Effect of hydrostatic pressure on unfolding of alpha-lactalbumin: Volumetric equivalence of the molten globule and unfolded state

The effect of pressure on the unfolding of bovine alpha-lactalbumin was inv estigated by ultraviolet absorption methods. The change of molar volume ass ociated with unfolding, Delta V, was measured in the presence or absence of guanidine hydrochloride at pH 7. The Delta V was estimated to be -63 cm(3) /mol in the absence of a chemical denaturant. While in the presence of guan idine hydrochloride (GuHCl), it was found that Delta V was -66 cm(3)/mol at 25 degrees C and was independent of the concentration of GuHCl, despite th e fact that the molten globule fraction in the total unfolding product decr eased with the increase of GuHCl concentration. The results indicate that t he volume of alpha-lactalbumin only changes at the transition from a native to a molten globule state, and almost no volume change has been found duri ng the transition from a molten globule to the unfolded state.