The adsorption of proteins at the solid/liquid interface

Adhesion of Norovirus to Surfaces: Contribution of Thermodynamic and Molecular Properties Using Virus-Like Particles

The aim of the study was to assess human norovirus and feline calicivirus (FCV) surface free energy, hydrophobicity, and ability to interact with fresh foods and food-contact surfaces. Virus-like particles (VLPs) of human norovirus (GI.1 and GII.4) and FCV were produced, purified, and analyzed for their surface free energy, hydrophobicity, and the total interfacial free energy of interaction [Formula: see text] with lettuce, strawberry, polyethylene, and stainless steel. GII.4 VLPs were further tested for adhesion at different pH, ionic strengths, and temperature. All the VLPs and the test materials showed low surface energies, as well as hydrophobic characters except for GI.1. Nearly all [Formula: see text] values were propitious for spontaneous adhesion. GII.4 VLPs adsorbed almost indifferently to stainless steel, polyethylene, and lettuce. Isoelectric point and high temperature generally promoted adhesion while ionic strength effect was surface-dependant. According to this study, all the materials assessed are of low-energy and hydrophobic nature except GI.1 VLPs. Interfacial free energies of interaction were favorable for spontaneous adhesion ([Formula: see text] < 0) of all VLPs to the test materials, except for GI.1 VLPs to both stainless steel and straweberry. It is also found that norovirus adhesion is more sensitive to physicochemical conditions than to surface character itself.

Nonspecific Binding-Fundamental Concepts and Consequences for Biosensing Applications

Nature achieves differentiation of specific and nonspecific binding in molecular interactions through precise control of biomolecules in space and time. Artificial systems such as biosensors that rely on distinguishing specific molecular binding events in a sea of nonspecific interactions have struggled to overcome this issue. Despite the numerous technological advancements in biosensor technologies, nonspecific binding has remained a critical bottleneck due to the lack of a fundamental understanding of the phenomenon. To date, the identity, cause, and influence of nonspecific binding remain topics of debate within the scientific community. In this review, we discuss the evolution of the concept of nonspecific binding over the past five decades based upon the thermodynamic, intermolecular, and structural perspectives to provide classification frameworks for biomolecular interactions. Further, we introduce various theoretical models that predict the expected behavior of biosensors in physiologically relevant environments to calculate the theoretical detection limit and to optimize sensor performance. We conclude by discussing existing practical approaches to tackle the nonspecific binding challenge in vitro for biosensing platforms and how we can both address and harness nonspecific interactions for in vivo systems.

The Effect of Radiolabeling of Human Fibrinogen on Its Adsorption Behaviour on a Polystyrene Surface

Human fibrinogen (HFB) was labeled with different radioactive labels (Technetium −99m and Iodine −125) in various ways. Characterization by chromatographic and electrophoretic methods did not show differences between the labeled and the nonlabeled proteins.

The effect of the label and the labeling method on the adsorption behaviour of 99mTc and 125l labeled HFB at a polystyrene surface was investigated.

In all cases labeled HFB showed preferential adsorption as compared to nonlabeled HFB. The preferential adsorption was expressed in terms of a factor ø (van der Scheer et al. 1978a), which will be 1, when no preferential adsorption occurs. 99mTc – and 125| – HFB showed ø values from 1.48 – 1.88. It is concluded that only meaningful adsorption experiments with labeled proteins can be performed when the possible occurrence of preferential adsorption has been investigated by appropriate methods. The results of prior work on protein adsorption at biomaterials using radiolabeled proteins have to be reconsidered.

A microfluidic thermometer: Precise temperature measurements in microliter- and nanoliter-scale volumes

Measuring the temperature of a sample is a fundamental need in many biological and chemical processes. When the volume of the sample is on the microliter or nanoliter scale (e.g., cells, microorganisms, precious samples, or samples in microfluidic devices), accurate measurement of the sample temperature becomes challenging. In this work, we demonstrate a technique for accurately determining the temperature of microliter volumes using a simple 3D-printed microfluidic chip. We accomplish this by first filling “microfluidic thermometer” channels on the chip with substances with precisely known freezing/melting points. We then use a thermoelectric cooler to create a stable and linear temperature gradient along these channels within a measurement region on the chip. A custom software tool (available as online Supporting Information) is then used to find the locations of solid-liquid interfaces in the thermometer channels; these locations have known temperatures equal to the freezing/melting points of the substances in the channels. The software then uses the locations of these interfaces to calculate the temperature at any desired point within the measurement region. Using this approach, the temperature of any microliter-scale on-chip sample can be measured with an uncertainty of about a quarter of a degree Celsius. As a proof-of-concept, we use this technique to measure the unknown freezing point of a 50 microliter volume of solution and demonstrate its feasibility on a 400 nanoliter sample. Additionally, this technique can be used to measure the temperature of any on-chip sample, not just near-zero-Celsius freezing points. We demonstrate this by using an oil that solidifies near room temperature (coconut oil) in a microfluidic thermometer to measure on-chip temperatures well above zero Celsius. By providing a low-cost and simple way to accurately measure temperatures in small volumes, this technique should find applications in both research and educational laboratories.

Chromatographic and traditional albumin isotherms on cellulose: a model for wound protein adsorption on modified cotton

Albumin is the most abundant protein found in healing wounds. Traditional and chromatographic protein isotherms of albumin binding on modified cotton fibers are useful in understanding albumin binding to cellulose wound dressings. An important consideration in the design of cellulosic wound dressings is adsorption and accumulation of proteins like albumin at the solid-liquid interface of the biological fluid and wound dressing fiber. To better understand the effect of fiber charge and molecular modifications in cellulose-containing fibers on the binding of serum albumin as observed in protease sequestrant dressings, albumin binding to modified cotton fibers was compared with traditional and chromatographic isotherms. Modified cotton including carboxymethylated, citrate-crosslinked, dialdehyde and phosphorylated cotton, which sequester elastase and collagenase, were compared for their albumin binding isotherms. Albumin isotherms on citrate-cellulose, cross-linked cotton demonstrated a two-fold increased binding affinity over untreated cotton. A comparison of albumin binding between traditional, solution isotherms and chromatographic isotherms on modified cellulose yielded similar equilibrium constants. Application of the binding affinity of albumin obtained in the in vitro protein isotherm to the in vivo wound dressing uptake of the protein is discussed. The chromatographic approach to assessment of albumin isotherms on modified cellulose offers a more rapid approach to evaluating protein binding on modified cellulose over traditional solution approaches.

A comparison of covalent immobilization and physical adsorption of a cellulase enzyme mixture

This paper reports the first use of a linker-free covalent approach for immobilizing an enzyme mixture. Adsorption from a mixture is difficult to control due to varying kinetics of adsorption, variations in the degree of unfolding and competitive binding effects. We show that surface activation by plasma immersion ion implantation (PIII) produces a mildly hydrophilic surface that covalently couples to protein molecules and avoids these issues, allowing the attachment of a uniform monolayer from a cellulase enzyme mixture. Atomic force microscopy (AFM) showed that the surface layer of the physically adsorbed cellulase layer on the mildly hydrophobic surface (without PIII) consisted of aggregated enzymes that changed conformation with incubation time. The evolution observed is consistent with the existence of transient complexes previously postulated to explain the long time constants for competitive displacement effects in adsorption from enzyme mixtures. AFM indicated that the covalently coupled bound layer to the PIII-treated surface consisted of a stable monolayer without enzyme aggregates, and became a double layer at longer incubation times. Light scattering analysis showed no indication of aggregates in the solution at room temperature, which indicates that the surface without PIII-treatment induced enzyme aggregation. A model for the attachment process of a protein mixture that includes the adsorption kinetics for both surfaces is presented.

Protein adsorption on derivatives of hyaluronic acid and subsequent cellular response

The modulation of biological interactions with artificial surfaces is a vital aspect of biomaterials research. Serum protein adsorption onto photoreactive hyaluronic acid (Hyal-N(3)) and its sulfated derivative (HyalS-N(3)) was analyzed to determine extent of protein interaction and protein conformation as well as subsequent cell adhesion. There were no significant (p < 0.01) differences in the amount of protein adsorbed to the two polymers; however, proteins were found to be more loosely bound on HyalS-N(3) compared with Hyal-N(3). Fibronectin was adsorbed onto HyalS-N(3) in such an orientation as to allow the availability of the cell binding region, while there was more restricted access to this region on fibronectin adsorbed onto Hyal-N(3). This was confirmed by reduced cell adhesion on fibronectin precoated Hyal-N(3) compared with fibronectin precoated HyalS-N(3). Minimal cell adhesion was observed on albumin and serum precoated Hyal-N(3). The quartz crystal microbalance confirmed that specific cell-surface interactions were experienced by cells interacting with the fibronectin precoated polymers and serum precoated HyalS-N(3).

Influences of properties of protein and adsorption surface on removal kinetics of protein adsorbed on metal surface by H2O2-electrolysis treatment

"H(2)O(2)-electrolysis" treatment is an alternative method for removing proteinaceous materials that are adsorbed to metal surfaces. The method is based on the generation of hydroxyl radicals by electrolysis of hydrogen peroxide and the subsequent decomposition of organic substances adhering to the metal surface. We herein investigated the influence of some parameters on the kinetics of protein removal by H(2)O(2)-electrolysis. These parameters included the properties of proteins and the type of metal surface. Sixteen types of proteins and nine types of metal surfaces were used. The removal of adsorbed protein from a metal surface during the treatment was monitored by ellipsometry. Apparent first-order rate constants for removal, k(c)(l), for various adsorption and treatment conditions were determined. The k(c)(l) value varied markedly with the type of protein and was also influenced by the pH used in the adsorption. The isoelectric point (pI) of protein used was found to be a major factor. The amount of adsorbed protein removed by a unit amount of (·)OH was larger for a metal surface with a lower pI. The impact of the properties of the protein and metal surface on the removal kinetics are discussed, focusing on relationships with the adsorption characteristics of the protein.

Fibrinogen adsorption onto macroporous polymeric surfaces: correlation with biocompatibility aspects

The present work focus on the adsorption of fibrinogen (Fgn) on to the semi-interpenetrating polymer networks (IPNs) of polyethylene glycol (PEG) and poly(2-hydroxyethyl methacrylate-co-acrylonitrile) and attempts to correlate the adsorption behaviour of proteins to the blood compatible aspects of the polymeric surfaces. The semi-IPNs were prepared by copolymerizing 2-hydroxyethyl methacrylate and acrylonitrile in the presence of PEG and a crosslinker ethyleneglycol dimethacrylate (EGDMA). The prepared spongy gels were characterized by FTIR and Environmental Scanning Electron Microscopy (ESEM) for structural and morphological analysis. The prepared semi IPNs were studied for their water sorption capacity and the data were utilized to evaluate network parameters such as average molecular weight between crosslinks (M(c)) and crosslink density (q). The adsorption of Fgn was carried out on to the prepared polymeric matrices and static and dynamic aspects of the adsorption process were investigated. The adsorption process was also studied as a function of pH and ionic strength of the protein solution and chemical architecture of the semi IPN. The antithrombogenic properties of the IPN's were also judged and correlated with water sorption and protein adsorption findings.

Parameters influencing the stealthiness of colloidal drug delivery systems

Over the last few decades, colloidal drug delivery systems (CDDS) such as nano-structures have been developed in order to improve the efficiency and the specificity of drug action. Their small size permits them to be injected intravenously in order to reach target tissues. However, it is known that they can be rapidly removed from blood circulation by the immune system. CDDS are removed via the complement system and via the cells of the mononuclear phagocyte system (MPS), after their recognition by opsonins and/or receptors present at the cell surface. This recognition is dependent on the physicochemical characteristics of the CDDS. In this study, we will focus on parameters influencing the interactions of opsonins and the macrophage plasma membrane with the surface of CDDS, whereby parameters of the polymer coating become necessary to provide good protection.

Molecular determinants of biocompatibility

The biocompatibility of medical implants dictates the fate of almost all medical devices. It is well established that medical devices trigger a variety of adverse tissue responses, such as inflammation, fibrosis, infection and thrombosis. However, the mechanisms involved in biomaterial-mediated tissue responses remain largely unknown. The lack of such knowledge hinders the development of biomaterials with better biocompatibility and safety. The aim of this review is to summarize our current understanding of the processes governing foreign body reactions to tissue-contact devices. Obviously, this information is urgently needed for assisting the rational design of materials or medical devices to minimize undesirable tissue reactions upon implantation and, in addition, to promote the wound healing process.

Negatively charged sol-gel column with stable electroosmotic flow for online preconcentration of zwitterionic biomolecules in capillary electromigration separations

A negatively charged sol-gel coating was developed for on-line preconcentration of zwitterionic biomolecules in capillary electrophoresis (CE), using asparagine and myoglobin as representative zwitterionic bioanalytes. The sol-gel coating was created by using a solution containing three precursors: mercaptopropyltrimethoxysilane (MPTMS), tetramethoxysilane (TMOS), and n-octadecyltriethoxysilane (C18-TEOS). The resulting sol-gel coating contained chemically bonded mercaptopropyl functional groups that were further oxidized by hydrogen peroxide to the corresponding sulfonic acid moieties. Such a surface-bonded sol-gel coating can carry a negative charge over a wide range of pH due to the presence of deprotonated sulfonic acid groups. Under favorable pH conditions, the negatively charged sol-gel coating can facilitate the extraction of positively charged analytes from a zwitterionic sample through electrostatic interaction. This principle was employed to extract myoglobin and asparagine by passing aqueous samples of these zwitterionic analytes through a negatively charged sol-gel column. The extracted analytes were then desorbed and focused via local pH change and stacking. The local pH change was accomplished by passing a buffer solution with a pH above the solute p/ value, while a dynamic pH junction between the sample solution and the background electrolyte was utilized to facilitate solute focusing. The sorption/desorption phenomena could, perhaps, also be explained on the basis of ion-exchange and local pH junction effects. On-line preconcentration and analysis results obtained on sulfonated sol-gel columns were compared with those obtained on an uncoated fused silica capillary of identical dimensions using conventional sample injections. Using UV detection, the presented sample preconcentration technique provided a sensitivity enhancement factor (SEF) on the order of 3 x 10(3) for myoglobin, and 7 x 10(3) for asparagine.

Enzyme induced biodegradation of polycarbonate-polyurethanes: dose dependence effect of cholesterol esterase

The current study has investigated the influence of esterase activity (80-400units/ml) on the biodegradation of polycarbonate-urethanes (PCNUs) by cholesterol esterase (CE), with a particular interest in studying the influence of different hard segment structures and their contribution to sensitizing the polymer towards enzyme catalyzed hydrolysis. Polycarbonate based polyurethanes were synthesized with varying hard segment content as well as hard segment chemistry based on three different diisocyanates, 1,6-hexane diisocyanate (HDI), 4,4'-methylene bisphenyl diisocyanate (MDI) and 4,4-methylene biscyclohexyl diisocyanate (HMDI). The effect of different chemistry on surface contact angle was measured in order to define the relative chemical nature of the surfaces. The enzyme dose response was found to be lower when hard segment content in the polymer was high. There was a very strong dependence on enzyme concentration for polyurethanes with different hard segment chemistry, despite the fact that the nature of the hydrolysable polycarbonate segment remained the same. The PCNU which showed the most dramatic dependence on enzyme concentration was synthesized with HMDI. At low enzyme concentration (80units/ml) this material was the most stable of the polymers while at elevated CE concentration (400units/ml) the polymer underwent a catastrophic breakdown. The findings suggested that protein binding on the surfaces was saturated even though enzyme degradation did not achieve saturation on any of the surfaces. The role of protein binding in modulating the hydrolytic action of the enzymes at different activity levels highlights a need for further study in this area.

Neutron Reflectivity Study of the Adsorption of beta-Lactoglobulin at a Hydrophilic Solid/Liquid Interface

Adsorption of a globular protein on a hydrophilic solid/liquid interface was investigated. Neutron reflectivity was used to determine structural information on an adsorbed beta-lactoglobulin layer at a hydrophilic silicon surface. The thickness of the protein film was found to be compatible with the diameter of the native protein, indicating that possible conformational changes of the protein during adsorption are not gross enough to alter the shape of the protein. The amount adsorbed is consistent with that derived by our ellipsometry measurements, obtained in similar experiments. Evidence for significant H-D exchange in the adsorbed protein was found. Copyright 1999 Academic Press.

Matrix-assisted laser desorption ionization mass spectrometry: a new tool for probing interactions between proteins and metal surfaces. Use in dental implantology

The fixation in the bone of an artificial titanium tooth root is believed to be initiated by the rapid adsorption of the proteins present in the surgical cavity on the titanium surface. The study of this adsorption should make it possible to predict the osseointegration capacities of new implant surface treatments. We describe here a new method, based on matrix-assisted laser desorption ionization-mass spectrometry (MALDI-MS), for quantifying proteins adsorbed on titanium surfaces fully identical to these designed for implantology. The key step of this method is a new MALDI-MS sample preparation allowing the adsorbed proteins to be removed from the surface and to be homogeneously dispersed in the matrix crystals. The adsorption of a model protein (lysozyme) on two titanium surfaces (polished and sandblasted) was studied in order to evaluate the method. The absolute MALDI-MS intensity was shown to vary linearly with the amount of adsorbed lysozyme. After dipping the titanium surfaces for different times in lysozyme solutions at different concentrations, the maximum amount of adsorbed lysozyme was measured by MALDI-MS and was shown to correspond to a lysozyme monolayer, which is consistent with results described in the literature.

Adsorption of Bovine Serum Albumin and Lysozyme on Hydrophobic Calcium Hydroxyapatites

The adsorption of bovine serum albumin (BSA) and lysozyme (LSZ) to oleyl phosphate(OP)-grafted calcium hydroxyapatite (OP-CaHAP) with different degrees of hydrophobicity, ranging the number of surface oleyl group per unit nm2 (nO) from 0 to 2.60, was investigated. The pronounced effects of the hydrophobic moiety of adsorbent on protein adsorption were observed. The saturated amount of adsorbed BSA (ns) was increased up to nO = 0.6 by an enlargement of hydrophobic interaction between hydrophobic CaHAP particle and proteins. However, ns decreased at nO >/= 1.3 by increasing the electrostatic repulsive force between negatively charged BSA and OP-CaHAP particles. On the other hand, the ns value of LSZ was continuously increased up to nO = 2.0 and saturated by increasing either the hydrophobic interaction or the electrostatic attraction of positively charged LSZ and negatively charged OP-grafted CaHAPs. The BSA adsorption experiment revealed that the effect of positively charged adsorption sites on the exposed ac or bc crystal faces (C-sites) of the CaHAPs is screened by the OP-groups grafted on their particle surfaces. Copyright 1999 Academic Press.

Plasma protein adsorption on biodegradable microspheres consisting of poly(D,L-lactide-co-glycolide), poly(L-lactide) or ABA triblock copolymers containing poly(oxyethylene). Influence of production method and polymer composition

Biodegradable particulate systems have been considered as parenteral drug delivery systems. The adsorption of plasma proteins on micro- and nanoparticles is determined by the surface properties and may, in turn, strongly influence the biocompatibility and biodistribution of both carriers. In the present study the influence of the polymer composition and the production method of microspheres on the in vitro plasma protein adsorption were investigated using two-dimensional electrophoresis (2-DE). Microparticles were prepared from poly(l-lactide) (l-PLA), poly(d,l-lactide-co-glycolide) (PLGA), and ABA triblock copolymers containing hydrophilic poly(oxyethylene) (B-blocks) domains connected to hydrophobic polyesters (A-blocks). Two different microencapsulation methods were employed, namely the w/o/w emulsion solvent evaporation method and the spray-drying technique. It could be demonstrated that the polymer composition and, especially, the encapsulation technique, influenced the interactions with plasma proteins significantly. For example, the percentages of several apolipoproteins in the plasma protein adsorption patterns of spray-dried PLGA- and l-PLA-particles were distinctly higher when compared to the adsorption patterns of the particles produced by the w/o/w-technique. Some adsorbed proteins were found to be characteristic or even specific for particles produced by the same method or consisting of identical polymers. Polyvinyl alcohol used as stabilizer in the w/o/w-technique may decisively influence the surface properties relevant for protein adsorption. The plasma protein adsorption on particles composed of ABA copolymers was drastically reduced when compared to microspheres made from pure polyesters. The adsorption patterns of ABA-particles were dominated by albumin. The plasma protein adsorption patterns detected on the different microspheres are likely to affect their in vivo performance as parenteral drug delivery systems.

Structure and reactivity of water at biomaterial surfaces

Molecular self association in liquids is a physical process that can dominate cohesion (interfacial tension) and miscibility. In water, self association is a powerful organizational force leading to a three-dimensional hydrogen-bonded network (water structure). Localized perturbations in the chemical potential of water as by, for example, contact with a solid surface, induces compensating changes in water structure that can be sensed tens of nanometers from the point of origin using the surface force apparatus (SFA) and ancillary techniques. These instruments reveal attractive or repulsive forces between opposing surfaces immersed in water, over and above that anticipated by continuum theory (DLVO), that are attributed to a variable density (partial molar volume) of a more-or-less ordered water structure, depending on the water wettability (surface energy) of the water-contacting surfaces. Water structure at surfaces is thus found to be a manifestation of hydrophobicity and, while mechanistic/theoretical interpretation of experimental results remain the subject of some debate in the literature, convergence of experimental observations permit, for the first time, quantitative definition of the relative terms 'hydrophobic' and 'hydrophilic'. In particular, long-range attractive forces are detected only between surfaces exhibiting a water contact angle theta > 65 degrees (herein defined as hydrophobic surfaces with pure water adhesion tension tau O = gamma O cos theta < 30 dyn/cm where gamma O is water interfacial tension = 72.8 dyn/cm). Repulsive forces are detected between surfaces exhibiting theta < 65 degrees (hydrophilic surfaces, tau O > 30 dyn/cm). These findings suggest at least two distinct kinds of water structure and reactivity: a relatively less-dense water region against hydrophobic surfaces with an open hydrogen-bonded network and a relatively more-dense water region against hydrophilic surfaces with a collapsed hydrogen-bonded network. Importantly, membrane and SFA studies reveal a discrimination between biologically-important ions that preferentially solubilizes divalent ions in more-dense water regions relative to less-dense water regions in which monovalent ions are enriched. Thus, the compelling conclusion to be drawn from the collective scientific evidence gleaned from over a century of experimental and theoretical investigation is that solvent properties of water within the interphase separating a solid surface from bulk water solution vary with contacting surface chemistry. This interphase can extend tens of nanometers from a water-contacting surface due to a propagation of differences in self association between vicinal water and bulk-phase water. Physicochemical properties of interfacial water profoundly influence the biological response to materials in a surprisingly straightforward manner when key measures of biological activity sensitive to interfacial phenomena are scaled against water adhesion tension tau O of contacting surfaces. As examples, hydrophobic surfaces (tau O < 30 dyn/cm) support adsorption of various surfactants and proteins from water because expulsion of solute from solution into the interphase between bulk solid and solution phases is energetically favorable. Adsorption to hydrophobic surfaces is driven by the reduction of interfacial energetics concomitant with replacement of water molecules at the surface by adsorbed solute (surface dehydration). Hydrophilic surfaces (tau O > 30 dyn/cm) do not support adsorption because this mechanism is energetically unfavorable. Protein-adsorbing hydrophobic surfaces are inefficient contact activators of the blood coagulation cascade whereas protein-repellent hydrophilic surfaces are efficient activators of blood coagulation. Mammalian cell attachment is a process distinct from protein adsorption that occurs efficiently to hydrophilic surfaces but inefficiently to hydrophobic surfaces. (ABSTRACT TRUNCATED)

Tryptophan, Tryptophan-Leucine, and BSA Adsorption at an Oil-Water Interface

Adsorption of the amino acid tryptophan, the peptide tryptophan-leucine and the protein bovine serum albumin (BSA) at buffered water-oleyl alcohol interfaces has been studied by ellipsometry. Tryptophan-leucine solutions showed a systematic change in ellipticity with concentration indicating an adsorption of 0.5 mg/m2 for a solution concentration of 1 g/L and evidence of saturation at that concentration. BSA showed an adsorption of 0.45 mg/m2 for a concentration of 3.5 g/L with no sign of saturation. There appeared to be little adsorption of tryptophan at the interface.

Attractive adsorbate interaction in biological surface reactions

Recent studies on ferritin adsorption have revealed strong adsorbate interactions during adsorption (H. Nygren, Biophys. J., 65 (1993) 1508). In the present study, efforts were made to quantify the attractive lateral adsorbate interaction and to describe the effect of such interactions on the time- and concentration dependence of biological surface reactions. Attractive forces between ligands increase the time that they are considered neighbours to other ligands. Thus, the inter-ligand interaction effects their distribution and this can be measured experimentally by the pair correlation function g(r). The attractive lateral interaction was shown to affect the surface adsorption of ferritin, virus particles and bacteria, but played a minor role in the binding of antibodies to surface-immobilised antigen.

Logarithmic growth in surface adsorption

A review is made of experimental data on surface adsorption of particles and polymers from water solutions, their analysis and interpretation in terms of general theoretical models of surface adsorption. A characteristic isotherm and kinetics is found and defined as logistic growth. The discussion is focused on literature indicating a possibility of describing logistic growth by using statistical (probabilistic) models based on the mean stay time of molecules on the surface. The statistical approach is further elaborated as follows: ligands arriving at a surface have a binary choice-to bind or to become reflected. Since the number of attempts to bind, n, will be high we can use the true mean of the binomial distribution to describe the reaction and write: S = n a; where S is the number of successful attempts and a is the probability of binding. The probability, a, will depend on the site density and on the sticking probability of the ligand at the binding site. Several experimental studies show that surface reactions have a nonlinear time- and concentration dependence and can be described by a Boltzmann factor of the form, I(1-e-t/tau); where I is the flux of ligands to the surface and tau = stay-time. The exponential form indicates that the reactions are self-dependent, and a statistical model for description of such reactions will be of the form: S(t) = N(o)(1-2-alpha t) (2-beta t); where N(o) is the number of molecules present in the system, alpha relates to the probability of positive cooperativity or t-dependent binding, and beta relates to the probability of desorption.

Contact activation of the plasma coagulation cascade. II. Protein adsorption to procoagulant surfaces

A study of blood protein adsorption to procoagulant surfaces utilizing a coagulation time assay, contact angles, Wilhelmy balance tensiometry, and electron spectroscopy (ESCA) is presented. Using a new contact angle method of measuring protein adsorption termed "adsorption mapping" it was demonstrated that protein-adsorbent surfaces were inefficient activators of the intrinsic pathway of the plasma coagulation cascade whereas water-wettable, protein-repellent surfaces were efficient procoagulants. Repeated use of fully water-wettable (spreading) glass procoagulants in the coagulation time assay demonstrated that putative "activating sites" were not consumed in the coagulation of platelet-poor porcine plasma. Furthermore, these procoagulant surfaces retained water-wettable surface properties after incubation with blood proteins and saline rinse. The interpretation of these observations was that plasma and serum proteins were not adsorbed to water-wettable surfaces. However, ESCA of these same surfaces revealed the presence of a thin protein layer. Wilhelmy balance tensiometry resolved these seemingly divergent observations by demonstrating that protein was "associated" with a bound hydration layer, but not formally adsorbed through a surface dehydration or ionic interaction mechanism.

Neutron reflection study of bovine beta-casein adsorbed on OTS self-assembled monolayers

Specular neutron reflection has been used to determine the structure and composition of bovine beta-casein adsorbed on a solid surface from an aqueous phosphate-buffered solution at pH 7. The protein was adsorbed on a hydrophobic monolayer self-assembled from deuterated octadecyltrichlorosilane solution on a silicon (111) surface. A two-layer structure formed consisting of one dense layer of thickness 23 +/- 1 angstroms and a surface coverage of 1.9 milligrams per square meter adjacent to the surface and an external layer protruding into the solution of thickness 35 +/- 1 angstroms and 12 percent protein volume fraction. The structure of the (beta-casein) layer is explained in terms of the charge distribution in the protein.