Combined NMR-observation of cold denaturation in supercooled water and heat denaturation enables accurate measurement of ΔCp of protein unfolding

Cold and heat denaturation of the double mutant Arg 3→Glu/Leu 66→Glu of cold shock protein Csp of Bacillus caldolyticus was monitored using 1D 1H NMR spectroscopy in the temperature range from −12°C in supercooled water up to +70°C. The fraction of unfolded protein, fu, was determined as a function of the temperature. The data characterizing the unfolding transitions could be consistently interpreted in the framework of two-state models: cold and heat denaturation temperatures were determined to be −11°C and 39°C, respectively. A joint fit to both cold and heat transition data enabled the accurate spectroscopic determination of the heat capacity difference between native and denatured state, ΔCp of unfolding. The approach described in this letter, or a variant thereof, is generally applicable and promises to be of value for routine studies of protein folding.