Micro-phase separation explains the abrupt structural change of denatured globular protein gels on varying the ionic strength or the pH

Aqueous solutions of globular proteins gel after heat-induced denaturation. The structure of beta-lactoglobulin gels was studied as a function of the pH (2-8) and the NaCl concentration (0-1 M) over a wide range of length scales (1 nm-100 mm) using a combination of scattering techniques and microscopy. The gel structure depended on the strength of the electrostatic interaction that was varied by changing the charge density of the proteins (pH) or the screening length (ionic strength). Homogeneous so-called finely stranded gels were formed at strong interaction, while more heterogeneous so-called particulate gels were formed at weak interaction. It is shown that phase separation of growing irreversibly bound protein aggregates explains the abrupt transition between the two structures for small changes of the ionic strength or the pH. Phase separation was limited to the formation of protein rich domains with a radius of several microns that associated into the particulate gels. The structure of homogeneous gels is shown to be determined by the amplitude of the (critical) concentration fluctuations of protein aggregates that were frozen-in by gelation. The characteristic length scales of the homogeneous gels varied between about 20 nm and 1 mu m.