Depletion-flocculation in oil-in-water emulsions using fibrillar protein assemblies.

This paper shows that low concentrations of beta-lactoglobulin fibrils can induce depletion-flocculation in beta-lactoglobulin-stabilized oil-in-water emulsions. The minimum required fibril concentration for flocculation was determined experimentally for fibril lengths of about 3 and 0.1 microm. The minimum fibril concentration for flocculation is two orders of magnitude higher for the short fibrils than for the long ones. These experimental results correspond well with a theoretical prediction based on a model of spinodal decomposition. In addition, rheological measurements were performed, verifying that flocculation was induced by a depletion mechanism. The results of this study show that the minimum concentration required for depletion-flocculation can be tuned by varying the length of the fibrils.

Mesoscopic properties of semiflexible amyloid fibrils.

We have determined the contour length, persistence length, bending rigidity, and critical percolation concentration for semiflexible amyloid fibrils formed from the globular proteins beta-lactoglobulin, bovine serum albumin, and ovalbumin. The persistence length was estimated using an adjusted random contact model for highly charged semiflexible chains. We have found contour lengths in the range of 50 nm to 10 microm and persistence lengths in the range of 16 nm to 1.6 microm. This wide range of contour and persistence lengths and the ease of preparation of these amyloid fibrils make them ideal model systems for the study of semiflexible polymers.

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.

A new multistep Ca2+-induced cold gelation process for beta-lactoglobulin.

The objective of this study was to obtain beta-lactoglobulin (beta-lg) gels at very low protein concentrations using a new multistep Ca(2+)-induced cold gelation process. In the conventional cold gelation process, salt free beta-lg solutions were heated at neutral pH, cooled, and cross-linked by adding salts. In our new process, first, long linear beta-lg fibrils were formed at pH 2. Solutions of these fibrils were cooled, and subsequently, the pH was adjusted to 7 or 8. Transmission electron microscopy studies showed that the long linear fibrils formed at pH 2 were stable when the pH was adjusted to 7 or 8. In the final step, the fibrils were cross-linked using CaCl(2). Using rheological measurements, the critical percolation concentration was determined. In the new multistep cold gelation process, the critical percolation concentration was an order of magnitude lower than in the conventional cold gelation method.

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.

Effect of electrostatic interactions on the percolation concentration of fibrillar beta-lactoglobulin gels.

The effect of electrostatic interactions on the critical percolation concentration (c(p)) of fibrillar beta-lactoglobulin gels at pH 2 was investigated using rheological measurements, transmission electron microscopy (TEM), and performing conversion experiments. A decreasing c(p) with increasing ionic strength was found. The fraction of nonaggregated beta-lactoglobulin was independent of ionic strength in the regime of 0.01-0.08 M. TEM experiments showed long fibrils (2-7 microm) for ionic strengths between 0.01 and 0.08 M. Since both the conversion of monomers and the contour length of the fibrils were independent of ionic strength (0.01-0.08M), the linear increase of c(p) with the Debye length can be attributed purely to an increase of electrostatic repulsion between the fibrils. This increase is explained in terms of an adjusted random contact model which takes into account the charge and semiflexibility of the fibrils.