Bovine serum albumin pre-aggregates in dispersion undergo a reentrant aggregation phenomenon upon adding trivalent counterion Y3+ found by dynamic light scattering experiment

i.e. the aggregation of BSA-PAs is very slow at low counterion concentration

and it increases as the counterion concentration increases

while aggregation decreases as counterion concentration further increases. This reentrant condensation phenomenon is attributed to the short-ranged repulsive hydration force originating from increased dipoles generated by excess multivalent counterion binding. Small-angle light scattering and TEM (transmission electron microscope) results indicate that the incompact filamentous aggregates are formed at low and high counterion concentration

and the compact globular aggregates are formed at intermediated counterion concentration

which further supports the exist of hydration force at high counterion concentration. Hydration force occurs mainly on charged or polar patches of protein to protect it from aggregating; the aggregation of the filamentous BSA-PAs at hydrophobic patches at the two ends is still possible.