Volume changes associated with guanidine hydrochloride, temperature, and ethanol induced unfolding of lysozyme

We studied the guanidine hydrochloride (GdnHCl)-, temperature-, and ethanol-induced unfolding of lysozyme using high-precision densitometric measurements, aiming to characterize and compare the volume changes, Δν(o), accompanying the unfolding of a protein simultaneously by different means, that is, by GdnHCl, temperature, and an organic cosolvent, EtOH. The data obtained are also compared with other means of unfolding, such as high-pressure- and dimethyl sulfoxide (DMSO)-induced denaturation. To aid in interpreting the temperature dependence of the apparent specific volume of lysozyme, we have also carried out pressure perturbation (PPC) and differential scanning calorimetry (DSC) measurements under the same solution conditions. The PPC method allows the detection of very small volume changes with high accuracy. Next to the strong temperature dependence of Δν(o), the volume changes associated with the unfolding of the protein are found to be very sensitive to the type of denaturation. The apparent specific volume decreases upon the heat- and GdnHCl-induced denaturation. The observed volume change for the GdnHCl-induced denaturation is 60% larger (i.e., more negative) than that obtained for thermal denaturation. Conversely, the apparent specific volume increases by an order of magnitude and becomes positive upon ethanol-induced denaturation, similar to the aprotic organic solvent, DMSO. Hence, depending on the type of denaturant (temperature, pressure, chemical denaturants, or cosolvents), positive and negative volume changes of unfolding are found, which can--at least in part--be attributed to the formation of different unfolded state structures (including clustering) of lysozyme. The standard Gibbs energy changes upon denaturation, ΔG(D)(o), for the various perturbation parameters are found to be similar, however, if extrapolated to zero cosolvent concentration.