Competitive adsorption of albumin against collagen at solution-air and solution-polyethylene interfaces

Lysozyme adsorption studies at the silica/water interface using dual polarization interferometry

Lysozyme adsorption at the silica/water interface has been studied using a new analytical technique called dual polarization interferometry. This laboratory-based technique allows the build up or removal of molecular layers adsorbing or reacting on a lightly doped silicon dioxide (silica) surface to be measured in terms of thickness and refractive index changes with time. Lysozyme adsorption was studied at a range of concentrations from 0.03 to 4.0 g dm(-3) and at both pH 4 and pH 7. Adsorbed layers ranging from 14 to 43 +/- 1 A in thickness and 0.21 to 2.36 +/- 0.05 mg m(-2) in mass coverage were observed at pH 4 with increasing lysozyme concentration, indicating a strong deformation of the monolayer over the low concentration range and the formation of an almost complete sideways-on bilayer toward the high concentration of 4 g dm(-3). At pH 7, the thickness of adsorbed layers varied from 16 to 54 +/- 1 A with significantly higher surface coverage (0.74 to 3.29 +/- 0.05 mg m(-2)), again indicating structural deformation during the initial monolayer formation, followed by a gradual transition to bilayer adsorption over the high concentration end. The pH recycling performed at a fixed lysozyme concentration of 1.0 g dm(-3) indicated a broadly reversible adsorption regardless of whether the pH was cycled from pH 7 to pH 4 and back again or vice versa. These observations are in good agreement with earlier studies undertaken using neutron reflection although the fine details of molecular orientations in the layers differ subtly.

The influence of protein adsorption and surface modifying macromolecules on the hydrolytic degradation of a poly(ether-urethane) by cholesterol esterase

Previous investigations have demonstrated that the inflammatory cell derived enzyme, cholesterol esterase (CE) could degrade polyurethanes (PUs) by hydrolyzing ester and urethane bonds. Studies that have investigated the development of protective coatings for PUs have reported that the polymer degradation of polyester-urethanes (PESUs) can be reduced with the use of fluorine containing surface modifying macromolecules (SMMs). Since these latter studies were carried out in the presence of relatively pure enzyme, it has not been shown if SMMs would still provide an enhanced inhibitory effect if surfaces were pre-exposed to plasma proteins. This would be more representative of the in vivo scenario since protein adsorption would occur before the appearance of monocyte-derived macrophages which would be a primary source of esterase activities. The current investigation has focused on studying the influence of fibrinogen (Fg) as a simple model of protein adsorption in order to assess the effect of CE in combination with protein on polyether-urethane (PEU) surfaces. The materials were prepared with and without SMMs, and were pre-coated with Fg prior to carrying out biodegradation studies. The pre-adsorption of Fg onto the modified and non-modified surfaces provided a significant delay in the hydrolytic action of CE onto the PEU substrates. However, the effect was gone by 70 days and by the 126th day of incubation, both Fg coated and non-Fg coated groups had the same level of degradation. The difference between Fg coated and non-coated substrates was much smaller for materials containing SMMs. In addition, the pre-adsorption of Fg did not alter the SMMs' ability to provide a more biostable surface over the 4 month incubation period.

Fluorinated surface-modifying macromolecules: modulating adhesive protein and platelet interactions on a polyether-urethane

Polyether-urethanes (PEUs) have been the materials of choice for the manufacture of conventional blood-contacting devices. Nevertheless, biostability and blood compatibility are still among the principal limitations in their long-term application. Studies investigating the development of protective coatings for PEUs have shown that degradation can be reduced with the use of fluorinated surface-modifying macromolecules (SMMs). It has also been hypothesized that SMM-modified PEU surfaces may exhibit improved blood compatibility because other studies have shown a modulation in fibrinogen adsorption onto these surfaces. To determine the blood compatibility of a PEU-containing fluorinated SMMs, a series of in vitro experiments were designed to study the pattern of protein adsorption from plasma and then to assess the nature of platelet adhesion and activation on each substrate. Western blot analysis as well as single protein studies revealed that the dominant "adhesive proteins" [fibrinogen (Fg), fibronectin (Fnc), and vitronectin (Vnc)] were adsorbed on two of the SMM-containing PEUs in lower amounts relative to unmodified base. Platelet adhesion and activation data further highlighted the differences among the various substrates. It was shown that the unmodified base had a higher number of adhered platelets relative to the SMM-modified surfaces, and that of the SMM-containing substrates, which showed the lowest levels of adhesive proteins also, exhibited significantly lower platelet densities. Close morphological examination further revealed that platelets residing on these latter substrates were not appreciably activated. Based on the current evidence, it is believed that the fluorinated SMMs demonstrate good potential for the development of surfaces with minimal thrombogenic character in in vivo applications.

Native and Hydrophobically Modified Human Immunoglobulin G at the Air/Water Interface

The adsorption of human immunoglobulin G (IgG) at the air/water interface was monitored both by the in situ radiotracer technique using [(14)C] labeled IgG and by surface tension measurements. The results reveal that adsorption of IgG from single protein systems displays bimodality due to molecular rearrangements at the interface. Above the threshold value of 1.5x10(-2) mg/ml solution concentration, adsorbed IgG molecules reoriented from the side-on to the end-on configuration. The existence of a lag time which did not appear in Gamma=f(t) curves, was observed in Pi=f(t) relationships at low protein concentrations and was due to the limits of the surface pressure technique to detect protein adsorption. The adsorption of native IgG was also carried out in the presence of a hydrophobized IgG obtained by grafting capryloyl residues to its lysine groups by reaction with N-hydroxysuccinimide ester of caprylic acid, which yielded 19 covalently bound alkyl chains to the IgG molecule (19C(8)-IgG). This modified IgG exhibited enhanced adsorption at the air/water interface, as manifested by its increased adsorption efficiency relative to the native protein. Sequential and competitive adsorption experiments from binary mixtures of native IgG and 19C(8)-IgG clearly demonstrate that the displacement of the native protein from the air/water interface strongly depended on the manner of how 19C(8)-IgG and native IgG competed with each other. When the two proteins competed simultaneously, 19C(8)-IgG predominantly occupied the available area but when native IgG was adsorbed first, for 2 h, the sequentially adsorbed 19C(8)-IgG was incapable of substantially displacing it from the interface. Copyright 2001 Academic Press.

On-line deproteinization by adsorption of proteins on a polyethylene powder pre-column for the determination of Na, K, Mg and Ca in human serum by high-performance liquid chromatography

Deproteinization of human serum was carried out on-line using a polyethylene powder cartridge as pre-column. The serum sample, after dilution, was injected into the chromatograph, and when passing through the cartridge the proteins were adsorbed by the polyethylene. A protein-free eluate was carried to the analytical column while the pre-column was washed with methanol and water to elute the adsorbed proteins by changing the pump channels. After washing, the pre-column was conditioned with the eluent to receive the next sample. Deproteinization was evaluated off- and on-line using pooled serum, and testing the protein residue after passing through a polyethylene cartridge with the Comassie Brilliant Blue reaction. Parameters such as serum volume, eluent and washing solution were investigated. Sodium, potassium, magnesium and calcium determinations were performed by high-performance liquid chromatography with conductimetric detection. The proposed method is suitable for the determination of these cations in serum samples without further treatments. The total analysis time was about 20 min and a linear range from 0.3 to 1.2 mg/l for sodium, potassium, magnesium and calcium was observed. The method was characterized by a precision of about 95% and recoveries from spiked samples were between 96 and 102%. The results for serum samples analysed by the proposed method were compared with photometry and atomic absorption spectrometry.

Orientation of cell adhesion and growth on patterned heterogeneous polystyrene surface

Studies of neurite outgrowth or cell migration, two important processes in neuronal networks formation, are facilitated by cell culture models capable of orientating cellular growth and of designing a well-defined cellular pattern. Heterogeneous polystyrene surfaces composed of oxygen plasma-treated stripes (PSox) with a low hydrophobicity separated by non-treated areas (PS) have these properties. In this study, to guide cell growth, we developed a cell culture model using these supports and we identified the molecular factors involved in cellular orientation. When the heterogeneous supports were not coated, proteins from a serum culture medium were required for cells to line up on PSox. On the other hand, cell orientation on coated surfaces was clearly influenced by competitive adsorption of adhesive proteins such as fibronectin or collagen and anti-adhesive molecules as pluronic F68 or albumin. Attachment factors were adsorbed on PSox stripes while adsorption of anti-adhesive molecules on the most hydrophobic PS areas prevented cell adhesion or growth. Thus, we describe the preparation of a cell culture substrate that succeeded in orientating cell growth and that led to a line of cells on adhesive PSox stripes ranging from 2 to 100 microns width.

The Adsorption of Lysozyme at the Silica-Water Interface: A Neutron Reflection Study

The adsorption of lysozyme (chicken egg white) from aqueous solution on to the hydrophilic silica surface and the variation of interfacial structure with solution conditions have been studied by neutron reflection. The accurate determination of the adsorbed layer thicknesses in combination with the dimension of the globular structure of lysozyme allows us to postulate the mean structural conformation of the lysozyme molecules within the adsorbed layer. It was found that the adsorption was completely reproducible with respect to lysozyme concentration, but it was irreversible. The effect of ionic strength on the adsorption of lysozyme was examined at pH 7 and at a bulk lysozyme concentration of 0.03 g dm-3. The adsorbed layer was not affected by changes in ionic strength when the total ionic strength was below 0.05 M, but above this concentration addition of NaCl gradually reduced the amount of lysozyme adsorbed. Complete removal of adsorbed lysozyme was achieved when the total ionic strength was above 0.5 M. The effect of solution pH on the amount of lysozyme adsorbed was characterized by varying the pH in cycles at fixed lysozyme concentrations. Adsorption was found to be completely reversible with respect to pH over a wide protein concentration range. The level of surface excess was dominated by the electrostatic repulsion between lysozyme molecules within the adsorbed layers, rather than the attraction between the surface and lysozyme. The lysozyme layer structure along the surface normal was characterized by varying the isotopic composition of the water. At pH 7 a monolayer 30 +/- 2 Å thick was formed when the lysozyme concentration was below 0.03 g dm-3, indicating that the lysozyme was adsorbed with its long axis parallel to the surface (sideways-on). At higher concentrations the thickness of the layer changed to 60 +/- 2 Å, suggesting the formation of a bilayer of lysozyme molecules in the sideways-on configuration. When the lysozyme concentration is above 1 g dm-3 the surface excess within the inner layer is sufficiently high that repulsion within the adsorbed layer becomes significant and the molecules start to tilt towards longways-on adsorption. At pH 4, the electrostatic repulsion between the adsorbed molecules is stronger than at pH 7, resulting in a lower surface excess and a tilting away from the sideways-on configuration at lower surface concentration. Copyright 1998 Academic Press.

Influence of Substrate Hydrophobicity on the Adsorption of Collagen in the Presence of Pluronic F68, Albumin, or Calf Serum

The influence of Pluronic F68 [a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) copolymer surfactant], serum albumin (HSA), and fetal calf serum (FCS) on the adsorption of type I collagen by polymer substrates was investigated using radiolabeling and XPS analysis. Three different kinds of polystyrene substrates with increasing level of hydrophobicity were used. Change in the state of hydration of the sorbent and protein surfaces appears to be the main driving force for collagen adsorption. Pluronic F68 strongly reduces collagen adsorption, the reduction being more pronounced with higher substrate hydrophobicity. This explains why epithelial cell adhesion on substrates preconditioned with a solution of Pluronic F68 and collagen is strongly influenced by substrate hydrophobicity. Collagen adsorption is also reduced in the presence of HSA and FCS, but the reduction and its sensitivity to substrate hydrophobicity are lower than with Pluronic F68.

Differential surface binding of albumin, immunoglobulin G and fibrinogen

Protein-protein interactions, as well as the nature of the surface, significantly affect the activity of a specific protein towards a defined surface. Indications are that protein-protein associations may affect antibody detectability, but in some cases this is the result of altered antigenic accessibility rather than physical removal of the molecule. The antibody binding patterns are also quite variable over an entire methyl-silanol wettability gradient on silicon, suggesting that the surface itself is affecting protein-protein and protein-protein-surface associations. Ellipsometric studies were carried out on the gradients which were incubated in single; binary and tertiary physiological concentration solutions of human albumin, immunoglobulin G (IgG) and fibrinogen. The ellipsometric-antibody detectability of the proteins on such surfaces were found to be variable, depending upon the location on the gradient and the order and combination in which the proteins were presented to the surface. Radiolabelled proteins were studied on discrete regions of these gradients. Competitive effects of albumin were found to be inhibitory (negative) with respect to IgG adsorption on hydrophobic surfaces, while enhancing IgG deposition on hydrophilic surfaces (positive). Scanning force microscopy in the so-called tapping mode indicates that proteins, particularly IgG, organize themselves differently with respect to surfaces, depending upon the nature of the surface and the presence of other proteins.