The density and refractive index of adsorbing protein layers

The structure of the adsorbing layers of native and denatured proteins (fibrinogen, gamma-immunoglobulin, albumin, and lysozyme) was studied on hydrophilic TiO(2) and hydrophobic Teflon-AF surfaces using the quartz crystal microbalance with dissipation and optical waveguide lightmode spectroscopy techniques. The density and the refractive index of the adsorbing protein layers could be determined from the complementary information provided by the two in situ instruments. The observed density and refractive index changes during the protein-adsorption process indicated the presence of conformational changes (e.g., partial unfolding) in general, especially upon contact with the hydrophobic surface. The structure of the formed layers was found to depend on the size of the proteins and on the experimental conditions. On the TiO(2) surface smaller proteins formed a denser layer than larger ones and the layer of unfolded proteins was less dense than that adsorbed from the native conformation. The hydrophobic surface induced denaturation and resulted in the formation of thin compact protein films of albumin and lysozyme. A linear correlation was found between the quartz crystal microbalance measured dissipation factor and the total water content of the layer, suggesting the existence of a dissipative process that is related to the solvent molecules present inside the adsorbed protein layer. Our measurements indicated that water and solvent molecules not only influence the 3D structure of proteins in solution but also play a crucial role in their adsorption onto surfaces.     

Investigation of Bovine Serum Albumin (BSA) Attachment onto Self-Assembled Monolayers (SAMs) Using Combinatorial Quartz Crystal Microbalance with Dissipation (QCM-D) and Spectroscopic Ellipsometry (SE)

Understanding protein adsorption kinetics to surfaces is of importance for various environmental and biomedical applications. Adsorption of bovine serum albumin to various self-assembled monolayer surfaces including neutral and charged hydrophilic and hydrophobic surfaces was investigated using in-situ combinatorial quartz crystal microbalance with dissipation and spectroscopic ellipsometry. Adsorption of bovine serum albumin varied as a function of surface properties, bovine serum albumin concentration and pH value. Charged surfaces exhibited a greater quantity of bovine serum albumin adsorption, a larger bovine serum albumin layer thickness, and increased density of bovine serum albumin protein compared to neutral surfaces at neutral pH value. The quantity of adsorbed bovine serum albumin protein increased with increasing bovine serum albumin concentration. After equilibrium sorption was reached at pH 7.0, desorption of bovine serum albumin occurred when pH was lowered to 2.0, which is below the isoelectric point of bovine serum albumin. Our data provide further evidence that combinatorial quartz crystal microbalance with dissipation and spectroscopic ellipsometry is a sensitive analytical tool to evaluate attachment and detachment of adsorbed proteins in systems with environmental implications.     

Circular dichroism studies on conformational changes in protein molecules upon adsorption on ultrafine polystyrene particles

The conformational changes in well-characterized model proteins [bovine ribonuclease A (RNase A), horseradish peroxidase, sperm-whole myoglobin, human hemoglobin, and bovine serum albumin (BSA)] upon adsorption on ultrafine polystyrene (PS) particles have been studied using circular dichroism (CD) spectroscopy. These proteins were chosen with special attention to molecular flexibility. The ultrafine PS particles were negatively charged and have average diameters of 20 or 30 nm. Utilization of these ultrafine PS particles makes it possible to apply the CD technique to determine the secondary structure of proteins adsorbed on the PS surface. Effects of protein properties and adsorption conditions on the extent of the changes in the secondary structure of protein molecules upon adsorption on ultrafine PS particles were studied. The CD spectrum changes upon adsorption were significant in the "soft" protein molecules (myoglobin, hemoglobin, and BSA), while they were insingnificant in the "rigid" proteins (RNase A and peroxidase). The soft proteins sustained a marked decrease in alpha-helix content upon adsorption. Moreover, the native alpha-helix content, which is given as the percentage of the alpha-helix content in the free proteins, of adsorbed BSA was found to decrease with decreasing pH and increase with increasing adsorbed amount. These observations confirm some well-known hypotheses for the confirmational chages in protein molecules upon adsorption. (c) 1992 John Wiley & Sons, Inc.     

Thermal and Structural Stability of Adsorbed Proteins

Experimental evidence suggests that proteins adsorbed to hydrophobic surfaces at low coverages are stabilized relative to the bulk. For larger coverages, proteins unfold and form β-sheets. We performed computer simulations on model proteins and found that: 1), For weakly adsorbing surfaces, unfolded conformations lose more entropy upon adsorption than folded ones. 2), The melting temperature, both in the bulk and at surfaces, decreases with increasing protein concentration because of favorable interprotein interactions. 3), Proteins in the bulk show large unfolding free energy barriers; this barrier decreases at stronger adsorbing surfaces. We conjecture that typical experimental temperatures appear to be below the bulk melting temperature for a single protein, but above the melting temperature for concentrated protein solutions. Purely thermodynamic factors then explain protein stabilization on adsorption at low concentrations. However, both thermodynamic and kinetic factors are important at higher concentrations. Thus, proteins in the bulk do not denature with increasing concentration due to large kinetic barriers, even though the aggregated state is thermodynamically preferred. However, they readily unfold upon adsorption, with the surface acting as a heterogeneous catalyst. The thermal behavior of proteins adsorbed to hydrophobic surfaces thus appears to follow behavior independent of their chemical specificity.     

Protein adsorption at solid-liquid interfaces: Part II--Adsorption from binary protein mixture

Extent of adsorption of proteins at alumina-water interface from solutions containing binary mixture of beta-lactoglobulin and bovine serum albumin (BSA), beta-lactoglobulin and gelatin, and gelatin and bovine serum albumin has been estimated as functions of protein concentrations at varying pH, ionic strength, temperature and weight fraction ratios of protein mixture. The extent of adsorption (gamma lacw) of lactoglobulin in the presence of BSA increases with increase of protein concentration (Clac) until it reaches a maximum but a fixed value gamma lacw(m). Extent of adsorption gamma serw also initially increases with increase of protein concentrations until it reaches maximum value gamma serw(m). Beyond these protein concentrations, adsorbed BSA is gradually desorbed due to the preferential adsorption of lactoglobulin from the protein mixture. In many systems, gamma serw at high protein concentrations even becomes negative due to the strong competition of BSA and water for binding to the surface sites in the presence of lactoglobulin. For lactoglobulin-gelatin mixtures, adsorption of both proteins is enhanced as protein concentration is increased until limiting values for adsorption are reached. Beyond the limiting value, lactoglobulin is further accumulated at the interface without limit when protein concentration is high. For gelatin-albumin mixtures, extent of gelatin adsorption increases with increase in the adsorption of BSA. The limit for saturation of adsorption for gelatin is not reached for many systems. At acid pH, adsorbed BSA appears to be desorbed from the surface in the presence of gelatin. From the results thus obtained the role of electrostatic and hydrophobic effects in controlling the adsorption process has been analysed.     

Hydrolysis of butteroil by immobilized lipase using a hollow-fiber reactor: part I. lipase adsorption studies

Adsorption of proteins from a crude preparation containing a lipase from Aspergillus niger on microporous polypropylene hollow fibers was studied at six different temperatures. Langmuir isotherms accurately describe the overall adsorption equilibria. Lipase is selectively adsorbed relative to the other proteins in the crude preparation. Hence, immobilization also provides further purification of the lipase. The predictions of the Langmuir model for the change in the specific activity of lipase upon adsorption are consistent with experimental results. The loading capacity of the hollow fibers decreases and the adsorption constant increases as temperature is increased. This effect is more significant in the case of lipolytic activity than it is for the total amount of adsorbed protein. Small, positive enthalpy changes are associated with the adsorption of lipase on these hydrophobic membranes.     

Measurement of the secondary structure of adsorbed protein by circular dichroism. 1. Measurements of the helix content of adsorbed melittin.

A new circular dichroism (CD) technique is presented which quantifies, in situ, the changes in protein and peptide secondary structure upon adsorption at the quartz/liquid interface. Far-UV CD spectra of adsorbed proteins were recorded from several quartz interfaces contained in a specially constructed cell. Adsorbed, oriented alpha-helical spectra were recorded from hydrophilic and hydrophobic quartz using the bee venom peptide, melittin, which can be induced into an alpha-helical, tetrameric conformation in solution. The hydrophobic quartz provides a model system for oil-in-water emulsions and cell membranes. Surface concentrations were determined by radio-counting and were dependent on the nature of the surface. The characterization of these spectra has been partly achieved using far-UV CD spectra obtained from melittin adsorbed onto hydrophilic colloidal silica particles, where orientation effects are eliminated. Analysis of these spectra reveals considerable denaturation of the helical structures upon adsorption. Surface concentrations from the silica were determined from adsorption isotherms. The surface orientation of adsorbed melittin was dependent on the state of aggregation and hence degree of helicity of the molecule. These results support a model for the mode of action of melittin in lysing membranes.     

Protein precipitation and denaturation by dimethyl sulfoxide

Solvent conditions play a major role in a wide range of physical properties of proteins in solution. Organic solvents, including dimethyl sulfoxide (DMSO), have been used to precipitate, crystallize and denature proteins. We have studied here the interactions of DMSO with proteins by differential refractometry and amino acid solubility measurements. The proteins used, i.e., ribonuclease, lysozyme, beta-lactoglobulin and chymotrypsinogen, all showed negative preferential DMSO binding, or preferential hydration, at low DMSO concentrations, where they are in the native state. As the DMSO concentration was increased, the preferential interaction changed from preferential hydration to preferential DMSO binding, except for ribonuclease. The preferential DMSO binding correlated with structural changes and unfolding of these proteins observed at higher DMSO concentrations. Amino acid solubility measurements showed that the interactions between glycine and DMSO are highly unfavorable, while the interactions of DMSO with aromatic and hydrophobic side chains are favorable. The observed preferential hydration of the native protein may be explained from a combination of the excluded volume effects of DMSO and the unfavorable interaction of DMSO with a polar surface, as manifested by the unfavorable interactions of DMSO with the polar uncharged glycine molecule. Such an unfavorable interaction of DMSO with the native protein correlates with the enhanced self-association and precipitation of proteins by DMSO. Conversely, the observed conformational changes at higher DMSO concentration are due to increased binding of DMSO to hydrophobic and aromatic side chains, which had been newly exposed on protein unfolding.     

Influence of substrate pattern on the adsorption of HP lattice proteins

With the highly simplified hydrophobic-polar model representation of a protein, we can study essential qualitative physics without an unnecessarily large computational overhead. Using Wang-Landau sampling in conjunction with a set of efficient Monte Carlo trial moves, we studied the adsorption of short HP lattice proteins on various simple patterned substrates and in particular for checkered patterned surfaces. A set of single-site mutated HP proteins is used to investigate the role of hydrophobicity of a protein chain and surface pattern for substrates with various pattern cell sizes relative to the protein’s native configuration. For most cases, we found that the adsorption transition occurs at a lower temperature, while the hydrophobic core formation is less affected. The flattening procedure after the HP protein is adsorbed is more sensitive to the change in surface patterns and single-site mutations. These observations stay valid for both strongly and weakly attractive surfaces.     

Ordering of adsorbed proteins

Adapting a hard hexagon model to describing protein adsorption, we show by using Monte Carlo simulations that the ordering of adsorbed proteins may strongly depend on the relative location of the hydrophilic and hydrophobic patches on their surface. Specifically, proteins may form dimers, trimers, small rings, and zig-zag or straight chains or islands consisting of such fragments.     

A study of the nature of plasma protein adsorption on the surface of perfluorocarbon emulsions stabilized by different triblock copolymers

The composition of plasma proteins adsorbed on the surface of perfluorocarbon emulsions stabilized by different triblock copolymers and their quantitative ratio were analyzed. The results allowed us to describe three types of protein adsorption on the surface of emulsion droplets. Opsonin proteins prevailed during the first type of adsorption. Their adsorption occurred on a dense and inactive layer of triblock copolymers. The second type of adsorption occurred due to the hydrophobic effect on a dense and mobile layer, with low-molecular-weight proteins being predominantly adsorbed (monoadsorption). The third type of adsorption occurred on a loose layer of triblock copolymers. In this case, the adsorption of a large amount of proteins with a molecular weight of 10–500 kDa was observed, while the total molecular weight was distributed over a large number of proteins.     

Reduced protein adsorption at solid interfaces by sugar excipients

Sugar excipients are shown to reduce the adsorption of ribonuclease A, bovine serum albumin, and hen egg white lysozyme at the liquid-solid interface. The amount of protein adsorbed decreased as the concentration of the sugar increased. At the same sugar concentration, the ability of sugars to reduce protein adsorption followed the trend: trisaccharides > disaccharides > 6-carbon polyols > monosaccharides. This trend in adsorbed protein amounts among sugars was explained by stabilization of the protein native state in solution by the sugar excipients. The heat of solution of the amorphous saccharide was found to correlate with the amount of protein adsorbed.     

A ubiquitin-protein ligase specific for type III protein substrates

A previously studied species of ubiquitin-protein ligase contains specific sites for the binding of basic (Type I) and bulky hydrophobic (Type II) NH2-terminal amino acid residues of protein substrates. We now describe another enzyme that ligates ubiquitin specifically to proteins that have NH2-terminal residues other than the above two categories (Type III substrates). The new species of ligase, that we call E3 beta, is separable from the formerly described ligase (termed E3 alpha) by affinity chromatography on protein substrate columns. E3 beta was partially purified from extracts of rabbit reticulocytes and was shown to be required for the breakdown of Type III proteins. Apart from its different substrate specificity, it resembles E3 alpha in some physical properties, in a requirement for ubiquitin carrier protein (E2) for conjugate formation, and in its action to ligate multiple ubiquitin units to the substrate protein. The denatured derivative of bovine pancreatic ribonuclease is a specific substrate for E3 alpha, while that of ribonuclease S-protein is a good substrate for E3 beta. Since S-protein is formed by the removal from ribonuclease of NH2-terminal S-peptide, it is suggested that E3 beta interacts with an NH2-terminal determinant exposed in ribonuclease S-protein.     

Applicability of airlift draft-tube fluidized bioreactors for binary protein mixture bioseparation

An airlift draft-tube fluidized bioreactor has been designed and tested for applications in protein bioseparation. Operating parameters and geometrical dimensions of the bioreactor were optimized to ensure fluid circulation in a defined cyclic pattern between the riser and the downcomer. The overall directionality of liquid flow generates homogeneous field of low shear and achieves good mixing efficiency. Bioseparation of proteins was achieved from solutions containing both BSA and BHb at different initial concentrations and at pH 7. Similar adsorption capacities of both proteins were observed in single protein adsorption experiments at pH 7. Compressibility of BHb allowed for high adsorption capacity, in addition to the hydrophobic interaction forces. Apparently the homogeneous and lower shear generated by the airlift bioreactor reduces the compressibility of adsorbed BHb. This allowed for higher BSA adsorption from solutions containing BSA and BHb mixtures. Conventional batch adsorption experiments showed more adsorption of BHb, which reduces bioseparation efficiency.     

Microcolumn capture and digestion of proteins combined with mass spectrometry for protein identification

A procedure has been developed for protein identification using mass spectrometry (MS) that incorporates sample cleanup, preconcentration, and protein digestion in a single-stage system. The procedure involves the adsorption of a protein, or protein mixture, from solution onto a hydrophobic resin that is contained within a microcolumn. Sample loading is accomplished by flowing the protein solution through the microcolumn, where the protein adsorbs to the hydrophobic surface. The protein is digested while still bound to the hydrophobic surface by flowing a buffered trypsin solution through the column bed. The peptide fragments are subsequently eluted for detection by MALDI or ESI-MS. The procedure is demonstrated using dilute protein samples containing high concentrations of salt, urea, and modest amount of sodium dodecyl sulfate relative to protein. Peptide fragments are also detected by MS from a 500 nM bacteriorhodopsin solution digested in a microcolumn. In this case, a combined cyanogen bromide/trypsin digestion was performed in-column. The procedure is applied to the MALDI-MS/MS identification of proteins present in an individual fraction collected by ion exchange HPLC separation of E. coli total cell extract. An additional application is illustrated in the analysis of a human plasma fraction. A total of 14 proteins, which were present in the sample at sub-micromolar concentrations, were identified from ESI-MS/MS. The microcolumn digestion procedure represents the next step toward a system for fully automated protein analysis through capture and digestion of the adsorbed protein on hydrophobic surfaces.     

Enantioselective enzymatic hydrolysis of racemic glycidyl esters by using immobilized porcine pancreas lipase with improved catalytic properties

The crude porcine pancreas lipase (PPL) extract is a mixture of several proteins (mainly lipases and esterases). In order to develop enzymatic catalysts with good catalytic properties for hydrolytic enantioselective reactions in aqueous homogeneous medium, we studied the immobilization of the different enzymes contained in the crude PPL extracts by selective sequential adsorption on hydrophobic supports bearing octyl, octadecyl and phenyl groups. Some minor proteins were selectively adsorbed on octyl and octadecyl supports while the most abundant lipase was adsorbed on the support bearing phenyl groups. The enantioselectivity of the different lipase derivatives were tested considering the hydrolysis of esters of 1,2-epoxi-1-propanol (glycidol). The most abundant lipase contained in the commercial crude PPL extract resulted almost inactive while some lipases contained in low concentrations displayed high activities and enantioselectivities. The most interesting results were obtained with a 28-kDa protein selectively adsorbed on octyl-agarose. With this enzyme derivative, the residual butyric ester of glycidol was recovered with 96% enantiomeric excess at 55% conversion.     

Protein instability during HIC: hydrogen exchange labeling analysis and a framework for describing mobile and stationary phase effects

Unfolding of marginally stable proteins is a significant factor in commercial application of hydrophobic interaction chromatography (HIC). In this work, hydrogen-deuterium isotope exchange labeling has been used to monitor protein unfolding on HIC media for different stationary phase hydrophobicities and as a function of ammonium sulfate concentration. Circular dichroism and Raman spectroscopy were also used to characterize the structural perturbations experienced by solution phase protein that had been exposed to media and by protein adsorbed on media. As expected, greater instability is seen on chromatographic media with greater apparent hydrophobicity. However, increased salt concentrations also led to more unfolding, despite the well-known stabilizing effect of ammonium sulfate in solution. A thermodynamic framework is proposed to account for the effects of salt on both adsorption and stability during hydrophobic chromatography. Using appropriate estimates of input quantities, analysis with the framework can explain how salt effects on stability in chromatographic systems may contrast with solution stability.     

Adsorption from saliva to silica and hydroxyapatite surfaces and elution of salivary films by SDS and delmopinol

The adsorption of proteins from human whole saliva (HWS) onto silica and hydroxyapatite surfaces (HA) was followed by quartz crystal microbalance with dissipation (QCM-D) and ellipsometry. The influence of different surface properties and adsorption media (water and PBS) on the adsorption from saliva was studied. The viscoelastic properties of the salivary films formed on the solid surfaces were estimated by the use of the Voigt-based viscoelastic film model. Furthermore, the efficiency of SDS and delmopinol to elute the adsorbed salivary film from the surfaces was investigated at different surfactant concentrations. A biphasic kinetic regime for the adsorption from saliva on the silica and HA surfaces was observed, indicating the formation of a rigidly coupled first layer corresponding to an initial adsorption of small proteins and a more loosely bound second layer. The results further showed a higher adsorption from HWS onto the HA surfaces compared to the silica surfaces in both adsorption media (PBS and water). The adsorption in PBS led to higher adsorbed amounts on both surfaces as compared to water. SDS was found to be more efficient in removing the salivary film from both surfaces than delmopinol. The salivary film was found to be less tightly bound onto the silica surfaces since more of the salivary film could be removed with both SDS and delmopinol compared to that from the HA surface. When adsorption took place from PBS the salivary layer formed at both surfaces seemed to have a similar structure, with a high energy dissipation implying that a softer salivary layer is built up in PBS as opposed to that in water. Furthermore, the salivary layers adsorbed from water solutions onto the HA were found to be softer than those on silica.     

Protein chromatography in neat organic solvents

Pure formamide and ethylene glycol are used instead of water as processing media for protein chromatography. A number of common proteins are freely soluble in these solvents and most do not undergo irrersible inactivation in them. Batch adsorption studies reveal that proteins readily adsorbed to various ion-exchangers in formamide and ethyline glycol and subsequently can be completely desorbed by adding inorganic salts (LiCl and NH(4)NO(3)) to the solvents. The idea of protein separations in formamide and ethylene glycol is illustrated by column chromatography and preparative separation of mixtures of (i) oxidized A and B chains of insulin and (ii) lysozyme and ribonuclease on the anion-exchanger triethylaminoethycellulose and the cation-exchanger phosphocellulose, respectively.     

FTIR and XPS studies of protein adsorption onto functionalized bioactive glass

Adsorption and structural changes that occur upon interaction between methemoglobin (MetHb) and 5-methyl-aminomethyl-uridine forming enzyme (MnmE) with the surface of a bioactive glass (BG) were investigated by Fourier Transform Infrared (FTIR) spectroscopy and X-ray Photoelectron Spectroscopy (XPS). The effect of glutaraldehyde (GA) as a coupling agent for protein adsorption on BG was also investigated. The comparative analysis of FTIR spectra recorded from lyophilized proteins and from bioactive glass surface after protein adsorption was considered in order to obtain information about the changes in the secondary structure of the proteins. XPS data were used to determine the surface coverage. The unfolding of adsorbed proteins due to interactions between the internal hydrophobic protein domains and the hydrophobic BG surface was evidenced. After adsorption, the amount of α-helix decreases and less ordered structures (turns, random coils and aggregates) are preponderant. These changes are less pronounced on the BG functionalized with GA, suggesting that the treatment with GA preserves significantly larger amounts of α-helices in the structure of both proteins after adsorption.