Adsorption of virus-like particles on ion exchange surface: Conformational changes at different pH detected by dual polarization interferometry

Disassembling of virus-like particles (VLPs) like hepatitis B virus surface antigen (HB-VLPs) during chromatographic process has been identified as a major cause of loss of antigen activity. In this study, dual polarization interferometry (DPI) measurement, together with chromatography experiments, were performed to study the adsorption and conformational change of HB-VLPs on ion exchange surface at three different pHs. Changes in pH values of buffer solution showed only minimal effect on the HB-VLPs assembly and antigen activity, while significantly different degree of HB-VLPs disassembling was observed after ion exchange chromatography (IEC) at different pHs, indicating the conformational change of HB-VLPs caused mainly by its interactions with the adsorbent surface. By creating an ion exchange surface on chip surface, the conformational changes of HB-VLPs during adsorption to the surface were monitored in real time by DPI for the first time. As pH increased from 7.0 to 9.0, strong electrostatic interactions between oppositely charged HB-VLPs and the ion exchange surface make the HB-VLPs spread thinly or even adsorbed in disassembled formation on the surface as revealed by significant decrease in thickness of the adsorbed layer measured by DPI. Such findings were consistent with the results of IEC experiments operated at different pHs, that more disassembled HB-VLPs were detected in the eluted proteins at pH 9.0. At low pH like pH 5.0, however, possible bi-layer adsorption was involved as evidenced by an adsorbed layer thickness higher than average diameter of the HB-VLPs. The “lateral” protein-protein interactions might be unfavorable and would make additional contribution to the disassembling of HB-VLPs besides the primary mechanism related to the protein-surface interactions; therefore, the lowest antigen activity was observed after IEC at pH 5.0. Such real-time information on conformational change of VLPs is helpful for better understanding the real mechanism for the disassembling of VLPs on the solid-liquid interface.