Irreversible unfolding of the neutral pH form of influenza hemagglutinin demonstrates that it is not in a metastable state

The thermal denaturation of the proteins of influenza virus has been measured by differential scanning calorimetry in the presence and absence of lipids as a function of scan rate. We have applied theories of irreversible thermodynamics to obtain the activation energy. In the presence of liposomes of dioleoylphosphatidylcholine with the ganglioside, GD(1a), the denaturation temperature of the hemagglutinin protein is lowered. This lowering of thermal stability is also reflected in the temperature dependence of the circular dichroism spectra. Quasi-elastic light scattering confirms that liposomes containing GD1a interact with the virus and inhibit the growth in the size of the particle as a function of temperature. Although the virus can fuse with the liposomes at higher temperatures, the enthalpy change for this process is not detectable. Our results also demonstrate that the compact folded structure of the influenza hemagglutinin protein is not a kinetically trapped metastable high-energy form.