Circular dichroism spectroscopy was used to measure the thermal unfolding of bovine pancreatic ribonuclease A (RNase A) with various concentrations of guanidine hydrochloride (GuHCl). A red shift in transition midpoint temperatures, T
m, occurred with increasing concentration of the strong protein denaturant. van Hoff enthalpy changes, ΔH°, were calculated and plotted as a function of T
m to determine the heat capacity change, ΔC
p, for denaturation. A value of 4.02 ± 0.02 kJ mol–1 K–1 was calculated from d( ΔH)/d(ΔT
m). Reported values for ΔC
p range from 4.2 to 9.6 kJ mol–1 K–1. The shift in T
m for RNase A with increasing concentration of GuHCl suggests that the protein is undergoing substantial changes in its secondary structure.
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