Electrostatically driven protein aggregation: beta-lactoglobulin at low ionic strength.
The aggregation of beta-lactoglobulin (BLG) at ambient temperature was studied using turbidimetry and dynamic light scattering in the range 3.8<pH<5.2 in 0.0045 M NaCl, and in the ionic strength range 0.0045-0.5 M at fixed pH=5.0. The initial rate of aggregation, taken as the initial slope of turbidity vs time, (dtau/dt)0, indicated maximum aggregation near pH 4.6 (below the isoelectric point of 5.2), but the dependence of the initial rate of aggregation on pH was highly asymmetric. At pH 5.0, (dtau/dt)0 strongly increased with a decrease in ionic strength I from 0.1 to 0.0045 M and was found to be nearly linear with 1/I. DLS measurements revealed an increase in particle size with time, with the appearance of bimodal distributions in which the fast and slow modes corresponded, respectively, to a BLG dimer and to larger aggregates in the 100-800 nm range. At conditions of slower aggregation, DLS revealed the consumption of dimers to form higher order aggregates with no intermediate species. Computer modeling (Delphi) was used to visualize the electrostatic potential around the dimer to elucidate the pH and ionic strength dependence of the initial aggregation rates. The aggregation process appears to comprise an initial fast consumption of the dimer, whose dependence on pH and I arises from the interaction of the positive and negative domains of interacting dimers, followed by the slow formation of much larger aggregates with relatively little sensitivity to pH and I. The open-ended nature of BLG aggregation is thought to arise from the asymmetry of the dimer charge distribution.