Importance of physical vs. chemical interactions in surface shear rheology

Abstract

The stability of adsorbed protein layers against deformation has in literature been attributed to the formation of a continuous gel-like network. This hypothesis is mostly based on measurements of the increase of the surface shear elasticity with time. For several proteins this increase has been attributed to the formation of intermolecular disulfide bridges between adsorbed proteins. However, according to an alternative model the shear elasticity results from the low mobility of the densely packed proteins. To contribute to this discussion, the actual role of disulfide bridges in interfacial layers is studied. Ovalbumin was thiolated with S-acetylmercaptosuccinic anhydride (S-AMSA), followed by removal of the acetylblock on the sulphur atom, resulting in respectively blocked (SX) and deblocked (SH) ovalbumin variants. This allows comparison of proteins with identical amino acid sequence and similar globular packing and charge distribution, but different chemical reactivity. The presence and reactivity of the introduced, deblocked sulfhydryl groups were confirmed using the sulfhydryl -disulfide exchange index (SEI). Despite the reactivity of the introduced sulfhydryl groups measured in solution, no increase in the surface shear elasticity could be detected with increasing reactivity. This indicates that physical rather than chemical interactions determine the surface shear behaviour. Further experiments were performed in bulk solution to study the conditions needed to induce covalent aggregate formation. From these studies it was found that mere concentration of proteins (to 200 mg/mL, equivalent to a surface concentration of around 2 mg/m 2 ) is not sufficient to induce significant aggregation to form a continuous network. In view of these results, it was concluded that the adsorbed layer should not be considered a gelled network of aggregated material (in analogy with three-dimensional gels formed from heating protein solutions). Rather, it would appear that the adsorbed proteins form a highly packed system of proteins with net-repulsive interactions. D

Interfacial properties of heat-treated ovalbumin

“…Whether the aggregates participate to interfacial film viscoelasticity via disulfide bonds formation cannot be excluded, but it probably play a secondary role as the number of exposed sulfhydryl groups (accessible to DTNB) is more or less independent of the heating time at 80 • C while the shear elastic constant at the plateau was significantly lower. This confirms the results obtained by Wierenga et al [30] showing that introduction of reactive sulfhydryl groups on the surface of ovalbumin molecule did not lead to increased shear elastic constant. In addition, the decreased of the shear elastic constant at the plateau value with prolonged heating time from 5 to 40 min at 80 • C while neither the kinetics of formation of intermolecular interactions in the interfacial layer nor the foam stability was modified indicated that the shear elastic constant at the plateau value does not constitute the most prevalent criteria to predict the foam stability of protein dispersions confirming previous results we obtained studying the interfacial properties of non-native structures of β-lactoglobulin.…”

“…5 gives the values recorded at equilibrium, 10 h after the beginning of absorption. Note that these values remained constant as soon as [30] and Pezennec et al [4] for the same protein concentration in the bulk (100 µg mL −1 ) and for pH conditions far from ovalbumin pI. These values are in the range for a monolayer of protein adsorbed at the air/water interface.…”

Compression and shear surface rheology in spread layers of β-casein and β-lactoglobulin

“…For β-lactoglobulin, the evidence points towards the fact that the surface layer is a physically jammed state for surface pressures above ∼10 mN/m. This is supported by the similarity of viscoelastic data between 2D hard colloids and β-lactoglobulin layers [12], by the fact that the threshold concentration for emergence of a shear modulus is so high [11], and finally by very recent experiments where the presence or absence of chemically active groups was shown not to play an important role [36].…”

Interfacial viscoelasticity, yielding and creep ringing of globular protein–surfactant mixtures

“…While nearly no sub-phase corrections are required for the data measured using the DWR geometry, in other instruments used for making interfacial measurements with lower characteristic values of the Boussinesq numbers, the sub-phase contributions to the torque must be carefully accounted for (see, for example, the papers by Mannheimer and Schechter 33 and Oh and Slattery 34 . Several studies 3,4,31,35 have noted the long time scales required for adsorbed interfacial layers to reach equilibrium, with Wierenga et al 35 waiting for as long as 24 hours. This effect is even more pronounced in surfactant-protein mixtures 31,36 .…”