Lateral diffusion of bovine serum albumin adsorbed at the solid-liquid interface

Molecular Structure and Conformation of Biodegradable Water-Soluble Polymers Control Adsorption and Transport in Model Soil Mineral Systems

Water-soluble polymers (WSPs) are used in diverse applications, including agricultural formulations, that can result in the release of WSPs to soils. WSP biodegradability in soils is desirable to prevent long-term accumulation and potential associated adverse effects. In this work, we assessed adsorption of five candidate biodegradable WSPs with varying chemistry, charge, and polarity characteristics (i.e., dextran, diethylaminoethyl dextran, carboxymethyl dextran, polyethylene glycol monomethyl ether, and poly-l-lysine) and of one nonbiodegradable WSP (poly(acrylic acid)) to sand and iron oxide-coated sand particles that represent important soil minerals. Combined adsorption studies using solution-depletion measurements, direct surface adsorption techniques, and column transport experiments over varying solution pH and ionic strengths revealed electrostatics dominating interactions of charged WSPs with the sorbents as well as WSP conformations and packing densities in the adsorbed states. Hydrogen bonding controls adsorption of noncharged WSPs. Under transport in columns, WSP adsorption exhibited fast and slow kinetic adsorption regimes with time scales of minutes to hours. Slow adsorption kinetics in soil may lead to enhanced transport but also shorter lifetimes of biodegradable WSPs, assuming more rapid biodegradation when dissolved than adsorbed. This work establishes a basis for understanding the coupled adsorption and biodegradation dynamics of biodegradable WSPs in agricultural soils.

Recent Advances in Fluorescence Recovery after Photobleaching for Decoupling Transport and Kinetics of Biomacromolecules in Cellular Physiology

Among the new molecular tools available to scientists and engineers, some of the most useful include fluorescently tagged biomolecules. Tools, such as green fluorescence protein (GFP), have been applied to perform semi-quantitative studies on biological signal transduction and cellular structural dynamics involved in the physiology of healthy and disease states. Such studies focus on drug pharmacokinetics, receptor-mediated endocytosis, nuclear mechanobiology, viral infections, and cancer metastasis. In 1976, fluorescence recovery after photobleaching (FRAP), which involves the monitoring of fluorescence emission recovery within a photobleached spot, was developed. FRAP allowed investigators to probe two-dimensional (2D) diffusion of fluorescently-labelled biomolecules. Since then, FRAP has been refined through the advancements of optics, charged-coupled-device (CCD) cameras, confocal microscopes, and molecular probes. FRAP is now a highly quantitative tool used for transport and kinetic studies in the cytosol, organelles, and membrane of a cell. In this work, the authors intend to provide a review of recent advances in FRAP. The authors include epifluorescence spot FRAP, total internal reflection (TIR)/FRAP, and confocal microscope-based FRAP. The underlying mathematical models are also described. Finally, our understanding of coupled transport and kinetics as determined by FRAP will be discussed and the potential for future advances suggested.

Biosupramolecular complexes of amphiphilic photosensitizers with human serum albumin and cucurbit[7]uril as carriers for photodynamic therapy

In the present work, we evaluated the supramolecular interactions between three photosensitizers, namely toluidine blue O (TBO, positively charged) and two fatty acid conjugates of 6 and 14 carbon atoms chain lengths (TBOC6 and TBOC14), with human serum albumin (HSA) and the macrocycle cucurbit[7]uril (CB[7]), alone or in combination within a biosupramolecular system as potential carriers of photosensitizers for Photodynamic therapy (PDT). Binding studies were carried out using photophysical and calorimetric techniques and accompanied with molecular docking simulations. Amphiphilic photosensitizers, particularly TBOC14, showed stronger binding to HSA and (CB[7]). Comparing the different delivery systems, (CB[7]) had a marginal effect on cell uptake and phototoxicity in HeLa cells, while HSA showed enhanced cell uptake with phototoxicities that depended on the photosensitizer. Despite low cell uptake, the combination of both (CB[7]) and HSA was the most phototoxic, which illustrates the potential of combining these systems for PDT applications.

Estimating and leveraging protein diffusion on ion-exchange resin surfaces

Protein mobility at solid-liquid interfaces can affect the performance of applications such as bioseparations and biosensors by facilitating reorganization of adsorbed protein, accelerating molecular recognition, and informing the fundamentals of adsorption. In the case of ion-exchange chromatographic beads with small, tortuous pores, where the existence of surface diffusion is often not recognized, slow mass transfer can result in lower resin capacity utilization. We demonstrate that accounting for and exploiting protein surface diffusion can alleviate the mass-transfer limitations on multiple significant length scales. Although the surface diffusivity has previously been shown to correlate with ionic strength (IS) and binding affinity, we show that the dependence is solely on the binding affinity, irrespective of pH, IS, and resin ligand density. Different surface diffusivities give rise to different protein distributions within the resin, as characterized using confocal microscopy and small-angle neutron scattering (length scales of micrometer and nanometer, respectively). The binding dependence of surface diffusion inspired a protein-loading approach in which the binding affinity, and hence the surface diffusivity, is modulated by varying IS. Such gradient loading increased the protein uptake efficiency by up to 43%, corroborating the importance of protein surface diffusion in protein transport in ion-exchange chromatography.

Single-molecule electrochemistry in nanochannels: probing the time of first passage

The diffusive mass transport of individual redox molecules was probed experimentally in microfabricated nanogap electrodes. The residence times for molecules inside a well-defined detection volume were extracted and the resulting distribution was compared with quantitative analytical predictions from random-walk theory for the time of first passage. The results suggest that a small number of strongly adsorbing sites strongly influence mass transport at trace analyte levels.

Moringa oleifera Seed Protein Adsorption to Silica: Effects of Water Hardness, Fractionation, and Fatty Acid Extraction

Motivated by the proposed use of cationic protein-modified sand for water filtration in developing nations, this study concerns the adsorption of Moringa oleifera seed proteins to silica surfaces. These proteins were prepared in model waters of varying hardness and underwent different levels of fractionation, including fatty acid extraction and cation exchange chromatography. Adsorption isotherms were measured by ellipsometry, and the zeta potentials of the resulting protein-decorated surfaces were measured by the rotating disk streaming potential method. The results indicate that the presence of fatty acids has little effect on the M. oleifera cationic protein adsorption isotherm. Adsorption from the unfractionated extract was indistinguishable from that of the cationic protein isolates at low concentrations but yielded significantly greater extents of adsorption at high concentrations. Adsorption isotherms for samples prepared in model hard and soft fresh waters were indistinguishable from each other over the measured bulk solution concentration range, but adsorption from hard or soft water was more extensive than adsorption from deionized water at moderate protein concentrations. Streaming potential measurements showed that adsorption reversed the net sign of the zeta potential of silica from negative to positive for all protein fractions and water hardness conditions at protein bulk concentrations as low as 0.03 μg/mL. This suggests that sands can be effectively modified with M. oleifera proteins using small amounts of seed extract under various local water hardness conditions. Finally, ellipsometry indicated that M. oleifera proteins adsorb irreversibly with respect to rinsing in these model fresh waters, suggesting that the modified sand would be stable on repeated use for water filtration. These studies may aid in the design of a simple, effective, and sustainable water purification device for developing nations.

Energy Landscape of Negatively Charged BSA Adsorbed on a Negatively Charged Silica Surface

We study the energy landscape of the negatively charged protein bovine serum albumin adsorbed on a negatively charged silica surface at pH 7. We use fully atomistic molecular dynamics (MD) and steered MD (SMD) to probe the energy of adsorption and the pathway for the surface diffusion of the protein and its associated activation energy. We find an adsorption energy ∼1.2 eV, which implies that adsorption is irreversible even on experimental time scales of hours. In contrast, the activation energy for surface diffusion is ∼0.4 eV so that it is observable on the MD simulation time scale of 100 ns. This analysis paves the way for a more detailed understanding of how a protein layer forms on biomaterial surfaces, even when the protein and surface share the same electrical polarity.

Advances, challenges and opportunities for point-of-need screening of mycotoxins in foods and feeds

Recent advances in analytical methods for mycotoxin screening in foods and feeds are reviewed, focusing on point-of-need detection using integrated devices.

Multivalent-Ion-Activated Protein Adsorption Reflecting Bulk Reentrant Behavior

Protein adsorption at the solid-liquid interface is an important phenomenon that often can be observed as a first step in biological processes. Despite its inherent importance, still relatively little is known about the underlying microscopic mechanisms. Here, using multivalent ions, we demonstrate the control of the interactions and the corresponding adsorption of net-negatively charged proteins (bovine serum albumin) at a solid-liquid interface. This is demonstrated by ellipsometry and corroborated by neutron reflectivity and quartz-crystal microbalance experiments. We show that the reentrant condensation observed within the rich bulk phase behavior of the system featuring a nonmonotonic dependence of the second virial coefficient on salt concentration c_{s} is reflected in an intriguing way in the protein adsorption d(c_{s}) at the interface. Our findings are successfully described and understood by a model of ion-activated patchy interactions within the framework of the classical density functional theory. In addition to the general challenge of connecting bulk and interface behavior, our work has implications for, inter alia, nucleation at interfaces.

Simple bioconjugate chemistry serves great clinical advances: albumin as a versatile platform for diagnosis and precision therapy

Albumin is the most abundant circulating protein in plasma and has recently emerged as a versatile protein carrier for drug targeting and for improving the pharmacokinetic profile of peptide or protein based drugs. Three drug delivery technologies related to albumin have been developed, which include the coupling of low-molecular weight drugs to exogenous or endogenous albumin, conjugating bioactive proteins by albumin fusion technology (AFT), and encapsulation of drugs into albumin nanoparticles. This review article starts with a brief introduction of human serum albumin (HSA), and then summarizes the mainstream chemical strategies of developing HSA binding molecules for coupling with drug molecules. Moreover, we also concisely condense the recent progress of the most important clinical applications of HSA-binding platforms, and specify the current challenges that need to be met for a bright future of HSA-binding.

Heterogeneous surfaces to repel proteins

The nonspecific adsorption of proteins is usually undesirable on solid surfaces as it induces adverse responses, such as platelet adhesion on medical devices, negative signals of biosensors and contamination blockage of filtration membranes. Thus, an important scheme in material science is to design and fabricate protein-repulsive surfaces. Early approaches in this field focused on homogeneous surfaces comprised of single type functionality. Yet, recent researches have demonstrated that surfaces with heterogeneities (chemistry and topography) show promising performance against protein adsorption. In this review, we will summarize the recent achievements and discuss the new perspectives in the research of developing and characterizing heterogeneous surfaces to repel proteins. The protein repulsion mechanisms of different heterogeneous surfaces will also be discussed in details, followed by the perspective and challenge of this emerging field.

Effects of polythiophene surface structure on adsorption and conformation of bovine serum albumin: a multivariate and multitechnique study

Protein interactions with surfaces of promising conducting polymers are critical for development of bioapplications. Surfaces of spin-cast and postbaked poly(3-alkylthiophenes), regiorandom P3BT, and regioregular RP3HT are examined prior to and after adsorption of model protein, bovine serum albumin, with time-of-flight secondary ion mass spectrometry, atomic force microscopy, and X-ray photoelectron spectroscopy. The multivariate method of principal component analysis applied to ToF-SIMS data maximizes information on subtle differences in surface chemistry: PCA reveals alkyl side chains and conjugated backbones, exposed for RP3HT and P3BT, respectively. Phase imaging AFM shows semicrystalline microstructure of RP3HT and amorphous morphology of P3BT films. A cellular-like pattern of proteins adsorbed on RP3HT develops with coverage to more uniform overlayer, observed always on P3BT. The amount of adsorbed protein, determined by XPS as a function of BSA concentration (up to 10 mg/mL), is ∼21% lower for RP3HT than P3BT (up to 1.1 mg/m(2)). Although PCA differentiates protein from polythiophene, relative protein surface composition evaluated from ToF-SIMS saturates rather than increases with amount of adsorbed BSA from XPS. This reflects ToF-SIMS sensitivity to outermost layer of proteins, enabling multivariate analysis of protein conformation or orientation. PCA distinguishes between amino acids characteristic for external regions of BSA adsorbed to P3BT and RP3HT. These amino acids are identified for P3BT and RP3HT as hydrophilic and hydrophobic, respectively, by relative hydrophobicity of amino acid side chains. Alternative identification with BSA domains fails, pointing to substrate-induced changes in conformation and degree of denaturation rather than orientation of adsorbed protein.

Monte Carlo simulation of protein adsorption on energetically heterogeneous surfaces

The modified triangular-well potential model was applied to incorporate the effect of surface energy on the adsorption of particles or proteins on energetically heterogeneous surfaces. The method is convenient in simulating the adsorption on heterogeneous surface of which different region possesses different free energy. Spherical particles with attractive forces were added on the surface and underwent surface diffusion before they were quenched in place. It was seen that the ratio of surface energies of two regions had to be greater than 10 in order to simulate the adsorption in which the particles were selectively adsorbed on a favorable area. At a fixed ratio of surface energies, the obtained structures were similar. If the ratio was less than 10, the probability of adsorption on any site on the surface was not much different so the adsorption would be homogeneous adsorption. The method, thus, could be applied widely to simulate the adsorption of various conditions.

A bottom-up approach to understanding protein layer formation at solid-liquid interfaces

A common goal across different fields (e.g. separations, biosensors, biomaterials, pharmaceuticals) is to understand how protein behavior at solid-liquid interfaces is affected by environmental conditions. Temperature, pH, ionic strength, and the chemical and physical properties of the solid surface, among many factors, can control microscopic protein dynamics (e.g. adsorption, desorption, diffusion, aggregation) that contribute to macroscopic properties like time-dependent total protein surface coverage and protein structure. These relationships are typically studied through a top-down approach in which macroscopic observations are explained using analytical models that are based upon reasonable, but not universally true, simplifying assumptions about microscopic protein dynamics. Conclusions connecting microscopic dynamics to environmental factors can be heavily biased by potentially incorrect assumptions. In contrast, more complicated models avoid several of the common assumptions but require many parameters that have overlapping effects on predictions of macroscopic, average protein properties. Consequently, these models are poorly suited for the top-down approach. Because the sophistication incorporated into these models may ultimately prove essential to understanding interfacial protein behavior, this article proposes a bottom-up approach in which direct observations of microscopic protein dynamics specify parameters in complicated models, which then generate macroscopic predictions to compare with experiment. In this framework, single-molecule tracking has proven capable of making direct measurements of microscopic protein dynamics, but must be complemented by modeling to combine and extrapolate many independent microscopic observations to the macro-scale. The bottom-up approach is expected to better connect environmental factors to macroscopic protein behavior, thereby guiding rational choices that promote desirable protein behaviors.

Interfacial protein-protein associations

While traditional models of protein adsorption focus primarily on direct protein-surface interactions, recent findings suggest that protein-protein interactions may play a central role. Using high-throughput intermolecular resonance energy transfer (RET) tracking, we directly observed dynamic, protein-protein associations of bovine serum albumin on polyethylene glycol modified surfaces. The associations were heterogeneous and reversible, and associating molecules resided on the surface for longer times. The appearance of three distinct RET states suggested a spatially heterogeneous surface - with areas of high protein density (i.e., strongly interacting clusters) coexisting with mobile monomers. Distinct association states exhibited characteristic behavior, i.e., partial-RET (monomer-monomer) associations were shorter-lived than complete-RET (protein-cluster) associations. While the fractional surface area covered by regions with high protein density (i.e., clusters) increased with increasing concentration, the distribution of contact times between monomers and clusters was independent of solution concentration, suggesting that associations were a local phenomenon, and independent of the global surface coverage.

Kinetics of epidermal growth factor/receptor binding on cells measured by total internal reflection/fluorescence recovery after photobleaching

Total internal reflection fluorescence (TIRF) microscopy is used to measure the dissociation kinetic rate of fluorescein-labeled epidermal growth factor from its specific receptors on the surface of intact but mildly fixed A431 human epidermoid cells in culture. Prior applications of TIRF microscopy have been limited to nonreceptor binding or to model membrane systems. The evanescent field excites fluorescence selectively at the surface of the cell proximal to the coverslip. "Prismless" epiillumination TIR is employed to avoid space limitations and is achieved by passing the excitation laser beam through a high (1.4)-aperture objective so that the light is incident at the glass/water interface beyond the critical angle. Long-term focus is maintained by a special feedback system. Of the possible effects that can influence the time course of the postbleach fluorescence recoveries-the EGF/receptor dissociation ratek 2, the bulk solution diffusion rate of EGF, and the cell surface motion of the receptors-we infer that the dissociation ratek 2 dominates. Several fitting schemes are compared and indicate the presence of a multiplicity of values fork 2, ranging from about 0.05 to 0.004 s(-1), with an average value of about 0.012 s(-1). These results compare well with values previously obtained by radiolabel/washing techniques. The significance of the results in terms of kinetic models and the advantages of the TIRF technique for these sorts of measurements are discussed.

Do clustering monoclonal antibody solutions really have a concentration dependence of viscosity?

Protein solution rheology data in the biophysics literature have incompletely identified factors that govern hydrodynamics. Whereas spontaneous protein adsorption at the air/water (A/W) interface increases the apparent viscosity of surfactant-free globular protein solutions, it is demonstrated here that irreversible clusters also increase system viscosity in the zero shear limit. Solution rheology measured with double gap geometry in a stress-controlled rheometer on a surfactant-free Immunoglobulin solution demonstrated that both irreversible clusters and the A/W interface increased the apparent low shear rate viscosity. Interfacial shear rheology data showed that the A/W interface yields, i.e., shows solid-like behavior. The A/W interface contribution was smaller, yet nonnegligible, in double gap compared to cone-plate geometry. Apparent nonmonotonic composition dependence of viscosity at low shear rates due to irreversible (nonequilibrium) clusters was resolved by filtration to recover a monotonically increasing viscosity-concentration curve, as expected. Although smaller equilibrium clusters also existed, their size and effective volume fraction were unaffected by filtration, rendering their contribution to viscosity invariant. Surfactant-free antibody systems containing clusters have complex hydrodynamic response, reflecting distinct bulk and interface-adsorbed protein as well as irreversible cluster contributions. Literature models for solution viscosity lack the appropriate physics to describe the bulk shear viscosity of unstable surfactant-free antibody solutions.

Apparent activation energies associated with protein dynamics on hydrophobic and hydrophilic surfaces

With the use of single-molecule total internal reflection fluorescence microscopy (TIRFM), the dynamics of bovine serum albumin (BSA) and human fibrinogen (Fg) at low concentrations were observed at the solid-aqueous interface as a function of temperature on hydrophobic trimethylsilane (TMS) and hydrophilic fused silica (FS) surfaces. Multiple dynamic modes and populations were observed and characterized by their surface residence times and squared-displacement distributions (surface diffusion). Characteristic desorption and diffusion rates for each population/mode were generally found to increase with temperature, and apparent activation energies were determined from Arrhenius analyses. The apparent activation energies of desorption and diffusion were typically higher on FS than on TMS surfaces, suggesting that protein desorption and mobility were hindered on hydrophilic surfaces due to favorable protein-surface and solvent-surface interactions. The diffusion of BSA on TMS appeared to be activationless for several populations, whereas diffusion on FS always exhibited an apparent activation energy. All activation energies were small in absolute terms (generally only a few kBT), suggesting that most adsorbed protein molecules are weakly bound and move and desorb readily under ambient conditions.

Subsurface influence on the structure of protein adsorbates as revealed by in situ X-ray reflectivity

The adsorption process of proteins to surfaces is governed by the mutual interactions among proteins, the solution, and the substrate. Interactions arising from the substrate are usually attributed to the uppermost atomic layer. This actual surface defines the surface chemistry and hence steric and electrostatic interactions. For a comprehensive understanding, however, the interactions arising from the bulk material also have to be considered. Our protein adsorption experiments with globular proteins (α-amylase, bovine serum albumin, and lysozyme) clearly reveal the influence of the subsurface material via van der Waals forces. Here, a set of functionalized silicon wafers enables a distinction between the effects of surface chemistry and the subsurface composition of the substrate. Whereas the surface chemistry controls whether the individual proteins are denatured, the strength of the van der Waals forces affects the final layer density and hence the adsorbed amount of proteins. The results imply that van der Waals forces mainly influence surface processes, which govern the structure formation of the protein adsorbates, such as surface diffusion and spreading.

Evidence of a mobile precursor state in nonspecific protein adsorption

We study the dynamics of nonspecific protein adsorption using nanometer-micrometer-scale patterns involving hydrophobic domains in hydrophilic matrices. We report the discovery of a critical requirement for the sizes of the hydrophobic/adhesive pads for protein adsorption: the area of each adhesive pad must be more than 2 orders of magnitude larger than the footprint of a protein molecule before irreversible adsorption occurs. We attribute this to the minimal surface area sampled by a mobile protein molecule in a precursor state before irreversible adsorption occurs. Kinetic analysis based on the precursor model quantitatively accounts for the experimental observation and reveals that the distance sampled by the mobile precursor state before irreversible adsorption increases with the size of the protein molecule.

Adsorption and diffusion of plasma proteins on hydrophilic and hydrophobic surfaces: effect of trifluoroethanol on protein structure

The aim of this work was to investigate the conformational changes and diffusion of adsorbed proteins (immunoglobulin G (IgG), fibrinogen (Fib) and human serum albumin (HSA)) on hydrophilic quartz and hydrophobized quartz (octadecyltrichlorosilane (OTS)) surfaces. Circular dichroism spectroscopy measurements have shown that IgG is the most stable protein after adsorption on hydrophilic quartz, whereas HSA and Fib unfold. The structural changes are dependent on adsorption time, initial protein concentration in bulk, and surface chemistry. The effect of trifluoroethanol (TFE) in recovering the original protein structure after adsorption was analyzed by total internal reflection fluorescence and fluorescence recovery after photobleaching (TIRF-FRAP). TIRF-FRAP experiments revealed a strong dependence of the surface chemistry on protein diffusion coefficients: proteins diffuse 4 times slower on hydrophobic surfaces than on hydrophilic surfaces. The diffusion coefficient of TFE at hydrophobic surfaces is 2 orders magnitude higher than at hydrophilic surfaces. However, protein desorption occurs faster on hydrophilic quartz than on OTS, proving that the strength of protein-surface interaction is weaker at hydrophilic surfaces. This result shows that desorption is determined by surface/protein chemistry and not by mass transfer limitations. FTIR-ATR results demonstrated that TFE interaction with adsorbed proteins is stronger at hydrophilic surfaces than at hydrophobic surfaces.

Particle tracking single protein-functionalized quantum dot diffusion and binding at silica surfaces

We evaluate commercial QD585 and QD605 streptavidin-functionalized quantum dots (QDs) for single-particle tracking microscopy at surfaces using total internal reflectance fluorescence and measure single QD diffusion and nonspecific binding at silica surfaces in static and flow conditions. The QD diffusion coefficient on smooth, near-ideal, highly hydroxylated silica surfaces is near bulk-solution diffusivity, as expected for repulsive surfaces, but many QD trajectories on rougher, less-than-ideal surfaces or regions display transient adsorptions. We attribute the binding to defect sites or adsorbates, possibly in conjunction with protein conformation changes, and estimate binding energies from the transient adsorption lifetimes. We also assess QD parameters relevant to tracking, including hydrodynamic radius, charge state, signal levels, blinking reduction with reducing solutions, and photoinduced blueing and bleaching.

Kinetics of adsorption and proteolytic cleavage of a multilayer ovalbumin film by subtilisin Carlsberg

Adsorption and proteolytic activity of the enzyme subtilisin Carlsberg have been studied on an immobilized, multilayer ovalbumin film. The cross-linked multilayer substrate permits protease adsorption to be examined unencumbered by the surface inhomogeneity typically observed in monolayer studies of protease surface kinetics. Decline of the protein film was measured over time using ellipsometry. Resulting kinetic data as a function of aqueous enzyme concentration and temperature were well fit by a Langmuir-Michaelis-Menten model for surface proteolysis. We observed that both the protein degradation kinetics and the in situ adsorption data were well described by the proposed model. The temperature dependence of the kinetic rate parameter yielded an activation energy of 12 kcal/mol. Further, the apparent Langmuir adsorption equilibrium constant of the enzyme at the protein/aqueous interface was 0.11 L/mg at 22 degrees C, 0.034 L/mg at 36 degrees C, and 0.011 L/mg at 50 degrees C. Although enzyme adsorption at a given aqueous enzyme concentration decreased at higher temperature, the enzyme cleaved the substrate more rapidly, leading to a net increase in the ovalbumin film degradation rate. We observed that the maximum enzyme coverage on the immobilized protein surface was approximately 40% of a close-packed monolayer at ambient temperature (22 degrees C).

Conformational flexibility of a model protein upon immobilization on self-assembled monolayers

The present study reports on the retention of conformational flexibility of a model allosteric protein upon immobilization on self-assembled monolayers (SAMs) on gold. Organothiolated SAMs of different compositions were utilized for adsorptive and covalent attachment of bovine liver glutamate dehydrogenase (GDH), a well-characterized allosteric enzyme. Sensitive fluorimetric assays were developed to determine immobilization capacity, specific activity, and allosteric properties of the immobilized preparations as well as the potential for repeated use and continuous catalytic transformations. The allosteric response of the free and immobilized forms towards ADP, L-leucine and high concentrations of NAD(+), some of the well-known activators for this enzyme, were determined and compared. The enzyme immobilized by adsorption or chemical binding responded similarly to the activators with a greater degree of activation, as compared to the free form. Also loss of activity involving the two immobilization procedures were similar, suggesting that residues essential for catalytic activity or allosteric properties of GDH remained unchanged in the course of chemical modification. A recently established method was used to predict GDH orientation upon immobilization, which was found to explain some of the experimental results presented. The general significance of these observations in connection with retention of native properties of protein structures upon immobilization on SAMs is discussed.

Controlling surface mobility in interdiffusing polyelectrolyte multilayers

The phenomenon of interdiffusion of polyelectrolytes during electrostatic layer-by-layer assembly has been extensively investigated in the past few years owing to the intriguing scientific questions that it poses and the technological impact of interdiffusion on the promising area of electrostatic assembly processes. In particular, interdiffusion can greatly affect the final morphology and structure of the desired thin films, including the efficacy and function of thin film devices created using these techniques. Although there have been several studies on the mechanism of film growth, little is known about the origin and controlling factors of interdiffusion phenomena. Here, we demonstrate a simple but robust method of observing the process of polyelectrolyte interdiffusion by adsorbing charged viruses onto the surface of polyelectrolyte multilayers. The surface mobility of the underlying polycation enables the close-packing of viruses adsorbed electrostatically to the film so as to achieve a highly packed structure. The ordering of viruses can be controlled by the manipulation of the deposition pH of the underlying polyelectrolyte multilayers, which ultimately controls the thickness of each layer, effective ionic cross-link density of the film, and the surface charge density of the top surface. Characterization of the films assembled at different pH values were carried out to confirm that increased quantities of the mobile polycation LPEI incorporated at higher pH adsorption conditions are responsible for the ordered assembly of viruses. The surface mobility of viruses atop the underlying polyelectrolyte multilayers was determined using fluorescence recovery after photobleaching technique, which leads to estimate of the diffusion coefficient on the order of 0.1 microm(2)/sec for FITC-labeled viruses assembled on polyelectrolyte multilayers.

Protein adsorption on materials for recording sites on implantable microelectrodes

Implantable microelectrodes have the potential to become part of neural prostheses to restore lost nerve function after nerve damage. The initial adsorption of proteins to materials for implantable microelectrodes is an important factor in determining the longevity and stability of the implant. Once an implant is in the body, protein adsorption takes place almost instantly before the cells reach the surface of an implant. The aim of this study was to identify an optimum material for electrode recording sites on implantable microelectrodes. Common materials for electrode sites are gold, platinum, iridium, and indium tin oxide. These, along with a reference material (titanium), were investigated. The thickness and the structure of adsorbed proteins on these materials were measured using a combination of atomic force microscopy and ellipsometry. The adsorbed protein layers on gold (after 7 and 28 days of exposure to serum) were the smoothest and the thinnest compared to all the other substrate materials, indicating that gold is the material of choice for electrode recording sites on implantable microelectrodes. However, the results also show that indium tin oxide might also be a good choice for these applications.

Sequential quenching of randomly deposited ellipsoids: Anisotropy and spatial patterns

We studied the sequential quenching of prolate ellipsoids on a homogeneous surface by using our proposed pair potential whose repulsive part was that of hard ellipses and attractive part was the r-6 pairwise attraction. Both the strength and range parameters for the attraction were functions of the orientations of the pair of ellipses and related by epsilon proportional, variantsigma-lambda. The parameter lambda determines the relative strength of the side-by-side and end-to-end attractions and thus plays an important role in determining the alignment of the particles. We adopted the value lambda=2.19 by using point-energy additivity to compute the minimum energies for both of these configurations for a pair of ellipsoids of revolution with aspect ratio of 2:1. We investigated the effect of temperature and the parameter lambda on the alignments of ellipses. Both radial distribution function, g(r), and orientational correlation function, G(r), show the expected longer ranges of orientational correlation at lower temperatures and show higher degree of orientational order for lambda=3.5 than lambda=2.19 and 0.10. This can also be seen in the examples of configurations showing that for lambda=3.5, ellipses are more aligned than lambda=2.19 and 0.1.

Aggregation of lysozyme and of poly(ethylene glycol)-modified lysozyme after adsorption to silica

Surface-induced aggregation is a common instability during protein storage, delivery and purification. This aggregation can lead to the formation of fibrils rich in intermolecular beta-sheet structure. Techniques to probe surface-clustering are limited. Here we use protein intrinsic fluorescence and thioflavin T probe fluorescence in a total internal reflection fluorescence (TIRF) sampling geometry to simultaneously monitor the kinetics of adsorption and aggregation for chicken egg lysozyme on a silica surface. We observe a slow surface-induced aggregation process that continues well after the lysozyme adsorption kinetics have plateaued. The rate of surface-induced aggregation is independent of the lysozyme concentration in solution. Consistent with the clustering observed via thioflavin T fluorescence, infrared amide I band spectra also show a 1.5-fold increase in intermolecular beta-sheet content upon lysozyme adsorption. Tryptophan emission spectra show no evidence for any tertiary structural change upon adsorption. Furthermore, we observe that the covalent modification of lysozyme with a single poly(ethylene glycol) (PEG) grafted chain does not inhibit aggregation on the surface, but a second PEG graft significantly inhibits the intermolecular beta-sheet formation.

Sub-monolayer growth by sequential deposition of particles

We studied the various stages in the preparation of a sub-monolayer film by the sequential deposition of particles (sequential quenching model) from very low to very high temperatures, a limit at which the system becomes equivalent to random sequential adsorption. Due to the finite size of the simulation box, only one cluster can be found in a system at very low temperatures (T* = 0.01 and 0.1) and its size grows linearly with increasing density. At higher temperatures (T* = 0.3 or higher), on the other hand, the same systems show a crossover from a nucleation regime to a growth regime. It is best revealed in a plot showing the distribution of monomers on the surface versus density, where the crossover region appears as a peak on a curve. At densities above the crossover, any new addition tends to contribute to the growth of an existing cluster rather than to nucleating new one.

Analysis of mass transport models for protein adsorption to cation exchanger by visualization with confocal laser scanning microscopy

The mass transfer of bovine serum albumin (BSA) to a cation exchanger, SP Sepharose FF, has been studied by finite batch adsorption experiments. The uptake curve was simulated with three mass transport models (i.e., effective pore diffusion model, surface diffusion model and Maxwell-Stefan model) incorporating the particle size distribution of the adsorbent particles. All the three models can simulate the uptake curves reasonably well. However, how well these models could simulate the real concentration profile within the adsorbent particle cannot be verified by the fitness of the models to the uptake curve. Thus, confocal laser scanning microscopy (CLSM) was used to visualize protein uptake to the porous adsorbent particles during the batch experiments. Using a fluorescent dye-labeled bovine serum albumin (BSA) for the dynamic adsorption experiments, the radial concentration profiles of the labeled BSA molecules into individual adsorbent particles at different times were obtained from the CLSM images. The protein distribution profiles within various particle diameters at different time were compared with the radial protein distributions predicted from the models. It reveals that surface diffusion model describes the intraparticle protein concentration profiles better than the other two models.

Lipase surface diffusion studied by fluorescence recovery after photobleaching

We have analyzed surface diffusion properties of a variant of Thermomyces lanuginosa lipase (TLL) on hydrophilic silica and silica methylated with dichlorodimethylsilane (DDS) or octadecyltrichlorosilane (OTS). For this study a novel method for analysis of diffusion on solid surfaces was developed. The method is based on fluorescence recovery after photobleaching using confocal microscopy. When a rectangular area of the sample was photobleached, fluorescence recovery could be analyzed as one-dimensional diffusion, resulting in simplified mathematical expressions for fitting the data. The method was initially tested by measuring bovine serum albumin diffusion on glass, which led to a diffusion coefficient in good correspondence to earlier reports. For the analysis of TLL diffusion, ellipsometry data of TLL adsorption were used to calibrate fluorescence intensity to surface density of lipase, enabling measurements of the diffusion coefficient at different surface densities. The average diffusion coefficient was calculated in two time intervals after adsorption. Mobile fraction and diffusion coefficient were lowest on the OTS surface, when extrapolated to infinite surface dilution. Moreover, the diffusion rate decreased with time on the hydrophobic surfaces. Our observations can be explained by the surface dependence on the distribution of orientations and conformations of adsorbed TLL, where the transition from the closed to the catalytically active open and more hydrophobic structure is important.

Application of fluorescence recovery after photobleaching to diffusion of a polyelectrolyte in a multilayer film

The diffusion coefficient, D, and the proportion of mobile molecules, p, is measured for a fluorescently labeled polyelectrolyte in a multilayer film using fluorescence recovery after photobleaching (FRAP). The film was composed of poly(L-lysine) (PLL) and hyaluronan (HA). The labeled polyelectrolyte (PLL(FITC)) was either deposited on top of the film or embedded within it. A circular area of diameter approximately 60 microm was bleached using a confocal laser scanning microscope. Because molecules do already diffuse during the bleaching step, the initial light intensity profile is not characteristic of a uniformly bleached circular area. A formalism is developed in which a simple mathematical representation of a measured profile serves as starting profile. This radial distribution is introduced in the equation describing the time evolution of the labeled molecule concentration under the hypothesis that the recovery results from pure, two-dimensional Brownian diffusion of the mobile molecules according to Fick's law. The analysis of a series of images taken at successive times after bleaching (up to approximately one hour) leads to estimates of D of the order of 0.2 microm(2) s(-1) for labeled molecules deposited on top of the film and a 5-fold smaller value for the molecules embedded in the film. However, p is remarkably insensitive to the position in the multilayer architecture (p approximately 0.40).