Surface viscosity, proteins

As is known, if one blows air bubbles in pure water, no foam is formed. On the other hand, if a detergent or protein (amphiphile) is present in the system, adsorbed surfactant molecules at the interface produce foam or soap bubble. Foam can be characterized as a coarse dispersion of a gas in a liquid, where the gas is the major phase volume. The foam, or the lamina of liquid, will tend to contract due to its surface tension, and a low surface tension would thus be expected to be a necessary requirement for good foam-forming property. Furthermore, in order to be able to stabilize the lamina, it should be able to maintain slight differences of tension in its different regions. Therefore, it is also clear that a pure liquid, which has constant surface tension, cannot meet this requirement. The stability of such foams or bubbles has been related to monomolecular film structures and stability. For instance, foam stability has been shown to be related to surface elasticity or surface viscosity, qs, besides other interfacial forces. 

The foam drainage, surface viscosity, and bubble size distributions have been reported for different systems consisting of detergents and proteins. Foam drainage was investigated by using an incident light interference microscope technique. 

Figure 8.12 illustrates the effect of complex formation between protein and polysaccharide on the time-dependent surface shear viscosity at the oil-water interface for the system BSA + dextran sulfate (DS) at pH = 7 and ionic strength = 50 mM. The film adsorbed from the 10 wt % solution of pure protein has a surface viscosity of t]s > 200 mPa s after 24 h. As the polysaccharide is not itself surface-active, it exhibited no measurable surface viscosity (t]s < 1 niPa s). But, when 10 wt% DS was introduced into the aqueous phase below the 24-hour-old BSA film, the surface viscosity showed an increase (after a further 24 h) to a value around twice that for the original protein film. Hence, in this case, the new protein-polysaccharide interactions induced at the oil-water interface were sufficiently strong to influence considerably the viscoelastic properties of the adsorbed biopolymer layer. 

The foam stability of /3-cas foams progressively decreased with added Tween 20. In contrast, there was a very sharp transition in equilibrium film thickness at R = 0.5. Surprisingly, surface diffusion of /3-cas was not detected at any R value in these films. This was unexpected since it has been reported that adsorbed layers of /3-cas are characterized by a very low surface viscosity [3], signifying that protein-protein interactions in /3-cas films are very weak. We had expected to observe surface diffusion either in the films stabilized by... 

Indeed, a direct relationship between the lifetimes of films and foams and the mechanical properties of the adsorption layers has been proven to exist [e.g. 13,39,61-63], A decrease in stability with the increase in surface viscosity and layer strength has been reported in some earlier works. The structural-mechanical factor in the various systems, for instance, in multilayer stratified films, protein systems, liquid crystals, could act in either directions it might stabilise or destabilise them. Hence, quantitative data about the effect of this factor on the kinetics of thinning, ability (or inability) to form equilibrium films, especially black films, response to the external local disturbances, etc. could be derived only when it is considered along with the other stabilising (kinetic and thermodynamic) factors. Similar quantitative relations have not been established yet. Evidence on this influence can be found in [e.g. 2,13,39,44,63-65]. 

AG is the free energy of activation for flow, h is Planck s constant, and A is the area per flow unit. This theory was applied to the surface viscosity of a number of proteins, measured as a function of surface pressure (MacRitchie, 1970). Plots of log tjs against II were found to be linear, enabling AG and A to be evaluated. The data, which are summarized in Table VI, show that the flow unit for all the proteins and poly-DL-alanine is a segment of approximately 6-8 amino acid residues and also that the free energy of activation for flow is similar for all... 

Crosslinking of protein monolayers by mercuric ion (MacRitchie, 1970) and silicic acid (Minones et al., 1973) has been reported. These studies are relevant to poisoning by heavy-metal ions and to silicosis, effects that seem likely to result from attack on the cell membrane proteins. Crosslinking by mercuric ion was detected by a spectacular increase in surface viscosity and a decrease in compressibility when a number of proteins (BSA, insulin, ovalbumin, and hemoglobin) were spread on 0.001 M mercuric chloride solution. Poly-DL-alanine was unaffected whereas poly-L-lysine and poly-L-glutamic acid were affected in a similar manner to the proteins, indicating that mercuric ion interacts with the ionizable carboxyl and amino groups on the protein side—chains. Silicic acid similarly caused protein monolayers... 

An important feature of the mosaic or subunit model of the molecular organization of lipid and lipid-protein films and membranes (4, 5,12) is the cohesion between subunits and between molecules within each subunit (2, 13). The latter can be measured as microviscosity by polarization of fluorescence (14) of the surface film the former is measured as surface viscosity by various techniques (15,16). The relevance of this parameter to the exploration of biological surfaces cannot be overemphasized. Surface viscosity, in fact, has also been invoked as a force stabilizing pulmonary alveoli (6, 7,13, 16). 

Information about fluidity and viscosity of bilayers of artificial and natural membranes has been obtained from electron spin resonance studies in which the mobility of the spin-labelled species along the surface plane of the membrane is determined (17). However, the monolayer of either lipid, protein, or lipid-protein systems at the air-water interface, makes an ideal model because several parameters can be measured simultaneously. Surface tension, surface pressure, surface potential, surface viscosity, surface fluorescence and microviscosities, surface radioactivity, and spectroscopy may be determined on the same film. Moreover, the films can be picked up on grids from which they may be observed by electron microscopy, studied further for composition, and analyzed for structure by x-ray diffraction and spectroscopy. This approach can provide a clear understanding of the function and morphology of the lipid and lipid-protein surfaces of experimental membranes. However, the first objective is to obtain molecular correlations of surface tension, pressure, potential, and viscosity. 

This manuscript correlates the chemical structure of selected lipids and proteins and their surface viscosity. Our studies probed certain concepts (Figure 1) how and if surface viscosity depends on MW, the... 

Surface viscosity was determined by a torsion oscillation method in which the period of oscillation of a paraffin-coated mica ring was measured on a clean water surface and lipid-covered surface (6, 13). Since the period of oscillation is proportional to the surface viscosity, the difference in period (At) between lipid or protein film and a clean aqueous surface measures viscosity and relates to either molecular area, film pressure, or time of interaction (t). The period values were repro-... 

Protein and Lipid-Protein Systems. The high surface viscosity of BSA and the low surface viscosity of RNase had been observed by a torsion rotational method that used an extremely large torque (2, 6). In the present experiments, the protein is dispersed in the aqueous subphase to a final concentration of 10 /xg/ml. Both film pressure and surface viscosity are measured as a function of time (to 40 min). The film pressure of BSA was 21 dynes/cm at mixing and remained constant for 40 min. Also, surface viscosity reached a high value instantaneously (although the first measurement was made at 5 min) and remained constant for 40 min. In contrast, RNase built up pressure slowly, from 3 dynes/cm... 

Figure 10. Influence of cholesterol on the surface viscosity of protein films adsorbed from 0.15M NaCl at 25° C. Protein BSA ( ) 10 fig/ml, RNase (O) 10 ng/ml. At 40 min, microliter quantities of lipid-DPL or cholesterol [J i g/id in chloroform-methanol (85 15)] were added on the protein film. The solvent alone did not affect either surface pressure or viscosity.

MW 702) > lactoside (MW 859). The high surface viscosity of BSA as compared with RNase is probably the result of structural characteristics of the protein rather than size. 

Lipid—Lipid and Lipid—Protein Interactions. The DPL—cholesterol and the protein—DPL systems are particularly amenable to interpretation using our membrane model. The high viscosity lattice of DPL can be broken by cholesterol (Figure 9), and the lattice of BSA can be broken by a lipid (e.g., DPL, Figure 10), with a marked loss of surface viscosity. This lattice collapse means formation of independent membrane subunits whose lateral valences are saturated within the subunit, thereby producing a fluid system (Figure 1A) the subunit could be a lipid-lipid system, as with DPL and cholesterol, or a lipid-protein system. The phenomenon of lattice collapse with loss of surface viscosity is impressive in the DPL-albumin system since individually both components have a high surface viscosity. 

In light of the experiment on the compression and expansion of the monolayer and the calculation on the depth of the dragged substrate, we conclude that not only is the coiling of the protein monolayer important in the observed effect but also the structure of the sheath of bound water below the monolayer (subphase) and the extent to which it is bound to the monolayer effect the surface viscosity. Thus we advance the following hypothesis of a non-structured and structured subphase. 

The membrane/protein interface with the bulk is dominated by the discontinuity of the physical chemical properties of the reaction space. On one side of the borderline there is a low viscosity, high dielectric constant matrix where rapid proton diffusion can take place. On the other side of the boundary, there is a low dielectric matrix that is covered by a large number of rigidly fixed charged residues. The dielectric boundary amplifies the electrostatic potential of the fixed charges and, due to their organization on the surface of proteins, a complex pattern of electrostatic potentials is formed. These local fields determine the specific reactivity of the domain, either with free proton or with buffer molecules. In this chapter we shall discuss both the general properties of the interface and the manner in which they affect the kinetics of defined domains. 

We report on the use of surface viscosity measurement at the planar oil—water interface to monitor time-dependent structural and compositional changes in films adsorbed from aqueous solutions of individual proteins and their mixtures. Results are presented for the proteins casein, gelatin, oC-lactalbumin and lysozyme at the n-hexadecane— water interface (pH 7, 25 °C). We find that, for a bulk protein concentration of 10 wt%, while the steady-state tension is invariably reached after 5—10 hours, steady-state surface shear viscosity is not reached even after 80—100 hours. Viscosities of films adsorbed from binary protein mixtures are found to be sensitively dependent on the structures of the proteins, their proportions in the bulk aqueous phase, the age of the film, and the order of exposure of the two proteins to the interface. 

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