Monitoring of lysozyme thermal denaturation by volumetric measurements and nanoDSF technique in the presence of N-butylurea.

The results of thermal studies of denaturation of hen egg white lysozyme (HEWL) in water and an aqueous solution of N-butylurea (BU) are presented. High-precision densimetric measurements were used to characterize and analyze the changes of the specific volume, v, during temperature elevation. The temperature of the midpoint of protein denaturation was also determined by nanoDSF technique (differential scanning fluorimetry). The densities of lysozyme solutions were measured at temperatures ranging from 298.15 to 353.15 K with an interval of 5 K at atmospheric pressure (0.1 MPa). The concentration of the protein covered the range from 2 to 20 mg per 1 ml of the solution. The optimal range of the concentration for the densimetric measurements was roughly estimated. In the transition region, the structural changes of the protein are accompanied by the biggest increase of ν values with temperature. Our measurements show that this effect can be monitored from volumetric data without precise determination of protein concentration. The results prove that a two-state model of denaturation could be used for data interpretation. Contrary to common misconception, the volumetric measurements suggest that the denatured protein does not necessarily need to be in a fully extended state. In this way, the 'protein volume paradox' could be explained. The surface area of the protein remains unchanged and thus the increase of the specific volume of the protein is relatively small. Additionally, the self-stabilizing effect of the protein in BU solution was reported. For the HEWL in pure water, this phenomenon was not observed.