Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “interfacial characterization”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 667 records · Page 37Linked to original sources
PubMed2005

Interfacial Langmuir-Blodgett assembly of straight and parallel aligned nanoribbons.

The fabrication of one-dimensional nanomaterials is currently an important issue in nanoscience and nanotechnology. Herein, we report the construction of long, straight, and parallel aligned nanoribbons, by the in situ assembly of a novel, triangular amphiphilic molecule at an air-water interface. The 1,3,5-trimesyl-tri-L-glutamic acid hexaester, which has an aromatic core with three hydrophilic arms bearing three amide units and ending with six ester groups, was spread on a water surface, forming a stable, ultrathin film with a plateau region in its isotherm. When the film was compressed to a surface pressures, which exceeded the plateau region, nanoribbons were formed. Atomic force microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, transmission electron microscopy, electron diffraction, and circular dichroism were used to characterize the film and the nanoribbons. The nanoribbons were formed due to the in situ transformation of a two-dimensional monolayer into a three-dimensional nanoarchitecture at the air-water interface. When the compound was spread onto the water surface, it first formed a monolayer. Compressing the film led to the overlap of the second layer on the monomolecular sublayer. This second layer formed nanoribbons in which the triangular molecules stood vertically (using two corners) on the sublayer. The nanoribbons were stabilized by the ordered packing of the triangular molecules, which were held together by intermolecular hydrogen bonding. The results present a facile procedure for constructing three-dimensional, regular nanoarchitectures in a two-dimensional environment.

Magnetic Resonance Spectroscopy↗
PubMed2001

Dynamic behavior of lung surfactant.

We have previously developed an adsorption-limited model to describe the exchange of lung surfactant and its fractions to and from an air-liquid interface in oscillatory surfactometers. Here we extend this model to allow for diffusion in the liquid phase. Use of the model in conjunction with experimental data in the literature shows that diffusion-limited transport i.s important for characterizing the transient period from the start of oscillations to the achievement of steady-state conditions. Matching previous data shows that upon high levels of film compression, large changes occur in adsorption rate, desorption rate, and diffusion constant, consistent with what one might expect if the subsurface region was greatly enriched in DPPC. Collapse of the surfactant film that occurs during compression leads to a .significant elevation of surfactant concentration immediately heneath the interface, consistent with the subsurface depot of surfactant that has heen postulated by other investigators. Modeling studies also uncovered a phenomenon of surfactant behavior in which the interfacial tension remains constant at its minimum equilibrium value while the film is compressed, hut without collapse of the film. The phenomenon was due to desorption of surfactant from the interface and termed "pseudo-film collapse.'' The new model also gave improved agreement with steady-state oscillatory cycling in a pulsating bubble surfactometer.

Adsorption↗
PubMed1981

Binding of human pancreatic carboxylic ester hydrolase to lipid interfaces.

Human pancreatic carboxylic ester hydrolase (EC 3.1.1.1), usually characterized by its activity on water-soluble substrates, is shown to catalyze reactions taking place at a lipid/water interface. The inhibition of tributyrin hydrolysis by 1-alcohols follows the pattern of a Langmuir adsorption isotherm. Experiments performed with siliconized glass beads show that human pancreatic carboxylic ester hydrolase is adsorbed on this artificial (or substitute) interface with a dissociation constant for the enzyme-glass beads complex equal to 1.5 . 10(-8) M. The average molecular area at saturation is 4375 A2. Tripropionin hydrolysis is increased by the presence of glass beads. These results are strong arguments in favor of the interfacial activity of pancreatic carboxylic ester hydrolase. The activation of the enzyme bound to the interface is very weak. Bile salts do not prevent the adsorption of carboxylic ester hydrolase on siliconized glass beads and increase strongly the hydrolysis rate of emulsified tributyrin.

Alcohols↗
PubMed2003

Formation of intermolecular beta-sheet structures: a phenomenon relevant to protein film structure at oil-water interfaces of emulsions.

Oil-in-water emulsions stabilized with beta-lactoglobulin (beta-lg) were made using a homogenizer or a high-speed blender. The protein was studied by Fourier transform infrared (FTIR) spectroscopy in the raw emulsion, in the bulk phase, and at the interface, as a function of pH, oil content, and homogenizing pressure. Results show that the amount of adsorbed protein varies with the available interfacial area. The protein that remains in the aqueous phase exhibit no spectral change, which suggests that homogenization causes no conformational modification or reversible ones. Strong and irreversible changes were observed in the adsorbed protein. Our findings reveal the formation of intermolecular antiparallel beta-sheets upon adsorption due to the protein self-aggregation. As deduced from transmission electronic microscopy, this surface aggregation leads to the formation of continuous and homogeneous membranes coating the globules. The structure of the adsorbed proteins is unaffected by the homogenizing pressures used in our study and slightly modified by the pH. FTIR spectroscopy allows to characterize the type of aggregates formed at the interface. An analysis of the spectra of beta-lg heat-induced gels shows that the aggregates at the interface are very close at a molecular scale to those that constitute particulate gels near the protein's isoelectric point. Since the type of aggregates is similar when the emulsion water phase is pure D(2)O and D(2)O at pD 4.4, the interface not only seems to induce aggregation, but seems to determine the type of aggregation as well. The mechanism that drives the formation of particulate aggregates (rather than fine-stranded ones) may reside in strong protein-protein interactions that are promoted by adverse oil-protein interactions.

Adsorption↗
PubMed1997

A single amino acid substitution in the pleckstrin homology domain of phospholipase C delta1 enhances the rate of substrate hydrolysis.

The pleckstrin homology (PH) domain has been postulated to serve as an anchor for enzymes that operate at a lipid/water interface. To understand further the relationship between the PH domain and enzyme activity, a phospholipase C (PLC) delta1/PH domain enhancement-of-activity mutant was generated. A lysine residue was substituted for glutamic acid in the PH domain of PLC delta1 at position 54 (E54K). Purified native and mutant enzymes were characterized using a phosphatidylinositol 4,5-bisphosphate (PI(4, 5)P2)/dodecyl maltoside mixed micelle assay and kinetics measured according to the dual phospholipid model of Dennis and co-workers (Hendrickson, H. S., and Dennis, E. A. (1984) J. Biol. Chem. 259, 5734-5739; Carmen, G. M., Deems, R. A., and Dennis, E. A. (1995) J. Biol. Chem. 270, 18711-18714). Our results show that both PLC delta1 and E54K bind phosphatidylinositol bisphosphate cooperatively (Hill coefficients, n = 2.2 +/- 0.2 and 2.0 +/- 0.1, respectively). However, E54K shows a dramatically increased rate of (PI(4, 5)P2)-stimulated PI(4,5)P2 hydrolysis (interfacial Vmax for PLC delta1 = 4.9 +/- 0.3 micromol/min/mg and for E54K = 31 +/- 3 micromol/min/mg) as well as PI hydrolysis (Vmax for PLC delta1 = 27 +/- 3.4 nmol/min/mg and for E54K = 95 +/- 12 nmol/min/mg). In the absence of PI(4,5)P2 both native and mutant enzyme hydrolyze PI at similar rates. E54K also has a higher affinity for micellar substrate (equilibrium dissociation constant, Ks = 85 +/- 36 microM for E54K and 210 +/- 48 microM for PLC delta1). Centrifugation binding assays using large unilamelar phospholipid vesicles confirm that E54K binds PI(4,5)P2 with higher affinity than native enzyme. E54K is more active even though the interfacial Michaelis constant (Km) for E54K (0.034 +/- 0.01 mol fraction PI(4,5)P2) is higher than the Km for native enzyme (0.012 +/- 0.002 mol fraction PI(4,5)P2). D-Inositol trisphosphate is less potent at inhibiting E54K PI(4,5)P2 hydrolysis compared with native enzyme. These results demonstrate that a single amino acid substitution in the PH domain of PLC delta1 can dramatically enhance enzyme activity. Additionally, the marked increase in Vmax for E54K argues for a direct role of PH domains in regulating catalysis by allosteric modulation of enzyme structure.

Blood Proteins↗
PubMed2001

The Electrical Double Layer at Hydrous Manganese Dioxide/Electrolyte Interface.

The interfacial properties of two hydrous oxides of manganese, namely an amorphous variety and a poorly crystalline synthetic birnessite, in different electrolyte media, have been studied by using a batch potentiometric titration procedure with a 72-h equilibration period. The data obtained by this procedure are more relevant to a natural water system than those obtained by the rapid titration method as reported earlier [J. Colloid Interface Sci. 131, 103 (1989).]. Although the points of zero charge do not show any major variations, the surface dissociation and complexation (with Na(+) in NaCl) constants differ with the method of titration. Both of the samples are characterized by their high surface charge (low DeltapK(a)) and surface potential, and as a result, complexation takes place possibly within the inner helmholtz plane. It is concluded that the triple-layer model is not applicable to this colloid system and that a basic Stern model would be more suitable in this case. Copyright 2001 Academic Press.

Journal Article↗
PubMed2004

Incorporation of beta-lactoglobulin in a lipid/porphyrin monolayer at the air--water interface.

A catanionic lipid/porphyrin monolayer was formed at the air-water interface by the tetra-anionic porphyrin, tetra-sodium-meso-tetra(4-sulfonatophenyl)porphine (TSPP), mixed with the cationic lipid dioctadecyldimethylammonium bromide (DODAB) in a 1:4 molar ratio. This binary mixture (TSPP/4DODAB) was used as the incorporation matrix of beta-lactoglobulin (betaLG). Binary and ternary systems (TSPP/4DODAB/zbetaLG, where z stands for the number of protein residues per TSPP) were characterized by surface pressure versus area (pi-A) measurements and by Brewster angle microscopy (BAM) observation at the air-water interface. Pi-A measurements and BAM images show that protein is incorporated in the expanded regime of the monolayer and is gradually expelled upon compression at high surface pressures. The successive compression-expansion cycles indicate that the protein under adsorbed to the floating film is reincorporated after the expansion of the monolayer. At low subphase pH, TSPP tends to aggregate decreasing the interaction with DODAB molecules. Electrostatic and hydrophobic interactions are responsible for the presence of betaLG at the interfacial film.

Animals↗
PubMed2005

Intrinsic structural and functional determinants within the amino acid sequence of mature pulmonary surfactant protein SP-B.

Pulmonary surfactant protein SP-B is absolutely required for proper function of surfactant in the alveoli, and is an important component of therapeutical surfactant preparations used to treat respiratory pathologies. To explore inherent structural and functional determinants within the amino acid sequence of mature SP-B, porcine SP-B has been subjected to extensive disulfide reduction under highly denaturing conditions and to cysteine carboxyamidomethylation, and the structure, lipid-protein interactions, and surface activity of this modified form have been characterized. Refolding of the reduced protein yielded a form (SP-Br) with secondary structure practically identical to that of the native disulfide-linked SP-B dimer. Reduced SP-Br exhibited higher structural flexibility than native SP-B, as indicated by a higher susceptibility of fluorescence emission to quenching by acrylamide and biphasic behavior during interaction of the protein with lipid bilayers and monolayers. SP-Br had, however, effects similar to those of native SP-B on the thermotropic properties of dipalmitoylphosphatidylcholine (DPPC) bilayers. SP-Br was more effective than native SP-B in promoting interfacial adsorption of phospholipid bilayers into interfacial films, presumably because of its higher structural flexibility, and retained the ability of native SP-B to stabilize DPPC interfacial films compressed to pressures near collapse against spontaneous relaxation. SP-Br also mimicked the behavior of native SP-B in lipid-protein films subjected to dynamic compression-expansion cycling in a captive bubble surfactometer, but only in the presence of phosphatidylglycerol (PG), the main anionic phospholipid in surfactant. The presence of PG appears to be required for SP-Br to acquire the appropriate tertiary folding to produce progressively more efficient lipid-protein films capable of reaching very high pressures upon limited compression with almost no hysteresis.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed2005

Remote and adjacent excited-state electron transfer at TiO2 interfaces sensitized to visible light with Ru(II) compounds.

The ruthenium polypyridyl compounds, Ru(dpp)2(deeb)(PF6)2 (Ru-deeb) and cis-Ru(dpp)2(eina)2(PF6)2 (Ru-eina), where dpp is 4,7-diphenyl-1,10-phenanthroline, deeb is 4,4'-diethyl ester-2,2'-bipyridine, and eina is 4-ethyl ester pyridine, have been prepared and characterized to sensitize nanocrystalline TiO2 (anatase) thin films. In neat acetonitrile at room temperature, Ru-deeb was emissive with lambdaem=675 nm, tau=780 ns, and emission quantum yield phiem=0.067, whereas Ru-eina was nonemissive with tau<10 ns. The short lifetime and observed photochemistry for Ru-eina are consistent with the presence of low-lying ligand-field (LF) excited states. The metal-to-ligand charge transfer (MLCT) excited states of Ru-deeb were found to be localized on the surface-bound deeb ligand, and on the remote dpp ligand for Ru-eina. Interfacial proton concentration was employed to tune the relative sensitizer-semiconductor energetics. Injection quantum yields, phiinj, varied from approximately 0.2 at pH=5 to approximately 1 at pH=1, with a slope of approximately 0.15/pH for both compounds. At pH=12, long-lived excited states were observed with phiinj<0.05. At pH<or=2, phiinj became temperature-dependent for Ru-eina, but not for Ru-deeb. A mechanism is proposed wherein population of LF states at elevated temperatures lowers phiinj.

Journal Article↗
PubMed2002

Phase separation rates of aqueous two-phase systems: correlation with system properties.

The kinetics of phase separation in aqueous two-phase systems have been investigated as a function of the physical properties of the system. Two distinct situations for the settling velocities were found, one in which the light, organic-rich (PEG) phase is continuous and the other in which the heavier, salt-rich (phosphate) phase is continuous. The settling rate of a particular system is a crucial parameter for equipment design, and it was studied as a function of measured viscosity and density of each of the phases as well as the interfacial tension between the phases. Interfacial tension increases with increasing tie line length. A correlation that describes the rate of phase separation was investigated. This correlation, which is a function of the system parameters mentioned above, described the behavior of the system successfully. Different values of the parameters in the correlation were fitted for bottom-phase-continuous and top-phase-continuous systems. These parameters showed that density and viscosity play a role in the rate of separation in both top continuous- and bottom continuous-phase regions but are more dominant in the continuous top-phase region. The composition of the two-phase system was characterized by the tie line length. The rate of separation increased with increasing tie line length in both cases but at a faster rate when the bottom (less viscous) phase was the continuous phase. These results show that working in a continuous bottom-phase region is advantageous to ensure fast separation.

Models, Chemical↗
PubMed2006

Benchmarking of dimeric threading and structure refinement.

The understanding of protein-protein interactions is a major goal in the postgenomic era. The prediction of interaction from sequence and the subsequent generation of full-length dimeric models is therefore of great interest especially because the number of structurally characterized protein-protein complexes is sparse. A quality assessment of a benchmark comprised of 170 weakly homologous dimeric target-template pairs is presented. They are predicted in a two-step method, similar to the previously described MULTIPROSPECTOR algorithm: each target sequence is assigned to a monomeric template structure by threading; then, those templates that belong to the same physically interacting dimer template are selected. Additionally we use structural alignments as the "gold standard" to assess the percentage of correctly assigned monomer and dimer templates and to evaluate the threading results with a focus on the quality of the alignments in the interfacial region. This work aims to give a quantitative picture of the quality of dimeric threading. Except for one, all monomer templates are identified correctly, but approximately 40% of the dimer templates are still problematic or incorrect. Preliminary results for three full-length dimeric models generated with the TASSER method show on average a significant improvement of the final model over the initial template.

Algorithms↗
PubMed1997

De novo design of native proteins: characterization of proteins intended to fold into antiparallel, rop-like, four-helix bundles.

The de novo design and characterization of a series of 51-residue helix-turn-helix peptides intended to dimerize into antiparallel four-stranded coiled coils is described. The sequence is based on a coiled coil heptad repeat Ncap-(Aa Zb Zc Ld Ze Zf Zg)3-turn- (Xa Zb Zc Ld Ze Zf Zg)3-Ccap-CONH2, where X is either Val or Ala. The overall topology was intended to be similar to that found in the Escherichia coli protein ROP. The design strategy included consideration of geometric complementarity of the packing of side chains within the hydrophobic core as well as the use of specific interfacial interactions, both of which were intended to favor the desired ROP-like topology. Additionally, the sequence was designed to destabilize potential alternative structures that might compete with the desired topology. The peptides (RLP-1, RLP-2, and RLP-3) assemble into stable alpha-helical dimers and exhibit the hallmarks of a native protein as judged by its spectroscopic properties, and the lack of binding to hydrophobic dyes. Also, the enthalpy and heat capacity changes upon denaturation were determined by measuring the temperature dependence of the CD spectra and confirmed by differential scanning calorimetry (DSC). The values determined by the two methods are in excellent agreement and are in the range of those of naturally occurring proteins of this size. These results suggest that it is now possible to design native-like helical proteins that should serve as templates for the further design of functional proteins.

Amino Acid Sequence↗
PubMed2005

Correlation between mechanical behavior of protein films at the air/water interface and intrinsic stability of protein molecules.

The relation between mechanical film properties of various adsorbed protein layers at the air/water interface and intrinsic stability of the corresponding proteins is discussed. Mechanical film properties were determined by surface deformation in shear and dilation. In shear, fracture stress, sigma(f), and fracture strain, gamma(f), were determined, as well as the relaxation behavior after macroscopic fracture. The dilatational measurements were performed in a Langmuir trough equipped with an infra-red reflection absorption spectroscopy (IRRAS) accessory. During compression and relaxation of the surface, the surface pressure, Pi, and adsorbed amount, Gamma (determined from the IRRAS spectra), were determined simultaneously. In addition, IRRAS spectra revealed information on conformational changes in terms of secondary structure. Possible correlations between macroscopic film properties and intrinsic stability of the proteins were determined and discussed in terms of molecular dimensions of single proteins and interfacial protein films. Molecular properties involved the area per protein molecule at Pi approximately 0 mN/m (A(0)), A(0)/M (M = molecular weight) and the maximum slope of the Pi-Gamma curves (dPi/dGamma). The differences observed in mechanical properties and relaxation behavior indicate that the behavior of a protein film subjected to large deformation may vary widely from predominantly viscous (yielding) to more elastic (fracture). This transition is also observed in gradual changes in A(0)/M. It appeared that in general protein layers with high A(0)/M have a high gamma(f) and behave more fluidlike, whereas solidlike behavior is characterized by low A(0)/M and low gamma(f). Additionally, proteins with a low A(0)/M value have a low adaptability in changing their conformation upon adsorption at the air/water interface. Both results support the conclusion that the hardness (internal cohesion) of protein molecules determines predominantly the mechanical behavior of adsorbed protein layers.

Adsorption↗
PubMed2005

Controlling nonspecific protein adsorption in a plug-based microfluidic system by controlling interfacial chemistry using fluorous-phase surfactants.

Control of surface chemistry and protein adsorption is important for using microfluidic devices for biochemical analysis and high-throughput screening assays. This paper describes the control of protein adsorption at the liquid-liquid interface in a plug-based microfluidic system. The microfluidic system uses multiphase flows of immiscible fluorous and aqueous fluids to form plugs, which are aqueous droplets that are completely surrounded by fluorocarbon oil and do not come into direct contact with the hydrophobic surface of the microchannel. Protein adsorption at the aqueous-fluorous interface was controlled by using surfactants that were soluble in fluorocarbon oil but insoluble in aqueous solutions. Three perfluorinated alkane surfactants capped with different functional groups were used: a carboxylic acid, an alcohol, and a triethylene glycol group that was synthesized from commercially available materials. Using complementary methods of analysis, adsorption was characterized for several proteins (bovine serum albumin (BSA) and fibrinogen), including enzymes (ribonuclease A (RNase A) and alkaline phosphatase). These complementary methods involved characterizing adsorption in microliter-sized droplets by drop tensiometry and in nanoliter plugs by fluorescence microscopy and kinetic measurements of enzyme catalysis. The oligoethylene glycol-capped surfactant prevented protein adsorption in all cases. Adsorption of proteins to the carboxylic acid-capped surfactant in nanoliter plugs could be described by using the Langmuir model and tensiometry results for microliter drops. The microfluidic system was fabricated using rapid prototyping in poly(dimethylsiloxane) (PDMS). Black PDMS microfluidic devices, fabricated by curing a suspension of charcoal in PDMS, were used to measure the changes in fluorescence intensity more sensitively. This system will be useful for microfluidic bioassays, enzymatic kinetics, and protein crystallization, because it does not require surface modification during fabrication to control surface chemistry and protein adsorption.

Adsorption↗
PubMed1984

Characterization and comparison of a Neurospora crassa RNase purified from cultures undergoing each of three different states of derepression.

Extracellular RNase N4 from Neurospora crassa is derepressible by limitation of any of the three nutrient elements obtainable from RNA. We have purified and characterized the enzyme from cultures grown under each of the three states of derepression. The purification procedure consisted of an ultrafiltration step, cation-exchange chromatography, and gel filtration. We found only one enzyme (N4) that hydrolyzed RNA at pH 7.5 in the presence of EDTA in culture filtrates from nitrogen-, phosphorus-, or carbon-limited cells. In all three cases, the enzymes were identical by polyacrylamide gel electrophoresis (Mr approximately 9,500) and by gel filtration (Mr approximately 10,000). There were no differences in thermal stability or pH optimum; all three cross-reacted with antibody to the nitrogen-depressed enzyme in interfacial ring and in Ouchterlony tests. Digestion of homopolyribonucleotides indicated that N4 preferentially cleaved phosphodiester bonds adjacent to guanine residues. Results indicate that the enzymes are very similar or identical and are probably products of the same gene. N4 appears to be homologous to guanine-specific RNases from other fungal sources.

Amino Acids↗
PubMedDate not supplied

Neutrophil adhesion on phosphorylcholine-containing polyurethanes.

Polyurethanes have been synthesized using glycerophosphorylcholine (GPC) as a chain extender. By altering the ratio of GPC to butanediol (BD), a series of polymers was obtained composed of different contents of phosphorylcholine. Bulk and surface characterization of the polymers was carried out. Differential scanning calorimetry and dynamic mechanical analysis showed that the polymer with the highest phosphorylcholine content (PU-GPC-20) had the lowest soft segment Tg and the highest tensile strength and Young's modulus among the polymers studied. This is due to the high degree of microphase separation in PU-GPC-20 as a result of by ionic aggregation and hydrogen bonding from the zwitterionic phosphorylcholine moiety. PU-GPC-20 contained approximately 20 wt%, of glycerophosphorylcholine. Dynamic contact angle analysis showed that these polymers, especially the ones with high phosphorylcholine content, rearranged themselves to minimize their interfacial tension upon contacting an aqueous environment. Under shear rates of from 20 to 120 s(-1), neutrophils did not adhere to PU-GPC-20. Under similar conditions neutrophil adhesion was observed only at 20 s(-1) on PU-GPC-10, PU-GPC-5 and on the control polyurethane (PU-base). Cell spreading was observed on the control polyurethane but not on any of the other surfaces. The incorporation of phosphorylcholine into the polyurethane backbone effectively reduced neutrophil adhesion and thus potentially could result in lower inflammatory and foreign body responses.

Absorption↗
PubMed2005

Effects of hydraulic pressure on the stability and transition of wetting modes of superhydrophobic surfaces.

The underlying mechanisms of stability, metastability, or instability of the Cassie-Baxter and Wenzel wetting modes and their transitions on superhydrophobic surfaces decorated with periodic micropillars are quantitatively studied in this article. Hydraulic pressure, which may be generated by the water-air interfacial tension of water droplets or external factors such as raining impact, is shown to be a key to understanding these mechanisms. A detailed transition process driven by increasing hydraulic pressure is numerically simulated. The maximum sustainable or critical pressure of the Cassie-Baxter wetting state on a pillarlike microstructural surface is formulated for the first time in a simple, unified, and precise form. This analytic result reveals the fact that reducing the microstructural scales (e.g., the pillars' diameters and spacing) is probably the most efficient measure needed to enlarge the critical pressure significantly. We also introduce a dimensionless parameter, the pillar slenderness ratio, to characterize the stability of either the Cassie-Baxter or the Wenzel wetting state and show that the energy barrier for transitioning from the Cassie-Baxter to the Wenzel wetting mode is proportional to both the slenderness ratio and the area fraction. Thus, the Cassie-Baxter wetting mode may collapse under a hydraulic pressure lower than the critical one if the slenderness ratio is improperly small. This quantitative study explains fairly well some experimental observations of contact angles that can be modeled by neither Wenzel nor Cassie-Baxter contact angles and eventually leads to our proposals for a mixed (or coexisting) wetting mode.

Journal Article↗
PubMed1998

Increase in phospholipase A2 activity towards lipopolymer-containing liposomes.

Phospholipase A2 (PLA2)-catalyzed hydrolysis of dipalmitoylphosphatidylcholine (DPPC) liposomes incorporated with submicellar concentrations of polyethyleneoxide covalently attached to dipalmitoylphosphatidylethanolamine (DPPE-PEG2000) has been studied in the gel-to-fluid transition region of the host DPPC lipid bilayer matrix. By means of fluorescence and light-scattering measurements, the characteristic PLA2 lag time has been determined as a function of lipopolymer concentration and temperature. The degree of lipid hydrolysis was followed using radioactive labeled lipids. Differential scanning calorimetry has been applied to characterize the thermodynamic phase behavior of the lipopolymer-containing liposomes. A remarkable lipopolymer concentration-dependent decrease in the lag time was observed over broad temperature ranges. The radioactive measurements demonstrate an increase in catalytic activity for increasing amounts of lipopolymers in the bilayer. Hence, the lipopolymers act as a promoter of PLA2 lipid hydrolysis resulting in a degradation of the bilayer structure and a concomitant destabilization of the liposomes. This behavior is in contrast to the generally observed protective and stabilization effect in biological fluids exerted by lipopolymers in polymer-grafted liposomes. It is proposed that the enhanced activity of the small water soluble and interfacially active enzyme may involve a non-uniform distribution of the lipopolymers in the lipid matrix due to a coupling between local lipid bilayer curvature and composition of the non-bilayer-preferring lipopolymers.

1,2-Dipalmitoylphosphatidylcholine↗