Curtis Frank - Academia.edu (original) (raw)

Papers by Curtis Frank

Research paper thumbnail of In SituOptical Studies of Flow-Induced Orientation in a Two-Dimensional Polymer Solution

Macromolecules, 1996

... Matthew C. Friedenberg, † Gerald G. Fuller,* Curtis W. Frank, and Channing R. Robertson. ... ... more ... Matthew C. Friedenberg, † Gerald G. Fuller,* Curtis W. Frank, and Channing R. Robertson. ... It was recently shown that this ratio of roller diameter to roller spacing, 0.625, provided the closest approximation to homogeneous extension for bulk flows. ...

Research paper thumbnail of Monolayers of Perfluoropolyethers with a Hydrophilic Head Group

Langmuir, 1994

Perfluoropoly(oxypropylene), F((CFz)30),((CFz)z)X, with a hydrophilic head group (X = -COOHI has ... more Perfluoropoly(oxypropylene), F((CFz)30),((CFz)z)X, with a hydrophilic head group (X = -COOHI has been investigated as a thin film spread onto the surface ofwater and as films transferred to solid substrates via the Langmuir-Blodgett technique. Between the onset of the isotherm and collapse, the polymer forms a continuous film of uniform thickness. The density of this film is the same as in the bulk polymer, and the film thickness depends on the area per molecule. The theory of tethered polymers predicts an entropic contribution to the free energy of the system due to deformation of the polymer coils. We propose a thermodynamic analysis in which we mathematically separate the contributions to the surface pressure from the head groups and from the stretching of the polymer chains. In addition, we investigate the influence of the subphase composition (aqueous solutions of CaC12, poly(ethylenimine), HC1, FeCl3, N H 3 ) and compare the isotherm with a model system for the isolated head group. Based on the thermodynamic analysis and the comparison of isotherms, we conclude that-in the case of a chain length of approximately 100 o-bonds-there is no significant entropic contribution from the polymer chains.

Research paper thumbnail of Orientation in a Fatty Acid Monolayer:� Effect of Flow Type

Research paper thumbnail of Novel Starlike Poly(methyl methacrylate)s by Controlled Dendritic Free Radical Initiation

Macromolecules, 1999

Polymers derived from radical-initiated polymerizations are of significant importance in industri... more Polymers derived from radical-initiated polymerizations are of significant importance in industrial processes, since the polymerization process is simple and applicable to a wide range of monomers. However, the control of molecular weights and molecular weight ...

Research paper thumbnail of Fluid Dynamics of Langmuir Films

Aps March Meeting Abstracts, Mar 1, 1996

The fluid dynamics of fatty acid monolayers subjected to extensional flows are described. The pol... more The fluid dynamics of fatty acid monolayers subjected to extensional flows are described. The polydomain structure of docosanoic acid is examined using Brewster angle microscopy and it is demonstrated that both molecular orientation and domain distortion can be measured in situ. The extensional flows are generated using a four roll mill that is placed through monolayers residing at the air-water interface situated on a Langmuir trough. Data are presented on two separate phases: the liquid expanded and the liquid condensed phases. Of these two phases, only the structure of the liquid expanded phase is observed to deform affinely with the imposed flow. The liquid condensed phase, on the other hand, produces a shear-banded structure with the bands oriented at plus and minus 45 degrees relative to the axis of strain in the flow. The shear bands contain molecules with a tilt axis that is orthogonal to the imposed strain. Ultimately, the shear bands grow and the entire surface eventually contains molecules that are oriented orthogonal to the stretching direction. This process is reversible, and changing the direction of the flow causes a rearrangment of the molecular tilt direction.

Research paper thumbnail of Interfacial Binding Dynamics of Bee Venom Phospholipase A2 Investigated by Dynamic Light Scattering and Quartz Crystal Microbalance

Langmuir, 2010

Bee venom phospholipase A 2 (bvPLA 2 ) is part of the secretory phospholipase A 2 (sPLA 2 ) famil... more Bee venom phospholipase A 2 (bvPLA 2 ) is part of the secretory phospholipase A 2 (sPLA 2 ) family whose members are active in biological processes such as signal transduction and lipid metabolism. While controlling sPLA 2 activity is of pharmaceutical interest, the relationship between their mechanistic actions and physiological functions is not well understood. Therefore, we investigated the interfacial binding process of bvPLA 2 to characterize its biophysical properties and gain insight into how membrane binding affects interfacial activation. Attention was focused on the role of membrane electrostatics in the binding process. Although dynamic light scattering experiments indicated that bvPLA 2 does not lyse lipid vesicles, a novel, nonhydrolytic activity was discovered. We employed a supported lipid bilayer platform on the quartz crystal microbalance with dissipation sensor to characterize this bilayer-disrupting behavior and determined that membrane electrostatics influence this activity. The data suggest that (1) adsorption of bvPLA 2 to model membranes is not primarily driven by electrostatic interactions; (2) lipid desorption can follow bvPLA 2 adsorption, resulting in nonhydrolytic bilayer-disruption; and (3) this desorption is driven by electrostatic interactions. Taken together, these findings provide evidence that interfacial binding of bvPLA 2 is a dynamic process, shedding light on how membrane electrostatics can modulate interfacial activation.

Research paper thumbnail of Deformation and Relaxation Processes of Mono- and Bilayer Domains of Liquid Crystalline Langmuir Films on Water

Langmuir, 1996

... Acknowledgment. JL thanks H. McConnell for helpful discussions and gratefully acknowledges th... more ... Acknowledgment. JL thanks H. McConnell for helpful discussions and gratefully acknowledges the support of the Alexander-von-Humboldt Foundation through a Feodor-Lynen-Fellowship. ... Electron.1985, 1, 97. (13) Benvegnu, DJ; McConnell, HM J. Phys. Chem.1992, 96, 6820. ...

Research paper thumbnail of Fluid Mechanics of Rinsing Flows

ABSTRACT The fluid mechanics of rinsing flows, describing the use of a jet of one liquid or air t... more ABSTRACT The fluid mechanics of rinsing flows, describing the use of a jet of one liquid or air to push away a second, coating liquid on a rigid flat surface, has been investigated. This phenomenon is common to everyday life but despite this, few studies have considered the rinsing flow process in detail. One experimental challenge is that the timescale of the process is short and does not reach a steady state. The present study has employed high speed imaging devices allowing the examination of the initial contact between the two fluids. It is found that these flows present interesting variations to the classic hydraulic jump with a variety of new phenomena due to the presence of the second, coating layer. For example, a systematic study varying the viscosity ratio between the two fluids reveals a Taylor-Saffman instability at the moving interface between fluid contacts when the viscosity ratio between the coating fluid and the jet fluid is higher than ~25. The hydraulic jump circle size is also a strong function of the viscosity ratio and exhibits a power-law relationship. In addition, the effect of the viscoelasticity of the coating fluid has been investigated in response to either water or air jets. Three classes of fluids with the same zero-shear viscosity have been investigated: glycerin-water Newtonian liquids, polymeric solutions, and worm-like micelle surfactant solutions. Analysis of the high speed videos reveal that the evolution of the hydraulic jump circle sizes have very different characteristics. For example, for water jet experiments, Newtonian liquids show an inverse exponential-like growth in circle size while polymeric solutions show a linear growth and for surfactant solutions, the circle size first increases then recoils due to the high elasticity of the fluid. In addition, it is observed that the Non-Newtonian liquids are able to suppress surface roughness in the hydraulic jump region due to a coupling of the extensional viscosities of these fluids and stagnation point flows in the vicinity of the hydraulic jump. Both the degree of roughness suppression and hydraulic jump circle size increase with the extensional viscosity of the solution. The effect of altering the hydrophobicity of the flat surfaces on the efficiency of the rinsing process has also been investigated using scanning electron micrographs and Auger spectroscopy. Under the same conditions, the efficiency is affected by the wettability of the rinsing fluid on the surface and is a function of the radial position from the jet.

Research paper thumbnail of Surface Shear Rheology of a Polymerizable Lipopolymer Monolayer

Langmuir, Mar 1, 2002

... Carlton F. Brooks, † ‡ Jochen Thiele, † § Curtis W. Frank,* † David F. O'Brien, Wolfgang... more ... Carlton F. Brooks, † ‡ Jochen Thiele, † § Curtis W. Frank,* † David F. O'Brien, Wolfgang Knoll, Gerald G. Fuller,* † and Channing R. Robertson †. Department of Chemical Engineering, Stanford University, Stanford, California ...

Research paper thumbnail of Mechanism of an Amphipathic α-Helical Peptide’s Antiviral Activity Involves Size-Dependent Virus Particle Lysis

Acs Chem Biol, 2009

E nd-stage liver disease (ESLD) is a major cause of worldwide morbidity and mortality. Viral infe... more E nd-stage liver disease (ESLD) is a major cause of worldwide morbidity and mortality. Viral infections, such as those resulting from the hepatitis C virus (HCV), are important etiologies. Current interferon-based therapies for HCV are inadequate for most patients; hence, a great need to develop and evaluate new potential antiviral strategies exists (1). Disrupting NS5A membrane association represents a new, attractive anti-HCV strategy because the protein has been genetically validated as essential for HCV RNA genome replication (2, 3). While studying the NS5A-derived AH peptide's binding interaction with cellderived and model membranes (3−5), we discovered a surprising property of the AH peptide: its ability to induce lysis of lipid vesicles as well as virus particles, thereby inhibiting de novo HCV infection. Dynamic light scattering (DLS) was employed to study the vesicle size dependency of AH peptidemediated vesicle lysis as a model system for virus particle lysis. From these studies, we identified a vesicle size range within which the AH peptide shows vesicle lysis potency. To further examine this lysis activity, we investigated how vesicle size affects the AH peptide's vesicle rupturing ability in order to better understand the mechanism of virus particle lysis. Previously, we described a novel method that employs the AH peptide to destabilize a layer of intact vesicles adsorbed on various substrates such as gold and titanium oxide. The AH peptide interac-tion causes vesicle rupture, transforming the adsorbed vesicles into a planar bilayer (5). We used the quartz crystal microbalance-dissipation (QCM-D) technique as a detection system with nanoscale mass range precision to measure the degree of vesicle rupture (6). This technique not only measures the quantitative binding mass that can be derived from the simple linear relationship, the Sauerbrey equation, between the crystal's resonant frequency change and the mass of the adlayer , but the viscoelasticity of the adlayer, providing insight into peptide conformational changes upon binding, can also be deduced by the damping of the crystal oscillation (6).

Research paper thumbnail of In Situ Precipitation of Silica in Dansyl-Labeled Poly(dimethylsiloxane) Elastomers

Chemistry of Materials, 1995

... Muter. 1993, 5, 422. ... (34) Shea, KJ; Stoddard, GJ Macromolecules 1991, 24, 1207. (35) Shea... more ... Muter. 1993, 5, 422. ... (34) Shea, KJ; Stoddard, GJ Macromolecules 1991, 24, 1207. (35) Shea, KJ; Stoddard, GJ; Shavelle, DM; Wakui, F.; Choate, (36) Seo, T.; Take, S.; Miwa, K.; Hamada, K.; Iijima, T. Macromol-(37) Holmes-Farley, SR; Whitesides, GM Langmuir 1986,2, 266. ...

Research paper thumbnail of Direct Visualization of Flow-Induced Anisotropy in a Fatty Acid Monolayer

Langmuir, 1996

Brewster angle microscopy is used to directly visualize the influence of an applied extensional f... more Brewster angle microscopy is used to directly visualize the influence of an applied extensional flow on the domain structure and molecular orientation of a docosanoic acid monolayer at the air-water interface. At a surface pressure of 12 mN/m and a subphase temperature of 15 °C (L 2 phase), extensional flow causes domain elongation parallel to the extension axis. A frequency domain analysis of the Brewster angle images indicates that the domains undergo an affine deformation in response to flow. AT 20 mN/m (L 2 phase), the flow modifies not only the domain structure of the monolayer but also the azimuthal orientation of the fatty acid molecules. This flow-alignment process is strain-rate dependent. Thus, flow can couple to the monolayer order over a variety of length scales.

Research paper thumbnail of The role of electrostatic interactions in protease surface diffusion and the consequence for interfacial biocatalysis

Langmuir, Nov 1, 2010

This study examines the influence of electrostatic interactions on enzyme surface diffusion and t... more This study examines the influence of electrostatic interactions on enzyme surface diffusion and the contribution of diffusion to interfacial biocatalysis. Surface diffusion, adsorption, and reaction were investigated on an immobilized bovine serum albumin (BSA) multilayer substrate over a range of solution ionic strength values. Interfacial charge of the enzyme and substrate surface was maintained by performing the measurements at a fixed pH; therefore, electrostatic interactions were manipulated by changing the ionic strength. The interfacial processes were investigated using a combination of techniques: fluorescence recovery after photobleaching, surface plasmon resonance, and surface plasmon fluorescence spectroscopy. We used an enzyme charge ladder with a net charge ranging from -2 to þ4 with respect to the parent to systematically probe the contribution of electrostatics in interfacial enzyme biocatalysis on a charged substrate. The correlation between reaction rate and adsorption was determined for each charge variant within the ladder, each of which displayed a maximum rate at an intermediate surface concentration. Both the maximum reaction rate and adsorption value at which this maximum rate occurs increased in magnitude for the more positive variants. In addition, the specific enzyme activity increased as the level of adsorption decreased, and for the lowest adsorption values, the specific enzyme activity was enhanced compared to the trend at higher surface concentrations. At a fixed level of adsorption, the specific enzyme activity increased with positive enzyme charge; however, this effect offers diminishing returns as the enzyme becomes more highly charged. We examined the effect of electrostatic interactions on surface diffusion. As the binding affinity was reduced by increasing the solution ionic strength, thus weakening electrostatic interaction, the rate of surface diffusion increased considerably. The enhancement in specific activity achieved at the lowest adsorption values is explained by the substantial rise in surface diffusion at high ionic strength due to decreased interactions with the surface. Overall, knowledge of the electrostatic interactions can be used to control surface parameters such as surface concentration and surface diffusion, which intimately correlate with surface biocatalysis. We propose that the maximum reaction rate results from a balance between adsorption and surface diffusion. The above finding suggests enzyme engineering and process design strategies for improving interfacial biocatalysis in industrial, pharmaceutical, and food applications.

Research paper thumbnail of Surface modification of interpenetrating polymer networks

Research paper thumbnail of New Ground for Organic Catalysis : A Ring-Opening Polymerization Approach to Hydrogels

Biomacromolecules, Nov 1, 2007

Herein, we describe an organocatalytic living polymerization approach to network and subsequent h... more Herein, we describe an organocatalytic living polymerization approach to network and subsequent hydrogel formation. Cyclic carbonate-functionalized macromolecules were ring-opened using an alcoholic initiator in the presence of an organic catalyst, amidine 1,8-diazabicyclo[5.4.0]undec-7-ene. A model reaction for the cross-linking identified monomer concentration-dependent reaction regimes, and enhanced kinetic control was demonstrated by introducing a co-monomer, trimethylene carbonate. The addition of the co-monomer facilitated near-quantitative conversion of monomer to polymer (>96%). Resulting poly(ethylene glycol) networks swell significantly in water, and an open co-continuous (water-gel) porous structure was observed by scanning electron microscopy. The organocatalytic ring-opening polymerization of cyclic carbonate functional macromonomers using alcoholic initiators provides a simple, efficient, and versatile approach to hydrogel networks.

Research paper thumbnail of Lignin Poly(Lactic Acid) Copolymers

Research paper thumbnail of Alpha-Helical Peptide-Induced Vesicle Rupture Revealing New Insight into the Vesicle Fusion Process As Monitored in Situ by Quartz Crystal Microbalance-Dissipation and Reflectometry

Analytical Chemistry, Jun 1, 2009

We have used simultaneous quartz crystal microbalance-dissipation (QCM-D) monitoring and four-det... more We have used simultaneous quartz crystal microbalance-dissipation (QCM-D) monitoring and four-detector optical reflectometry to monitor in situ the structural transformation of intact vesicles to a lipid bilayer on a gold surface. The structural transformation of lipid vesicles to a bilayer was achieved by introducing a particular amphipathic, alpha-helical (AH) peptide. The combined experimental apparatus allows us to simultaneously follow the acoustic and optical property changes of the vesicle rupturing process upon interaction with AH peptides. While QCM-D and reflectometry have similar sensitivities in terms of mass and thickness resolution, there are unique advantages in operating these techniques simultaneously on the same substrate. These advantages permit us to (1) follow the complex interaction between AH peptides and intact vesicles with both acoustic and optical mass measurements, (2) calculate the amount of dynamically coupled water during the interaction between AH peptides and intact vesicles, (3) demonstrate that the unexpectedly large increase of both adsorbed mass and the film's energy dissipation is mainly caused by swelling of the vesicles during the binding interaction with AH peptides, and (4) permit us to understand the structural transformation from intact vesicles to a bilayer via the AH peptide interaction by monitoring viscoelastic properties, acoustic mass, optical mass, and thickness changes of both the binding and destabilization processes. From the deduced "hydration signature" we followed the complex transformation of lipid assemblies. On the basis of this information, a mechanism of this structural transformation is proposed that provides new insight into the process of vesicle fusion on solid substrates.

Research paper thumbnail of Interfacial Biocatalysis on Charged and Immobilized Substrates: The Roles of Enzyme and Substrate Surface Charge

Langmuir, Jan 4, 2011

An enzyme charge ladder was used to examine the role of electrostatic interactions involved in bi... more An enzyme charge ladder was used to examine the role of electrostatic interactions involved in biocatalysis at the solid-liquid interface. The reactive substrate consisted of an immobilized bovine serum albumin (BSA) multilayer prepared using a layer-by-layer technique. The zeta potential of the BSA substrate and each enzyme variant was measured to determine the absolute charge in solution. Enzyme adsorption and the rate of substrate surface hydrolysis were monitored for the enzyme charge ladder series to provide information regarding the strength of the enzyme-substrate interaction and the rate of interfacial biocatalysis. First, each variant of the charge ladder was examined at pH 8 for various solution ionic strengths. We found that for positively charged variants the adsorption increased with the magnitude of the charge until the surface became saturated. For higher ionic strength solutions, a greater positive enzyme charge was required to induce adsorption. Interestingly, the maximum catalytic rate was not achieved at enzyme saturation but at an invariable intermediate level of adsorption for each ionic strength value. Furthermore, the maximum achievable reaction rate for the charge ladder was larger for higher ionic strength values. We propose that diffusion plays an important role in interfacial biocatalysis, and for strong enzyme-substrate interaction, the rate of diffusion is reduced, leading to a decrease in the overall reaction rate. We investigated the effect of substrate charge by varying the solution pH from 6.1 to 8.7 and by examining multiple ionic strength values for each pH. The same intermediate level of adsorption was found to maximize the overall reaction rate. However, the ionic strength response of the maximum achievable rate was clearly dependent on the pH of the experiment. We propose that this observation is not a direct effect of pH but is caused by the change in substrate surface charge induced by changing the pH. To prove this hypothesis, BSA substrates were chemically modified to reduce the magnitude of the negative charge at pH 8. Chemical modification was accomplished by the amidation of aspartic and glutamic acids to asparagine and glutamine. The ionic strength response of the chemically modified substrate was considerably different than that for the native BSA substrate at an identical pH, consistent with the trend based on substrate surface charge. Consequently, for substrates with a low net surface charge, the maximum achievable catalytic rate of the charge ladder was relatively independent of the solution ionic strength over the range examined; however, at high net substrate surface charge, the maximum rate showed a considerable ionic strength dependence.

Research paper thumbnail of Formation of Bilayer Disks and Two-Dimensional Foams on a Collapsing/Expanding Liquid-Crystal Monolayer

Langmuir, 1994

... Matthew C. Friedenberg, Gerald G. Fuller,' Curtis W. Frank, and Channing R. Robertson De... more ... Matthew C. Friedenberg, Gerald G. Fuller,' Curtis W. Frank, and Channing R. Robertson Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025 Received July 26,1993. In Final Form: November 17,199P ...

Research paper thumbnail of Influence of the head-to-head defect and the molecular weight on the α→γ solid-state transformation of poly(vinylidene fluoride)

Macromolecules, 1985

Crystallization of PVFz from the melt results in three peaks in the melting endotherm obtained by... more Crystallization of PVFz from the melt results in three peaks in the melting endotherm obtained by differential scanning calorimetry; thew correspond to a, crysy, and transformed y forms. An additional low-melting peak is attributed to the interlamellar crystallization that occurs during quenching. Areas of these peaks are determined and used to study the solid-state ay transformation. The extent of the transformation appears to increase with the percentage of the y spherulites in the crystalline regions but to decrease with the content of the interlamellar amorphous layers. The results support Lovinger's proposal that the transformation proceeds more rapidly in the longitudinal direction toward the a-nuclei than in the transverse direction. Moreover, the two competing factors-the percentage of the y spherulites and the interlamellar amorphous content-increase with either the reversed monomer (head-to-head defect) content or the molecular weight. As a result, the ay transformation is a complex function of the head-to-head defect concentration and the molecular weight.

Research paper thumbnail of In SituOptical Studies of Flow-Induced Orientation in a Two-Dimensional Polymer Solution

Macromolecules, 1996

... Matthew C. Friedenberg, † Gerald G. Fuller,* Curtis W. Frank, and Channing R. Robertson. ... ... more ... Matthew C. Friedenberg, † Gerald G. Fuller,* Curtis W. Frank, and Channing R. Robertson. ... It was recently shown that this ratio of roller diameter to roller spacing, 0.625, provided the closest approximation to homogeneous extension for bulk flows. ...

Research paper thumbnail of Monolayers of Perfluoropolyethers with a Hydrophilic Head Group

Langmuir, 1994

Perfluoropoly(oxypropylene), F((CFz)30),((CFz)z)X, with a hydrophilic head group (X = -COOHI has ... more Perfluoropoly(oxypropylene), F((CFz)30),((CFz)z)X, with a hydrophilic head group (X = -COOHI has been investigated as a thin film spread onto the surface ofwater and as films transferred to solid substrates via the Langmuir-Blodgett technique. Between the onset of the isotherm and collapse, the polymer forms a continuous film of uniform thickness. The density of this film is the same as in the bulk polymer, and the film thickness depends on the area per molecule. The theory of tethered polymers predicts an entropic contribution to the free energy of the system due to deformation of the polymer coils. We propose a thermodynamic analysis in which we mathematically separate the contributions to the surface pressure from the head groups and from the stretching of the polymer chains. In addition, we investigate the influence of the subphase composition (aqueous solutions of CaC12, poly(ethylenimine), HC1, FeCl3, N H 3 ) and compare the isotherm with a model system for the isolated head group. Based on the thermodynamic analysis and the comparison of isotherms, we conclude that-in the case of a chain length of approximately 100 o-bonds-there is no significant entropic contribution from the polymer chains.

Research paper thumbnail of Orientation in a Fatty Acid Monolayer:� Effect of Flow Type

Research paper thumbnail of Novel Starlike Poly(methyl methacrylate)s by Controlled Dendritic Free Radical Initiation

Macromolecules, 1999

Polymers derived from radical-initiated polymerizations are of significant importance in industri... more Polymers derived from radical-initiated polymerizations are of significant importance in industrial processes, since the polymerization process is simple and applicable to a wide range of monomers. However, the control of molecular weights and molecular weight ...

Research paper thumbnail of Fluid Dynamics of Langmuir Films

Aps March Meeting Abstracts, Mar 1, 1996

The fluid dynamics of fatty acid monolayers subjected to extensional flows are described. The pol... more The fluid dynamics of fatty acid monolayers subjected to extensional flows are described. The polydomain structure of docosanoic acid is examined using Brewster angle microscopy and it is demonstrated that both molecular orientation and domain distortion can be measured in situ. The extensional flows are generated using a four roll mill that is placed through monolayers residing at the air-water interface situated on a Langmuir trough. Data are presented on two separate phases: the liquid expanded and the liquid condensed phases. Of these two phases, only the structure of the liquid expanded phase is observed to deform affinely with the imposed flow. The liquid condensed phase, on the other hand, produces a shear-banded structure with the bands oriented at plus and minus 45 degrees relative to the axis of strain in the flow. The shear bands contain molecules with a tilt axis that is orthogonal to the imposed strain. Ultimately, the shear bands grow and the entire surface eventually contains molecules that are oriented orthogonal to the stretching direction. This process is reversible, and changing the direction of the flow causes a rearrangment of the molecular tilt direction.

Research paper thumbnail of Interfacial Binding Dynamics of Bee Venom Phospholipase A2 Investigated by Dynamic Light Scattering and Quartz Crystal Microbalance

Langmuir, 2010

Bee venom phospholipase A 2 (bvPLA 2 ) is part of the secretory phospholipase A 2 (sPLA 2 ) famil... more Bee venom phospholipase A 2 (bvPLA 2 ) is part of the secretory phospholipase A 2 (sPLA 2 ) family whose members are active in biological processes such as signal transduction and lipid metabolism. While controlling sPLA 2 activity is of pharmaceutical interest, the relationship between their mechanistic actions and physiological functions is not well understood. Therefore, we investigated the interfacial binding process of bvPLA 2 to characterize its biophysical properties and gain insight into how membrane binding affects interfacial activation. Attention was focused on the role of membrane electrostatics in the binding process. Although dynamic light scattering experiments indicated that bvPLA 2 does not lyse lipid vesicles, a novel, nonhydrolytic activity was discovered. We employed a supported lipid bilayer platform on the quartz crystal microbalance with dissipation sensor to characterize this bilayer-disrupting behavior and determined that membrane electrostatics influence this activity. The data suggest that (1) adsorption of bvPLA 2 to model membranes is not primarily driven by electrostatic interactions; (2) lipid desorption can follow bvPLA 2 adsorption, resulting in nonhydrolytic bilayer-disruption; and (3) this desorption is driven by electrostatic interactions. Taken together, these findings provide evidence that interfacial binding of bvPLA 2 is a dynamic process, shedding light on how membrane electrostatics can modulate interfacial activation.

Research paper thumbnail of Deformation and Relaxation Processes of Mono- and Bilayer Domains of Liquid Crystalline Langmuir Films on Water

Langmuir, 1996

... Acknowledgment. JL thanks H. McConnell for helpful discussions and gratefully acknowledges th... more ... Acknowledgment. JL thanks H. McConnell for helpful discussions and gratefully acknowledges the support of the Alexander-von-Humboldt Foundation through a Feodor-Lynen-Fellowship. ... Electron.1985, 1, 97. (13) Benvegnu, DJ; McConnell, HM J. Phys. Chem.1992, 96, 6820. ...

Research paper thumbnail of Fluid Mechanics of Rinsing Flows

ABSTRACT The fluid mechanics of rinsing flows, describing the use of a jet of one liquid or air t... more ABSTRACT The fluid mechanics of rinsing flows, describing the use of a jet of one liquid or air to push away a second, coating liquid on a rigid flat surface, has been investigated. This phenomenon is common to everyday life but despite this, few studies have considered the rinsing flow process in detail. One experimental challenge is that the timescale of the process is short and does not reach a steady state. The present study has employed high speed imaging devices allowing the examination of the initial contact between the two fluids. It is found that these flows present interesting variations to the classic hydraulic jump with a variety of new phenomena due to the presence of the second, coating layer. For example, a systematic study varying the viscosity ratio between the two fluids reveals a Taylor-Saffman instability at the moving interface between fluid contacts when the viscosity ratio between the coating fluid and the jet fluid is higher than ~25. The hydraulic jump circle size is also a strong function of the viscosity ratio and exhibits a power-law relationship. In addition, the effect of the viscoelasticity of the coating fluid has been investigated in response to either water or air jets. Three classes of fluids with the same zero-shear viscosity have been investigated: glycerin-water Newtonian liquids, polymeric solutions, and worm-like micelle surfactant solutions. Analysis of the high speed videos reveal that the evolution of the hydraulic jump circle sizes have very different characteristics. For example, for water jet experiments, Newtonian liquids show an inverse exponential-like growth in circle size while polymeric solutions show a linear growth and for surfactant solutions, the circle size first increases then recoils due to the high elasticity of the fluid. In addition, it is observed that the Non-Newtonian liquids are able to suppress surface roughness in the hydraulic jump region due to a coupling of the extensional viscosities of these fluids and stagnation point flows in the vicinity of the hydraulic jump. Both the degree of roughness suppression and hydraulic jump circle size increase with the extensional viscosity of the solution. The effect of altering the hydrophobicity of the flat surfaces on the efficiency of the rinsing process has also been investigated using scanning electron micrographs and Auger spectroscopy. Under the same conditions, the efficiency is affected by the wettability of the rinsing fluid on the surface and is a function of the radial position from the jet.

Research paper thumbnail of Surface Shear Rheology of a Polymerizable Lipopolymer Monolayer

Langmuir, Mar 1, 2002

... Carlton F. Brooks, † ‡ Jochen Thiele, † § Curtis W. Frank,* † David F. O'Brien, Wolfgang... more ... Carlton F. Brooks, † ‡ Jochen Thiele, † § Curtis W. Frank,* † David F. O'Brien, Wolfgang Knoll, Gerald G. Fuller,* † and Channing R. Robertson †. Department of Chemical Engineering, Stanford University, Stanford, California ...

Research paper thumbnail of Mechanism of an Amphipathic α-Helical Peptide’s Antiviral Activity Involves Size-Dependent Virus Particle Lysis

Acs Chem Biol, 2009

E nd-stage liver disease (ESLD) is a major cause of worldwide morbidity and mortality. Viral infe... more E nd-stage liver disease (ESLD) is a major cause of worldwide morbidity and mortality. Viral infections, such as those resulting from the hepatitis C virus (HCV), are important etiologies. Current interferon-based therapies for HCV are inadequate for most patients; hence, a great need to develop and evaluate new potential antiviral strategies exists (1). Disrupting NS5A membrane association represents a new, attractive anti-HCV strategy because the protein has been genetically validated as essential for HCV RNA genome replication (2, 3). While studying the NS5A-derived AH peptide's binding interaction with cellderived and model membranes (3−5), we discovered a surprising property of the AH peptide: its ability to induce lysis of lipid vesicles as well as virus particles, thereby inhibiting de novo HCV infection. Dynamic light scattering (DLS) was employed to study the vesicle size dependency of AH peptidemediated vesicle lysis as a model system for virus particle lysis. From these studies, we identified a vesicle size range within which the AH peptide shows vesicle lysis potency. To further examine this lysis activity, we investigated how vesicle size affects the AH peptide's vesicle rupturing ability in order to better understand the mechanism of virus particle lysis. Previously, we described a novel method that employs the AH peptide to destabilize a layer of intact vesicles adsorbed on various substrates such as gold and titanium oxide. The AH peptide interac-tion causes vesicle rupture, transforming the adsorbed vesicles into a planar bilayer (5). We used the quartz crystal microbalance-dissipation (QCM-D) technique as a detection system with nanoscale mass range precision to measure the degree of vesicle rupture (6). This technique not only measures the quantitative binding mass that can be derived from the simple linear relationship, the Sauerbrey equation, between the crystal's resonant frequency change and the mass of the adlayer , but the viscoelasticity of the adlayer, providing insight into peptide conformational changes upon binding, can also be deduced by the damping of the crystal oscillation (6).

Research paper thumbnail of In Situ Precipitation of Silica in Dansyl-Labeled Poly(dimethylsiloxane) Elastomers

Chemistry of Materials, 1995

... Muter. 1993, 5, 422. ... (34) Shea, KJ; Stoddard, GJ Macromolecules 1991, 24, 1207. (35) Shea... more ... Muter. 1993, 5, 422. ... (34) Shea, KJ; Stoddard, GJ Macromolecules 1991, 24, 1207. (35) Shea, KJ; Stoddard, GJ; Shavelle, DM; Wakui, F.; Choate, (36) Seo, T.; Take, S.; Miwa, K.; Hamada, K.; Iijima, T. Macromol-(37) Holmes-Farley, SR; Whitesides, GM Langmuir 1986,2, 266. ...

Research paper thumbnail of Direct Visualization of Flow-Induced Anisotropy in a Fatty Acid Monolayer

Langmuir, 1996

Brewster angle microscopy is used to directly visualize the influence of an applied extensional f... more Brewster angle microscopy is used to directly visualize the influence of an applied extensional flow on the domain structure and molecular orientation of a docosanoic acid monolayer at the air-water interface. At a surface pressure of 12 mN/m and a subphase temperature of 15 °C (L 2 phase), extensional flow causes domain elongation parallel to the extension axis. A frequency domain analysis of the Brewster angle images indicates that the domains undergo an affine deformation in response to flow. AT 20 mN/m (L 2 phase), the flow modifies not only the domain structure of the monolayer but also the azimuthal orientation of the fatty acid molecules. This flow-alignment process is strain-rate dependent. Thus, flow can couple to the monolayer order over a variety of length scales.

Research paper thumbnail of The role of electrostatic interactions in protease surface diffusion and the consequence for interfacial biocatalysis

Langmuir, Nov 1, 2010

This study examines the influence of electrostatic interactions on enzyme surface diffusion and t... more This study examines the influence of electrostatic interactions on enzyme surface diffusion and the contribution of diffusion to interfacial biocatalysis. Surface diffusion, adsorption, and reaction were investigated on an immobilized bovine serum albumin (BSA) multilayer substrate over a range of solution ionic strength values. Interfacial charge of the enzyme and substrate surface was maintained by performing the measurements at a fixed pH; therefore, electrostatic interactions were manipulated by changing the ionic strength. The interfacial processes were investigated using a combination of techniques: fluorescence recovery after photobleaching, surface plasmon resonance, and surface plasmon fluorescence spectroscopy. We used an enzyme charge ladder with a net charge ranging from -2 to þ4 with respect to the parent to systematically probe the contribution of electrostatics in interfacial enzyme biocatalysis on a charged substrate. The correlation between reaction rate and adsorption was determined for each charge variant within the ladder, each of which displayed a maximum rate at an intermediate surface concentration. Both the maximum reaction rate and adsorption value at which this maximum rate occurs increased in magnitude for the more positive variants. In addition, the specific enzyme activity increased as the level of adsorption decreased, and for the lowest adsorption values, the specific enzyme activity was enhanced compared to the trend at higher surface concentrations. At a fixed level of adsorption, the specific enzyme activity increased with positive enzyme charge; however, this effect offers diminishing returns as the enzyme becomes more highly charged. We examined the effect of electrostatic interactions on surface diffusion. As the binding affinity was reduced by increasing the solution ionic strength, thus weakening electrostatic interaction, the rate of surface diffusion increased considerably. The enhancement in specific activity achieved at the lowest adsorption values is explained by the substantial rise in surface diffusion at high ionic strength due to decreased interactions with the surface. Overall, knowledge of the electrostatic interactions can be used to control surface parameters such as surface concentration and surface diffusion, which intimately correlate with surface biocatalysis. We propose that the maximum reaction rate results from a balance between adsorption and surface diffusion. The above finding suggests enzyme engineering and process design strategies for improving interfacial biocatalysis in industrial, pharmaceutical, and food applications.

Research paper thumbnail of Surface modification of interpenetrating polymer networks

Research paper thumbnail of New Ground for Organic Catalysis : A Ring-Opening Polymerization Approach to Hydrogels

Biomacromolecules, Nov 1, 2007

Herein, we describe an organocatalytic living polymerization approach to network and subsequent h... more Herein, we describe an organocatalytic living polymerization approach to network and subsequent hydrogel formation. Cyclic carbonate-functionalized macromolecules were ring-opened using an alcoholic initiator in the presence of an organic catalyst, amidine 1,8-diazabicyclo[5.4.0]undec-7-ene. A model reaction for the cross-linking identified monomer concentration-dependent reaction regimes, and enhanced kinetic control was demonstrated by introducing a co-monomer, trimethylene carbonate. The addition of the co-monomer facilitated near-quantitative conversion of monomer to polymer (>96%). Resulting poly(ethylene glycol) networks swell significantly in water, and an open co-continuous (water-gel) porous structure was observed by scanning electron microscopy. The organocatalytic ring-opening polymerization of cyclic carbonate functional macromonomers using alcoholic initiators provides a simple, efficient, and versatile approach to hydrogel networks.

Research paper thumbnail of Lignin Poly(Lactic Acid) Copolymers

Research paper thumbnail of Alpha-Helical Peptide-Induced Vesicle Rupture Revealing New Insight into the Vesicle Fusion Process As Monitored in Situ by Quartz Crystal Microbalance-Dissipation and Reflectometry

Analytical Chemistry, Jun 1, 2009

We have used simultaneous quartz crystal microbalance-dissipation (QCM-D) monitoring and four-det... more We have used simultaneous quartz crystal microbalance-dissipation (QCM-D) monitoring and four-detector optical reflectometry to monitor in situ the structural transformation of intact vesicles to a lipid bilayer on a gold surface. The structural transformation of lipid vesicles to a bilayer was achieved by introducing a particular amphipathic, alpha-helical (AH) peptide. The combined experimental apparatus allows us to simultaneously follow the acoustic and optical property changes of the vesicle rupturing process upon interaction with AH peptides. While QCM-D and reflectometry have similar sensitivities in terms of mass and thickness resolution, there are unique advantages in operating these techniques simultaneously on the same substrate. These advantages permit us to (1) follow the complex interaction between AH peptides and intact vesicles with both acoustic and optical mass measurements, (2) calculate the amount of dynamically coupled water during the interaction between AH peptides and intact vesicles, (3) demonstrate that the unexpectedly large increase of both adsorbed mass and the film's energy dissipation is mainly caused by swelling of the vesicles during the binding interaction with AH peptides, and (4) permit us to understand the structural transformation from intact vesicles to a bilayer via the AH peptide interaction by monitoring viscoelastic properties, acoustic mass, optical mass, and thickness changes of both the binding and destabilization processes. From the deduced "hydration signature" we followed the complex transformation of lipid assemblies. On the basis of this information, a mechanism of this structural transformation is proposed that provides new insight into the process of vesicle fusion on solid substrates.

Research paper thumbnail of Interfacial Biocatalysis on Charged and Immobilized Substrates: The Roles of Enzyme and Substrate Surface Charge

Langmuir, Jan 4, 2011

An enzyme charge ladder was used to examine the role of electrostatic interactions involved in bi... more An enzyme charge ladder was used to examine the role of electrostatic interactions involved in biocatalysis at the solid-liquid interface. The reactive substrate consisted of an immobilized bovine serum albumin (BSA) multilayer prepared using a layer-by-layer technique. The zeta potential of the BSA substrate and each enzyme variant was measured to determine the absolute charge in solution. Enzyme adsorption and the rate of substrate surface hydrolysis were monitored for the enzyme charge ladder series to provide information regarding the strength of the enzyme-substrate interaction and the rate of interfacial biocatalysis. First, each variant of the charge ladder was examined at pH 8 for various solution ionic strengths. We found that for positively charged variants the adsorption increased with the magnitude of the charge until the surface became saturated. For higher ionic strength solutions, a greater positive enzyme charge was required to induce adsorption. Interestingly, the maximum catalytic rate was not achieved at enzyme saturation but at an invariable intermediate level of adsorption for each ionic strength value. Furthermore, the maximum achievable reaction rate for the charge ladder was larger for higher ionic strength values. We propose that diffusion plays an important role in interfacial biocatalysis, and for strong enzyme-substrate interaction, the rate of diffusion is reduced, leading to a decrease in the overall reaction rate. We investigated the effect of substrate charge by varying the solution pH from 6.1 to 8.7 and by examining multiple ionic strength values for each pH. The same intermediate level of adsorption was found to maximize the overall reaction rate. However, the ionic strength response of the maximum achievable rate was clearly dependent on the pH of the experiment. We propose that this observation is not a direct effect of pH but is caused by the change in substrate surface charge induced by changing the pH. To prove this hypothesis, BSA substrates were chemically modified to reduce the magnitude of the negative charge at pH 8. Chemical modification was accomplished by the amidation of aspartic and glutamic acids to asparagine and glutamine. The ionic strength response of the chemically modified substrate was considerably different than that for the native BSA substrate at an identical pH, consistent with the trend based on substrate surface charge. Consequently, for substrates with a low net surface charge, the maximum achievable catalytic rate of the charge ladder was relatively independent of the solution ionic strength over the range examined; however, at high net substrate surface charge, the maximum rate showed a considerable ionic strength dependence.

Research paper thumbnail of Formation of Bilayer Disks and Two-Dimensional Foams on a Collapsing/Expanding Liquid-Crystal Monolayer

Langmuir, 1994

... Matthew C. Friedenberg, Gerald G. Fuller,' Curtis W. Frank, and Channing R. Robertson De... more ... Matthew C. Friedenberg, Gerald G. Fuller,' Curtis W. Frank, and Channing R. Robertson Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025 Received July 26,1993. In Final Form: November 17,199P ...

Research paper thumbnail of Influence of the head-to-head defect and the molecular weight on the α→γ solid-state transformation of poly(vinylidene fluoride)

Macromolecules, 1985

Crystallization of PVFz from the melt results in three peaks in the melting endotherm obtained by... more Crystallization of PVFz from the melt results in three peaks in the melting endotherm obtained by differential scanning calorimetry; thew correspond to a, crysy, and transformed y forms. An additional low-melting peak is attributed to the interlamellar crystallization that occurs during quenching. Areas of these peaks are determined and used to study the solid-state ay transformation. The extent of the transformation appears to increase with the percentage of the y spherulites in the crystalline regions but to decrease with the content of the interlamellar amorphous layers. The results support Lovinger's proposal that the transformation proceeds more rapidly in the longitudinal direction toward the a-nuclei than in the transverse direction. Moreover, the two competing factors-the percentage of the y spherulites and the interlamellar amorphous content-increase with either the reversed monomer (head-to-head defect) content or the molecular weight. As a result, the ay transformation is a complex function of the head-to-head defect concentration and the molecular weight.