe80, His82, Ile84, His85, and Val86 are brought into proximity. The CRIB motif may therefore represent a minimal interfacial region in the complexes between Cdc42 and its effector proteins.

Amino Acid Sequence↗

PubMed1997

Wetting of a High-Energy Fiber Surface

The measurement of the equilibrium contact angle of a small droplet of fluid partially wetting a flat solid surface provides information on the solid-liquid interfacial energy. However, if the spreading power, S = gammaSV - (gammaSL + gammaLV), of the surface is positive the liquid spreads completely, no equilibrium contact angle exists, and the resulting thin film has an ultimate thickness determined by Van der Waal's forces. On a chemically identical solid surface with only the geometry changed to a cylinder the same droplet of fluid which completely wets the flat surface can provide an equilibrium conformation. The indefinite spreading tendency is inhibited and the equilibrium is not necessarily a thin sheathing film about the fiber, but can have a macroscopic profile. On a high energy cylindrical surface a barrelling type droplet is only approximately spherical in cross section. Near the three phase contact line the curvature can change sign and measurement of the contact angle becomes difficult. In this work we consider the theoretical profile for such droplets and calculate the extent to which decreasing the fiber radius changes the surface energy and the maximum slope of the profile. We suggest that measurements of the inflection angle in addition to the reduced thickness and reduced length of the droplet provide an improved means of characterizing droplet on fiber systems. Experiments are reported showing the changes in contact length, droplet height, and inflection angle for poly(dimethyl)siloxane oils on copper cylinders of different diameters. These cylinders are produced from the same initial copper wire by etching in sodium hydroxide to produce controlled diameters ranging from 0.07 to 0.49 mm. As the curvature increases with reducing diameter the influence of gravity diminishes and the shape increasingly conforms to a symmetric barreling droplet type. Furthermore, as the reduced volume of fluid increases the inflection angle increases from 7° to 30° while the contact angle remains at 0°. Consistency between measured values of equilibrium parameters are compared to the theoretical values which we compute numerically and the suggested radius and volume dependence of the inflection angle is confirmed.

Journal Article↗

PubMed1985

Miscibility, chain packing, and hydration of 1-palmitoyl-2-oleoyl phosphatidylcholine and other lipids in surface phases.

The miscibility of 1-palmitoyl-2-oleoyl phosphatidylcholine with triolein, 1,2-diolein, 1,3-diolein, 1(3)-monoolein, oleyl alcohol, methyl oleate, oleic acid, and oleyl cyanide (18:1 lipids) was studied at the argon-water interface. The isothermal phase diagrams for the mixtures at 24 degrees were characterized by two compositional regions. At the limit of miscibility with lower mol fractions of 18:1 lipid, the surface pressure was composition-independent, but above a mixture-specific stoichiometry, surface pressure at the limit of miscibility was composition-dependent. From the two-dimensional phase rule, it was determined that at low mol fractions of 18:1 lipids, the surface consisted of phospholipid and a preferred packing array or complex of phospholipid and 18:1 lipid, whereas, above the stoichiometry of the complex, the surface phase consisted of complex and excess 18:1 lipids. In both regions of the phase diagram, mixing along the phase boundary was apparently ideal allowing application of an equation of state described earlier (J. M. Smaby and H. L. Brockman, 1984, Biochemistry, 23:3312-3316). From such analysis, apparent partial molecular areas and hydrations for phospholipid, complex, and 18:1 lipid were obtained. Comparison of these calculated parameters for the complexed and uncomplexed states shows that the aliphatic moieties behave independently of polar head group. The transition of each 18:1 chain to the complexed state involves the loss of about one interfacial water molecule and its corresponding area. For 18:1 lipids with more than one chain another two water molecules per additional chain are present in both states but contribute little to molecular area. In contrast to 18:1 lipids, the phospholipid area and hydration change little upon complexation. The uniformity of chain packing and hydration behavior among 18:1 lipid species contrasts with complex stoichiometries that vary from 0.04 to 0.65. This suggests that the stoichiometry of the preferred packing array is determined by interactions involving the more polar moieties of the 18:1 lipids and the phospholipid.

Glycerides↗

PubMed2005

Adsorption and interaction of fibronectin and human serum albumin at the liquid-liquid interface.

The goal of this work was to investigate the dynamics of human plasma fibronectin (HFN) at the oil-water interface and to characterize its interactions with human serum albumin (HSA) by total internal reflection fluorescence microscopy (TIRFM). Among key results, we observed that fibronectin adsorption at the oil-water interface is rapid and essentially irreversible, even over short time scales. This may be due to the highly flexible nature of the protein, which allows its various domains to quickly attain energetically favorable conformations. On the other hand, HSA adsorption at the oil-water interface is relatively reversible at short times, and the protein is readily displaced by fibronectin even after HSA has been adsorbed at the interface for as long as 2 h. At longer adsorption times, HSA is able to more effectively resist complete displacement by fibronectin, although we observed significant fibronectin adsorption even under those conditions. Displacement of adsorbed fibronectin by HSA was negligible under all conditions. Fibronectin also adsorbs preferentially from a mixture of HFN and HSA, even when the concentration of HSA is substantially higher. This study is relevant to such emerging research thrusts as the development of biomimetic interfaces for a variety of applications, where there is a clear need for better understanding of the effects of interfacial competition, adsorption time scales, and extent of adsorption irreversibility on interfacial dynamics.

Adsorption↗

PubMed2005

Interaction preferences across protein-protein interfaces of obligatory and non-obligatory components are different.

BACKGROUND: A polypeptide chain of a protein-protein complex is said to be obligatory if it is bound to another chain throughout its functional lifetime. Such a chain might not adopt the native fold in the unbound form. A non-obligatory polypeptide chain associates with another chain and dissociates upon molecular stimulus. Although conformational changes at the interaction interface are expected, the overall 3-D structure of the non-obligatory chain is unaltered. The present study focuses on protein-protein complexes to understand further the differences between obligatory and non-obligatory interfaces. RESULTS: A non-obligatory chain in a complex of known 3-D structure is recognized by its stable existence with same fold in the bound and unbound forms. On the contrary, an obligatory chain is detected by its existence only in the bound form with no evidence for the native-like fold of the chain in the unbound form. Various interfacial properties of a large number of complexes of known 3-D structures thus classified are comparatively analyzed with an aim to identify structural descriptors that distinguish these two types of interfaces. We report that the interaction patterns across the interfaces of obligatory and non-obligatory components are different and contacts made by obligatory chains are predominantly non-polar. The obligatory chains have a higher number of contacts per interface (20 +/- 14 contacts per interface) than non-obligatory chains (13 +/- 6 contacts per interface). The involvement of main chain atoms is higher in the case of obligatory chains (16.9 %) compared to non-obligatory chains (11.2 %). The beta-sheet formation across the subunits is observed only among obligatory protein chains in the dataset. Apart from these, other features like residue preferences and interface area produce marginal differences and they may be considered collectively while distinguishing the two types of interfaces. CONCLUSION: These results can be useful in distinguishing the two types of interfaces observed in structures determined in large-scale in the structural genomics initiatives, especially for those multi-component protein assemblies for which the biochemical characterization is incomplete.

Animals↗

PubMed2006

Oscillating bubble SHG on surface elastic and surface viscoelastic systems: new insights in the dynamics of adsorption layers.

Surface rheology governs a great variety of interfacial phenomena such as foams or emulsions and plays a dominant role in several technological processes such as high-speed coating. Its major difference with bulk rheology resides in the high compressibility of the surface phase, which is the direct consequence of the molecular exchange between adsorbed and dissolved species. In analogy to bulk rheology, a complex surface dilational modulus, epsilon, which captures surface tension changes upon defined area changes of the surface layer, can be defined. The module epsilon is complex, and the molecular interpretation of the dissipative process that gives rise to the imaginary part of the module is subject to some controversy. In this contribution, we used the oscillating bubble technique to study the surface dilational modulus in the mid-frequency range. The dynamic state of the surface layer was monitored by a pressure sensor and by surface second-harmonic generation (SHG). The pressure sensor measures the real and imaginary part of the modulus while SHG monitors independently the surface composition under dynamic conditions. The experiment allows the assessment of the contribution of the compositional term to the surface dilational modulus epsilon. Two aqueous surfactant solutions have been characterized: a surface elastic and a surface viscoelastic solution. The elastic surface layer can be described within the framework of the extended Lucassen-van den Tempel Hansen model. The change in surface concentration is in phase with the relative area change of the surface layer, which is in strong contrast with the results obtained from the surface viscoelastic solution. Here, surface tension, area change, and surface composition are phase-shifted, providing evidence for a nonequilibrium state within the surface phase. The data are used to assess existing surface rheology models.

Journal Article↗

PubMed2003

Characterization of gas-liquid mass transfer phenomena in microtiter plates.

Gas-liquid mass transfer properties of shaken 96-well microtiter plates were characterized using a recently described method. The maximum oxygen transfer capacity (OTR(max)), the specific mass transfer area (a), and the mass transfer coefficient (k(L)) in a single well were determined at different shaking intensities (different shaking frequencies and shaking diameters at constant filling volume) and different filling volumes by means of sulfite oxidation as a chemical model system. The shape (round and square cross-sections) and the size (up to 2 mL maximum filling volume) of a microtiter plate well were also considered as influencing parameters. To get an indication of the hydrodynamic behavior of the liquid phase in a well, images were taken during shaking and the liquid height derived as a characteristic parameter. The investigations revealed that the OTR(max) is predominantly dependent on the specific mass transfer area (a) for the considered conditions in round-shaped wells. The mass transfer coefficient (k(L)) in round-shaped wells remains at a nearly constant value of about 0.2 m/h for all shaking intensities, thus within the range reported in the literature for surface-aerated bioreactors. The OTR(max) in round-shaped wells is strongly influenced by the interfacial tension, determined by the surface tension of the medium used and the surface properties of the well material. Up to a specific shaking intensity the liquid surface in the wells remains horizontal and no liquid movement can be observed. This critical shaking intensity must be exceeded to overcome the surface tension and, thus, to increase the liquid height and enlarge the specific mass transfer area. This behavior is solely specific to microtiter plates and has not yet been observed for larger shaking bioreactors such as shaking flasks. In square-shaped microtiter plate wells the corners act as baffles and cause a significant increase of OTR(max), a, and k(L). An OTR(max) of up to 0.15 mol/L/h can be reached in square-shaped wells.

Bacteriological Techniques↗

PubMed2003

Characterization of the membrane binding mode of the C2 domain of PKC epsilon.

PKCepsilon is a member of the group of novel PKCs that contain a C2 domain located in their N-terminal region. On the basis of recent structural studies, a series of mutants were prepared to increase our knowledge of the mechanism of the phospholipid binding site of this domain. The results revealed that this domain preferentially binds to phosphatidic acid- and phosphatidylserine-containing vesicles. Although the increase in affinity was linear in the case of phosphatidic acid, it became exponential when the vesicles contained increasing concentrations of phosphatidylserine. Site-directed mutagenesis studies showed that residues W23, R26, and R32 located in loop 1 and I89 and Y91 located in loop 3 are of critical importance when the binding is performed with phosphatidic acid-containing vesicles. Furthermore, when the same mutants were assayed with phosphatidylserine-containing vesicles, no binding was observed in any case, reflecting the smaller affinity of the C2 domain for phosphatidylserine-containing vesicles. A study of the ionic nature of the membrane interaction suggested that it is mainly driven by electrostatic interactions that are disrupted by very low salt concentrations. Differential scanning calorimetry experiments performed to ascertain whether this interaction affected the transition phase of the phosphatidic acid demonstrated that increasing concentrations of the protein lead to changes in the transition, with more than one peak appearing at lower temperatures, which suggests a weak interaction focused on the polar headgroup of the phospholids. In conclusion, a different membrane-binding mode from those previously described in other C2 domains has been found and is seemingly based on electrostatic, interfacial, and hydrophobic interactions without the participation of Ca(2+) ions.

Animals↗

PubMed1991

Structure and dynamics of dimyristoylphosphatidic acid/calcium complexes by 2H NMR, infrared, spectroscopies and small-angle x-ray diffraction.

The structural and dynamic properties of complexes of dimyristoylphosphatidic acid (DMPA) and calcium ions have been characterized by 2H NMR, Raman, and infrared spectroscopies and small-angle X-ray diffraction. All techniques used show that these complexes do not undergo a cooperative thermotropic phase transition. Small-angle X-ray diffraction unambiguously demonstrates that the structure of the lipid molecules of the DMPA/Ca2+ complexes remains lamellar even at a temperature as high as 85 degrees C. Raman results indicate that within this temperature range, only a few trans-gauche isomerizations of the C-C bonds of the phospholipid acyl chains arise in this system. The 2H NMR spectra indicate that the DMPA chains are highly motionally restricted up to 65 degrees C and that higher temperatures might activate some low-frequency overall motions of entire lamellar domains. Small-angle X-ray scattering and 2H NMR spectroscopy of 2H2O also show that the interaction of calcium with DMPA promotes an important dehydration of the lipid assembly, even though the latter technique clearly demonstrates that some water molecules remain strongly bond in the DMPA/Ca2+ complexes. The carbonyl stretching mode region of the infrared spectrum of DMPA/Ca2+ complexes suggests that these water molecules are trapped near the interfacial region of the lipid membrane and are hydrogen bonded with the carbonyl groups of the lipid. Finally, comparison of the phosphate stretching mode region of the infrared spectra of complexes of DMPA with calcium ions with those of model compounds provides strong evidence that calcium ions bind to both charges of the phosphate group of DMPA and form bridges between adjacent bilayers.

Glycerophospholipids↗

PubMed1984

Inhibitors of lipase activities in soybean and other oil seeds.

In the cotyledon extracts of seedlings of many oil seeds, including soybean, sunflower, cucumber, and peanut, the in vitro lipase activity was too low to account for the observed in vivo lipolysis. The low in vitro lipase activity was due to the presence of lipase inhibitors in the extracts. The inhibitors from soybean were characterized based on their effects on the hydrolysis of trilinolein by corn, pancreatic, and Rhizopus lipases. The inhibitors were not dialyzable and unaltered by RNase and beta-galactosidase treatment. However, they were sensitive to heating and protease digestion. The inhibitory effect of the inhibitors was expressed irrespective of the sequence of the addition of lipase, substrate, and inhibitors to the assay medium. The inhibitory effect was equally expressed when the inhibitors were added either before or after the lipase reaction had been in progress. The inhibitory effect of the inhibitors was independent of the amount of lipase present in the assay, but was dependent on the amount of substrate added. High substrate concentration eliminated totally the inhibitory effect of the inhibitors. Most of the inhibitors were recovered in the soluble fraction in subcellular fractionation. They were present in the 2-4S and not in the 7S, and 11S (storage proteins) protein fraction. There was a gradual decrease of the inhibitors in the cotyledons in the postgerminative growth. We suggest that the inhibitors are proteins which bind to the surface of the substrate micelles. The binding prevents the normal functioning of lipase which acts on the interfacial area between the aqueous medium and the micelle surface.

Journal Article↗

PubMed2005

Optical monitoring of bubble size and shape in a pulsating bubble surfactometer.

The pulsating bubble surfactometer (PBS) is often used for in vitro characterization of exogenous lung surfactant replacements and lung surfactant components. However, the commercially available PBS is not able to dynamically track bubble size and shape. The PBS therefore does not account for bubble growth or elliptical bubble shape that frequently occur during device use. More importantly, the oscillatory volume changes of the pulsating bubble are different than those assumed by the software of the commercial unit. This leads to errors in both surface area and surface tension measurements. We have modified a commercial PBS through the addition of an image-acquisition system, allowing real-time determination of bubble size and shape and hence the accurate tracking of surface area and surface tension. Compression-expansion loops obtained with the commercially available PBS software were compared with those provided by the image-analysis system for dipalmitoylphosphatidylcholine, Infasurf, and Tanaka lipids (dipalmitoylphosphatidylcholine-palmitoyloleoylphosphatidyl-glycerol-palmitic acid, 68:22:9) at concentrations of 0.1 and 1.0 mg/ml and at frequencies of 1 and 20 cycles/min. Whereas minimum surface tension as determined by the image-analysis system is similar to that measured by the commercially available software, the maximum surface tension and the shapes of the interfacial area-surface tension loops are quite different. Differences are attributable to bubble drift, nonsinusoidal volume changes, and variable volume excursions seen with the modified system but neglected by the original system. Image analysis reveals that the extent of loop hysteresis is greatly overestimated by the commercial device and that an apparent, rapid increase in surface tension upon film expansion seen in PBS loops is not observed with the image-analysis system. The modified PBS system reveals new dynamic characteristics of lung surfactant preparations that have not previously been reported.

Equipment Design↗

PubMed1997

Biochemical and biophysical characterization of human recombinant lecithin: cholesterol acyltransferase.

We established a Chinese hamster ovary cell line that constitutively expresses up to 5 mg/L of human recombinant lecithin: cholesterol acyltransferase (rLCAT). We purified the rLCAT to > 96% purity, and characterized it along with plasma LCAT (pLCAT) biochemically and biophysically. The recombinant enzyme is more heavily glycosylated and more heterogeneous in its carbohydrate content than the plasma enzyme, as revealed by differences in molecular weight and pI isoforms, determined by mass spectrometry and isoelectric focusing. Recombinant LCAT is half as active enzymatically as pLCAT. The difference in activity is due to differences in the catalytic rates rather than in the apparent K(m) values, suggesting that the binding of the rLCAT to interfaces is not altered by its different glycosylation pattern. Despite these differences, rLCAT has essentially the same intrinsic tryptophan fluorescence emission spectrum and far-UV CD spectrum as pLCAT, indicating that the tertiary and secondary structures of both enzyme forms are very similar. Both enzyme forms have a propensity to self-associate, and their multimers appear resistant to dissociation by SDS and dilution. The free energies of unfolding (delta G(H2O)) of rLCAT and pLCAT are 3.4 +/- 0.2 and 3.2 +/- 0.2 kcal/mol, respectively, as determined by guanidine hydrochloride denaturation monitored by fluorescence. These relatively low delta G(H2O) values support the notion that LCAT is capable of undergoing major conformational changes upon interaction with interfacial substrates.

Animals↗

PubMed2006

Retardation of crystallization-induced destabilization of PMF-in-water emulsion with emulsifier additives.

An oil-in-water (O/W) emulsion, in which the oil phase is semi-solid fat, is easily destabilized when stored below the crystallization temperature of the oil phase. Such destabilization, characterized by loss of fluidity at chilled temperature and oil-water separation after re-heating, is caused by inter-droplet bridging of fat crystals protruding out of the emulsion droplets. In the present study, we found that the simultaneous use of additives of highly hydrophobic sucrose oligoester (SOE; P-170) and highly hydrophilic SOE (P-1670) containing palmitic acid moiety remarkably retarded the crystallization-induced destabilization of the O/W emulsion that contains palm-mid-fraction (PMF) as the oil phase. Without the additives, destabilization occurred when the emulsion was cooled from 60 to 0 degrees C and kept at 0 degrees C for 1 day. Microscopic observation revealed that destabilization was caused by coalescence of the oil droplets, which was triggered by the growth of needle-shaped PMF crystals protruding out of the emulsion membranes. However, the addition of P-170 to PMF increased the crystallization temperature of PMF and at the same time retarded the destabilization. Furthermore, the simultaneous addition of P-170 and P-1670 retarded the crystallization-induced destabilization even more. Optical observation, DSC, and synchrotron radiation X-ray diffraction measurements indicated that the P-170 additive enhanced interfacial heterogeneous crystallization to form tiny PMF crystals in the droplets, and that the P-1670 additive retarded morphological change of the PMF crystals into long needle shapes in association with polymorphic transformation from alpha to beta'.

Crystallization↗

PubMed2005

Structure of the liquid-vapor interface of water-methanol mixtures as seen from Monte Carlo simulations.

Monte Carlo simulation of the vapor-liquid interface of water-methanol mixtures of different compositions, ranging from pure water to pure methanol, have been performed on the canonical (N, V, T) ensemble at 298 K. The analysis of the systems simulated has revealed that the interface is characterized by a double layer structure: methanol is strongly adsorbed at the vapor side of the interface, whereas this adsorption layer is followed at its liquid side by a depletion layer of methanol of lower concentration than in the bulk liquid phase of the system. The dominant feature of the interface has been found to be the adsorption layer in systems of methanol mole fractions below 0.2, and the depletion layer in systems of methanol mole fractions between 0.25 and 0.5. The orientation of the molecules located at the depletion layer is found to be already uncorrelated with the interface, whereas the methanol molecules of the adsorption layer prefer to align perpendicular to the interface, pointing straight toward the vapor phase by their methyl group. Although both the preference of the molecular plane for a perpendicular alignment with the interface and the preference of the methyl group for pointing straight to the vapor phase are found to be rather weak, the preference of the methyl group for pointing as straight toward the vapor phase as possible within the constraint imposed by the orientation of the molecular plane is found to be fairly strong. One of the two preferred orientations of the interfacial water molecules present in the neat system is found to disappear in the presence of methanol, because methanol molecules aligned in their preferred orientation can replace these water molecules in the hydrogen-bonding pattern of the interface.

Journal Article↗

PubMed2004

A microfluidics approach to the problem of creating separate solution environments accessible from macroscopic volumes.

We report on a microfluidic device that generates separate solution environments in macroscopic volumes. Spatially distinct patterns are created by emitting fluids from 16 different sources (closely spaced microchannels) into a solution-filled macroscopic chamber. The fluid in neighboring microchannels couples viscously in the macroscopic container, generating one single interdigitated stream. Scanning nanoelectrode amperometry was used for characterizing the concentration landscape and the diffusion zones between solutions running in parallel at different coordinates in the stream. These experiments were complemented by finite element simulations of the Navier-Stokes and mass transport equations to describe the velocity distributions and the diffusion behavior. For in channel flow velocities of 50 mm.s(-1), patterns could persist on the order of millimeters to centimeters in the open volume. The most narrow diffusion zones with widths less than 10 microm (5-95% concentration change) were found some tens of micrometers out in the macroscopic container. We demonstrate that a 14-microm-diameter nearly spherical object (biological cell) attached to a micropipet can be moved from one solution environment to another by a lateral displacement of only 8 microm. The device is suitable for applications where the solution environment around a microscopic or nanoscopic sensor needs to be changed multiple times, i.e., in order to build layered structures, for obtaining binding isotherms, and kinetic information, for example, on ion channels, enzymes, and receptors as well as in applications where different loci on an object need to be exposed to different environments or where complex solution environments need to be created for studies of interfacial chemistry between two streaming layers.

Journal Article↗

PubMed2005

Effects of simultaneously doped and deposited Ag on the photocatalytic activity and surface states of TiO2.

Ag-TiO2 catalysts with different Ag contents were prepared via a sol-gel method in the absence of light. Based on the characterizations of XRD, photoluminescence (PL), surface photovoltage spectroscopy (SPS), field-induced surface photovoltage spectroscopy (FISPS), and XPS as well as the evaluation of the photocatalytic activity for degrading rhodamine B(RhB) solutions, it was found that the Ag dopant promoted the phase transformation as well as had an inhibition effect on the growth of anatase crystallite. The PL and SPS intensities were decreased with increasing Ag content, indicating that the Ag dopant could effectively inhibit the recombination of the photoinduced electrons and holes. However, the active sites capturing the photoinduced electrons reduced, while the Ag content exceeded 5 mol %. At rather low Ag dopant concentrations, the migration and diffusion of Ag+ ions were predominant, while at rather high Ag dopant concentrations, the migration, diffusion, and reduction of Ag ions simultaneously occurred. The Ag-TiO2 photocatalysts with appropriate content of Ag (Ag species concentration is from about 3 to 5 mol %) possessed abundant electron traps so as to be favorable for the separation of the photoinduced electron-hole pairs, which could greatly enhance the activity of the photocatalysts. From the results of FISPS measurements, it could be found that the impurity bands and abundant surface states were introduced into the interfacial layer of TiO2 because of Ag simultaneously doping and depositing, which could improve the absorption capability for visible light of the photocatalysts.

Journal Article↗