Secondary structure analysis of HIV-1-gp41 in solution and adsorbed to aluminum hydroxide by Fourier transform infrared spectroscopy

The formulation of human vaccines often includes adjuvants such as aluminum hydroxide that are added to enhance the immune responses to vaccine antigens. However, these adjuvants may also affect the conformation of antigenic proteins. Such structural modifications could lead to changes in antigenicity such that suboptimal protective immune responses could be generated relative to those induced by the vaccine antigens alone. Here, we used attenuated total reflectance infrared spectroscopy (ATR-FTIR) to compare the secondary structures of recombinant HIV-1-gp41 (gp41) in solution or adsorbed to aluminum hydroxide. The gp41 secondary structure content was 72% α-helices and 28% β-sheets in 5 mM formate buffer p2H 2.5, while it was 66% β-sheets and 34% random coil in acetonitril/2H2O (95/5:v/v). A fully reversible conformational change of gp41 in acetonitril/2H2O (95/5:v/v) was observed upon addition of either 35 mM formate p2H 2.5 or 0.1% (w/v) detergent (Tween 20, Hecameg, Brij 35 or β-d-octyl-glucopyranoside). When gp41 was adsorbed to aluminum hydroxide in the presence of 0.1% (w/v) detergent, in either formate or in acetonitril/2H2O (95/5:v/v) its secondary structure remained stable and was identical to that of gp41 in 5 mM formate buffer p2H 2.5. The method described here could be applied for the characterization of gp41 conformers for use in immunological screening of antigens, and more generally to all antigenic proteins adsorbed to aluminum hydroxide.