C. Veerman

Mesostructure of fibrillar bovine serum albumin gels

The mesostructure of bovine serum albumin (BSA) at low pH was investigated. Rheological measurements were performed to determine the critical percolation concentration (c(p)). A decreasing c(p) with increasing ionic strength was found. Fibrils with a contour length of about 100-300 nm were found using transmission electron microscopy. The measured conversion of monomers into fibrils was independent of ionic strength (0.20-0.30 M). Dilution of BSA samples showed that the aggregation process is reversible and that there exists a critical concentration for the self-assembly of BSA. We explain the decreasing c(p) with increasing ionic strength in terms of an adjusted random contact model.

Rheology and structure of ovalbumin gels at low pH and low ionic strength

The objective of this study was to relate the rheological behavior of ovalbumin gels at low pH and low ionic strength to their mesoscopic structure, using rheological measurements and Transmission Electron Microscopy (TEM). Varying pH, ionic strength and protein concentration, we obtained transparent solutions, transparent gels, opaque gels, or turbid gels, upon heating and subsequent cooling of the ovalbumin solutions. At equal pH and increasing salt concentration we found an increase in turbidity, suggesting that the structure of the aggregates changed from linear or branched to more clustered aggregates. The gel strength increased with increasing salt concentration. A similar trend was observed at equal ionic strength and increasing pH. TEM micrographs of ovalbumin aggregates at pH 2 and 15 mM NaCl show that the ovalbumin aggregates are linear. Increasing the pH from 2 to 3.5 results in structures that are still linear, but with a higher degree of clustering. No random aggregates were observed. TEM micrographs of the gel phase at pH 3.5 and 30 mM NaCl (turbid) show that the structure consists of linear aggregates organized in large clusters of approximately 230–350 nm. At pH 2 and 30 mM NaCl (transparent) the gel consists of single strands with a diameter of about 3.3–3.9 nm, i.e. one or at most two monomers thick.

Irreversible self-assembly of ovalbumin into fibrils and the resulting network rheology

The self-assembly of ovalbumin into fibrils and resulting network properties were investigated at pH 2, as a function of ionic strength. Using transmission electron microscopy (TEM), the effect of ovalbumin concentration on the contour length was determined. The contour length was increasing with increasing ovalbumin concentration. TEM micrographs were made to investigate the effect of ionic strength on the contour length. In the measured ionic strength regime (0.01-0.035 M) fibrils of approximately equal length (+/-200 nm) were observed. TEM micrographs showed that the contour length of the fibrils, versus time after dilution, remained constant, which indicates that the self-assembly of ovalbumin is irreversible. Using the results of rheological measurements, we observed a decreasing critical percolation concentration with increasing ionic strength. We explain this result in terms of an adjusted random contact model for charged semiflexible fibrils. Hereby, this model has now been proven to be valid for fibril networks of beta-lg, BSA and, currently, for ovalbumin.

The effect of shear flow on the percolation concentration of fibrillar protein assemblies

The objective of this study was to investigate the effect of shear flow on the percolation concentration (cp) for solutions of fibrillar protein assemblies. Theoretical calculations were performed to obtain cp versus Peclet number. They were based on a random contact model for rodlike particles, making use of a shear dependent excluded volume per fibril. We found cp to increase with increasing Peclet number. Results of flow birefringence measurements were used to obtain the rotational diffusion coefficient at cp, which enables one to transform the theoretically obtained cp(Pe) into a prediction for cp versus shear rate. This prediction was used to fit viscosity measurements as a function of shear rate, near the percolation threshold. A satisfactory fit was found indicating that the percolation threshold, cp, as function of shear rate can be predicted by combining theory and optical measurements.

Properties of Fibrillar Food Protein Assemblies and their Percolating Networks

Properties of Fibrillar Protein Assemblies and their Percolating Networks. PhD thesis, Wageningen University, The Netherlands Keywords: bovine serum albumin, complex fluids, excluded volume, fibrils, gels, innovation, b-lactoglobulin, ovalbumin, percolation, proteins, rheology, rheo-optics, self-assembly, structure function relations. Abstract The objective of this thesis was to explore the assembly of food proteins into fibrils, and to describe the resulting percolating systems at rest and under shear flow, in terms of mesoscopic fibril properties. The effect of ionic strength on the percolation concentration for three different food proteins, namely b-lactoglobulin, bovine serum albumin and ovalbumin is described. The dependence of ionic strength on the percolation concentration was explained using an adjusted random contact model, in which the percolation concentration is related to the average number of contacts per particle, and the excluded volume of the rod. Also the contour length, persistence length, and bending rigidity for these three protein assemblies were determined, as well as the phase behaviour of b-lactoglobulin at low pH. A new multistep Ca2+-induced cold gelation process is described to prepare b-lactoglobulin gels at very low protein concentrations (0.07%). The behaviour of fibrillar assemblies of ovalbumin under oscillatory shear, close to the critical percolation concentration, was probed with the use of rheo-optical measurements and Fourier transform rheology. Also the effect of shear flow on the critical percolation concentration for solutions of fibrillar protein assemblies was investigated. Results of viscosity measurements were analysed using percolation theory, where the effect of shear flow was taken into account. The experimental results were compared with our theoretical calculations for the percolation concentration versus shear, based on a random contact model for rodlike particles, making use of a shear dependent excluded volume per fibril. In conclusion conditions leading to gel formation, in terms of mesoscopic fibril properties, under non-flow conditions have been discussed. The observed critical gelation concentration was explained in terms of an excluded volume per fibril (at zero shear). The influence of shear flow on this critical gelation concentration was also described. Here, the critical percolation concentration versus shear flow could again be expressed in terms of an excluded volume per fibril, in this case as a function of shear.