Three easy pieces.

Background: Differential scanning calorimetry is a powerful method that provides a complete thermodynamic characterization of the stability of a protein as a function of temperature. There are, however, circumstances that preclude a complete analysis of DSC data. The most common ones are irreversible denaturation transitions or transitions that take place at temperatures that are beyond the temperature limit of the instrument. Even for a protein that undergoes reversible thermal denaturation, the extrapolation of the thermodynamic data to lower temperatures, usually 25°C, may become unreliable due to difficulties in the determination of ΔCp.

Methods: The combination of differential scanning calorimetry and isothermal chemical denaturation allows reliable thermodynamic analysis of protein stability under less than ideal conditions.

Results And Conclusions: This paper demonstrates how DSC can be used in combination with chemical denaturation to address three different scenarios: 1) estimation of an accurate ΔCp value for a reversible denaturation using as a test system the envelope HIV-1 glycoprotein gp120; 2) determination of the Gibbs energy of stability in the region in which thermal denaturation is irreversible using HEW lysozyme at different pH values; and, 3) determination of Gibbs energy of stability for a thermostable protein, thermolysin.