The aggregation was also performed in VDB to investigate the role of the ionic strength in the aggregation process. The SAXS patterns for the AP205/ pMETAC 1:5 ratio at pH 7.0 and 8.0 result in an hcp structure with comparable dimensions reported for PBS conditions above (see Figure ). However, at pH 6.5, a reduced intensity of the Bragg peaks is observed. The role of the zeta potential on the AP205/pMETAC aggregation process in VDB was analyzed to understand the pH dependency. AP205 shows a negative zeta potential of -8.9 ± 0.6 mV at pH 7.0 (see Figure ). The addition of the cationic pMETAC triggers electrostatic attractions, and at an AP205 VLP/pMETAC weight ratio of 1:5, the magnitude of the zeta potential decreases from |-8.9 ± 0.6| to |-1.1 ± 0.5| mV. As previously shown, AP205 VLP is unstable at pH 6.5, aggregating into a partially ordered structure. Therefore, the destabilization of the superstructure at pH 6.5 arises upon reaching the IEP of AP205 VLP.