A new and simple procedure for the evaluation of the association of surfactants to proteins

Contrastive Study of the Foaming Properties of N-Acyl Amino Acid Surfactants with Bovine Serum Albumin and Gelatin

A detailed study on the foamability, foam stability, foam liquid-carrying capacity, and foam morphology of two N-acyl amino acid surfactants with bovine serum albumin (BSA) and gelatin were performed by foam scanning. The results showed that the foamability of the mixed system increased gradually and then tended to be stable with increasing surfactant concentration. The foamability of the high-concentration BSA system was stronger than that of the low-concentration BSA system. The foamability and foam stability of sodium N-lauroyl phenylpropanoic acid (N-C₁₂P)/BSA were better than those of sodium N-lauroyl propylamino acid (N-C₁₂A)/BSA, and the foamability and foam stability of N-C₁₂A/gelatin was better than those of N-C₁₂P/gelatin. The liquid-carrying capacity of the foam initially increased and then decreased with increasing time, and the maximum liquid-carrying capacity increased with increasing surfactant concentration. When the concentration of the surfactant was 8 mM, the drainage rate of N-C₁₂A/protein was higher than that of N-C₁₂P/protein. The morphology of the bubble gradually changed from spherical to polyhedron and the number of bubbles gradually decreased with time increasing. Differences in surfactant structure and protein type had an important effect on the number and area of foam.

Thermodynamics, interfacial pressure isotherms and dilational rheology of mixed protein-surfactant adsorption layers

Proteins and their mixtures with surfactants are widely used in many applications. The knowledge of their solution bulk behavior and its impact on the properties of interfacial layers made great progress in the recent years. Different mechanisms apply to the formation process of protein/surfactant complexes for ionic and non-ionic surfactants, which are governed mainly by electrostatic and hydrophobic interactions. The surface activity of these complexes is often remarkably different from that of the individual protein and has to be considered in respective theoretical models. At very low protein concentration, small amounts of added surfactants can change the surface activity of proteins remarkably, even though no strongly interfacial active complexes are observed. Also small added amounts of non-ionic surfactants change the surface activity of proteins in the range of small bulk concentrations or surface coverages. The modeling of the equilibrium adsorption behavior of proteins and their mixtures with surfactants has reached a rather high level. These models are suitable also to describe the high frequency limits of the dilational viscoelasticity of the interfacial layers. Depending on the nature of the protein/surfactant interactions and the changes in the interfacial layer composition rather complex dilational viscoelasticities can be observed and described by the available models. The differences in the interfacial behavior, often observed in literature for studies using different experimental methods, are at least partially explained by a depletion of proteins, surfactants and their complexes in the range of low concentrations. A correction of these depletion effects typically provides good agreement between the data obtained with different methods, such as drop and bubble profile tensiometry.

Tyrosine fluorescence probing of the surfactant-induced conformational changes of albumin

Tyrosine fluorescence in native proteins is known to be effectively quenched, whereas its emission increases upon proteins’ unfolding.

Adsorption of proteins at the solution/air interface influenced by added nonionic surfactants at very low concentrations for both components. 2. Effect of different surfactants and theoretical model

The influence of the addition of the nonionic surfactants dodecyl dimethyl phosphine oxide (C12DMPO), tetradecyl dimethyl phosphine oxide (C14DMPO), decyl alcohol (C10OH), and C10EO5 at concentrations between 10(-5) and 10(-1) mmol/L to solutions of β-casein (BCS) and β-lactoglobulin (BLG) at a fixed concentration of 10(-5) mmol/L on the surface tension is studied. It is shown that a significant decrease of the water/air surface tension occurs for all the surfactants studied at very low concentrations (10(-5)-10(-3) mmol/L). All measurements were performed with the buoyant bubble profile method. The dynamics of the surface tension was simulated using the Fick and Ward-Tordai equations. The calculation results agree well with the experimental data, indicating that the equilibration times in the system studied do not exceed 30 000 s, while the time required to attain the equilibrium on a plane surface is by one order of magnitude higher. To achieve agreement between theory and experiment for the mixtures, a supposition was made about the influence of the concentration of nonionic surfactant on the adsorption activity of the protein. The adsorption isotherm equation of the protein was modified accordingly, and this corrected model agrees well with all experimental data.

Evaluation of the number of binding sites in proteins from their intrinsic fluorescence: limitations and pitfalls

Changes in the intrinsic protein fluorescence with the additive concentration provide one of the most employed methodologies for the evaluation of the binding constant and the number of binding sites. In the last years, more than 175 studies have been published where the double logarithmic plot shown below is used toward determining the number of equivalent binding sites (n). Log [(F° - F)/F] = log K + n log [Q0 ]. However, the value of n evaluated by this procedure is unrelated to the number of equivalent binding sites; rather it represents the stoichiometry of the binding step. The confusion on the meaning of n arises upon assuming that the binding process is represented by the forward and backward elementary steps shown below, implying that binding of the n solutes takes place simultaneously, i.e. there are no intermediate species. nQ + P ⇆ Qn P. The conclusion that n is unrelated to the number of equivalent binding sites is supported by the fact that in all the systems considered (99% of them) n values are close to one and much smaller than those obtained by ultrafiltration. It is then remarkable, the profusion of publications in peer-reviewed, specialized journals including a conceptual error that confuses Hill's coefficient and/or the stoichiometry of the binding step with the number of independent binding sites. Here, we discuss the origin of this common misconception and provide alternative methods to determine the number of binding sites.

Development of a fluorescence model for the binding of medium- to long-chain perfluoroalkyl acids to human serum albumin through a mechanistic evaluation of spectroscopic evidence

A novel model for measuring the strength of perfluoroalkyl acid (PFAA) binding to human serum albumin (HSA) by use of the protein's native fluorescence is described. The model is derived from published properties of HSA and its interactions with other surfactants; it is consistent with these properties and experimental observations. The model's validity has been tested with both medium- to long-chain PFAAs (perfluoroheptanoate, perfluorooctanoate, perfluorononanoate, perfluorodecanoate, perfluoroundecanoate, perfluorohexanesulfonate, and perfluorooctanesulfonate) and short-chain PFAAs (perfluorohexanoate and perfluorobutanesulfonate). These experiments confirm the model as a valid description for the binding of medium- to long-chain PFAAs to HSA. Results indicate at least 2-3 PFAAs bind to each protein with affinity on the order of 10(4) M(-1). These binding strengths exhibit a dependence on protein concentration. Measured PFAA binding constants are approximately 10% of those values reported for fatty acids of similar chain length; correcting for protein concentration suggests the binding strengths may be as low as 2-3% of the corresponding fatty acids' affinities. Like fatty acids, the carboxylate PFAAs exhibit a trend of generally increasing binding strength with increased chain length. The model does not appear valid for the binding of short-chain PFAAs to HSA. Hill binding coefficients, fluorescence intensity measurements, and wavelengths of maximum emission suggest short-chain PFAAs associate with HSA differently and fail to promote the same conformational changes in the protein's tertiary structure as the medium- to long-chain PFAAs.

Binding of perfluorocarboxylates to serum albumin: a comparison of analytical methods

Perfluorochemicals are globally pervasive contaminants that are persistent, bioaccumulative, and toxic. Perfluorocarboxylic acids (PFCAs) with 8-13 carbons accumulate in the liver and blood of aquatic organisms; PFCA-protein interactions may explain this accumulation pattern. Here, the interactions between PFCAs with 8-11 carbons and serum albumin are examined using three experimental approaches: surface tension titrations, (19)F NMR spectroscopy, and fluorescence spectroscopy. Surface tension titrations indicate complex formation at high (mM) PFCA concentrations. Secondary association constants ranging from 10(2) to 10(4) M(-1) were determined from (19)F NMR titrations at high PFCA:albumin mole ratios. Fluorescence measurements indicate that PFCA-albumin interactions alter the protein conformation at low PFCA:albumin mole ratios (up to 5:1) and suggest two binding classes with association constants around 10(5) and 10(2) M(-1). While (19)F NMR and fluorescence provide both qualitative and quantitative information about PFCA-albumin interactions, surface tension provides only qualitative information. Limitations associated with instrumentation and methods require high PFCA concentrations in both surface tension and (19)F NMR experiments; in contrast, fluorescence allows for analysis of a wider range of PFCA concentrations and PFCA:albumin mole ratios. Results from this study indicate that fluorescence, though an indirect method, offers a more comprehensive picture of the nature of PFCA-albumin interactions.

Physico-chemical studies of molecular interactions between non-ionic surfactants and bovine serum albumin

Surfactants, particularly non-ionic types, are often added to prevent and/or minimize protein aggregation during fermentation, purification, freeze-drying, shipping, and/or storage. In this work we have investigated the interactions between two non-ionic surfactants (Tween 20 and Tween 80) and bovine serum albumin (BSA), as model protein, using surface tension, fluorescence measurements and computational analysis. The results showed that, in both cases, the surface tension profile of the surfactants curve is modified upon addition of the protein, and the CMC values of Tween 20 and Tween 80 in the presence of protein are higher than the CMC values of the pure surfactants. The results indicate that although Tween 20 and Tween 80 do not greatly differ in their chemical structures, their interactions with BSA are of different nature, with distinct binding sites. Measurements at different protein concentrations showed that the interactions are also dependent on the protein aggregation state in solution. It was found from fluorescence studies that changes observed in both the intensity and wavelength of the tryptophan emission are probably caused by modifications of tryptophan environment due to surfactant binding, rather than by direct interaction. Based on a computational analysis of a BSA three-dimensional model, we hypothesize about the binding mechanism of non-ionic surfactant to globular protein, which allowed us to explain surface tension profiles and fluorescence results.

Sodium dodecyl sulfate promoting a cooperative association process of sodium cholate with bovine serum albumin

Sodium cholate (NaC) was used as a representative bile salt in the process of cooperative binding to bovine serum albumin (BSA) in a mixture with sodium dodecyl sulfate (SDS). The experiments were performed in 0.02 M Tris-HCl buffer solution (pH 7.50), in the presence of 0.1% BSA and at 25 degrees C. The aim of this study is to provide information on the performance of the BSA in the promotion of cooperative binding of sodium cholate promoted by the presence of SDS. The method used to monitor the binding was based on the analysis of the effect of SDS and NaC concentrations and their mixtures upon the fluorescence intensity of the BSA tryptophan residues. Plots of the fluorescence emission bands in terms of the A0/A ratio vs surfactant concentrations, where A0 and A represent the areas of emission bands in the presence and absence of the surfactants, respectively, were drawn in order to investigate the surfactant interaction with the protein. An alternative methodology, the specific conductivity vs surfactant concentration plots, was used, which involves mixtures of SDS and NaC to investigate the association processes, through the determination of the critical aggregation concentration (cac, when in the presence of protein) and the critical micellar concentration (cmc). The results led to a general conclusion that as the mixed micellar aggregates become richer in the bile salt monomer, the tendency to lose the reactivity with the protein increases. According to our results, a clear evidence of the predomination of BSA-SDS-NaC complexes is found only for the SDS molar fraction above approximately 0.6, and below this fraction a tendency toward free mixed micelles starts to predominate.

Probing Structural Changes of Proteins Incorporated into Water-in-Oil Emulsions

The applicability of different techniques, that is, Differential Scanning Calorimetry (DSC), Fourier Transform Infrared Spectroscopy (FTIR), and intrinsic tryptophan fluorescence, for probing the structural changes of proteins in the water-in-oil emulsions are investigated using nondefatted bovine (BSA) and human serum albumin (HSA) as model proteins. FTIR shows that the overall secondary structure of the proteins changes to some extent, 12% for BSA and 9% for HSA, when these are incorporated into the emulsion. There was no evidence of changes in the distribution of secondary structural elements apart from the changes in overall secondary structure. A blue shift of 12 to 14 nm in the fluorescence emission maximum was observed for proteins in the emulsion and 3 to 11 nm in the simulated interior of the aqueous phase, thus indicating structural changes around the tryptophan residues. DSC scans indicated that the domains in the proteins change because the shape of the transition peaks changes, when the proteins were incorporated into the emulsions. The total enthalpy decreases for BSA and HSA when these are incorporated into the emulsion, and some changes to the transition temperatures are observed. All the applied techniques supplement each other to give a more complete picture of the structural changes in proteins in intact water-in-oil emulsions.

Effects of sodium dodecyl sulfate on the conformation and hemolytic activity of St I and St II, two isotoxins purified from Stichodactyla helianthus

The effect of sodium dodecyl sulfate (SDS) upon the conformation and hemolytic activity of St I and St II strongly depends on its concentration. At relatively low surfactant concentrations (ca. 0.5-5mM range) the surfactant leads to the formation of aggregates, as suggested by the turbidity observed even at relatively low (micromolar range) protein concentrations. In this surfactant range, the proteins show an increase in intrinsic fluorescence intensity and reduced quenching by acrylamide, with an almost total loss of its hemolytic activity. At higher surfactant concentrations the protein adducts disaggregates. This produces a decrease in fluorescence intensity, increase in quenching efficiency by acrylamide, loss of the native tertiary conformation (as reported by the near UV-CD spectra), and increase in alpha-helix content (as evidenced by the far UV-CD spectra). However, and in spite of these substantial changes, the toxins partially recover their hemolytic activity. The reasons for this recovering of the activity at high surfactant concentrations is discussed.

Effect of a zwitterionic surfactant (HPS) on the conformation and hemolytic activity of St I and St II, two isotoxins purified from Stichodactyla helianthus

N-hexadecyl-N-N'-dimethyl-3-ammonio-1-propane-sulfonate (BPS) is a zwitterionic surfactant that readily binds to sticholysins I and II, two sea toxins isolated from Stichodactyla helianthus. The binding constants, evaluated from changes in fluorescence intensities elicited by the surfactant, are approximately 0.5-0.7 microM(-1). The binding of the surfactant changes the conformation of the tertiary protein, without significant changes in its secondary structure, as reported from far-ultraviolet circular dichroism spectra. The changes elicited by HPS lead to loss of the native conformation (as reported from near-ultraviolet circular dichroism spectra) and to a shift of the intrinsic protein fluorescence toward longer wavelengths, an increase in fluorescence intensities and lifetimes, and a faster quenching by acrylamide. All these changes are indicative of a more expanded tertiary conformation. Despite this, the toxins fully retain their hemolytic activities, indicating that spectroscopic changes can be poor predictors of toxin activity.