Strong impact of ionic strength on the kinetics of fibrilar aggregation of bovine β-lactoglobulin

We investigate the effect of ionic strength on the kinetics of heat-induced fibrilar aggregation of bovine beta-lactoglobulin at pH 2.0. Using in situ light scattering we find an apparent critical protein concentration below which there is no significant fibril formation for all ionic strengths studied. This is an independent confirmation of our previous observation of an apparent critical concentration for 13 mM ionic strength by proton NMR spectroscopy. It is also the first report of such a critical concentration for the higher ionic strengths. The critical concentration decreases with increasing ionic strength. Below the critical concentration mainly dead-end species that cannot aggregate anymore are formed. We prove that for the lowest ionic strength this species consists of irreversibly denatured protein. Atomic force microscopy studies of the morphology of the fibrils formed at different ionic strengths show shorter and curvier fibrils at higher ionic strength. The fibril length distribution changes non-monotonically with increasing ionic strength. At all ionic strengths studied, the fibrils had similar thicknesses of about 3.5 nm and a periodic structure with a period of about 25 nm.