Is the molten globule a third thermodynamic state of protein? The example of alpha-lactalbumin.

Thermal and denaturant-induced transitions of the acid molten globule state of bovine alpha-lactalbumin (acid [A] state) are analyzed by scanning calorimetry, titration calorimetry, viscosimetry, and derivative spectroscopy. A denaturant-induced heat effect of the A state is shown by a calorimetric difference titration of the A-state versus unfolded (reduced) alpha-lactalbumin. However, changes of viscosity and derivative spectra do not parallel the heat effect. At thermal denaturation monitored by derivative spectroscopy and scanning microcalorimetry the presence of a gradual transition in alpha-lactalbumin A state is shown. The results are consistent with the existence of tertiary interactions in the A state and the absence of a cooperative unfolding transition of the molten globule. The results do not support the idea that the molten globule is a third thermodynamic state.