Denaturation of phycocyanin by urea and determination of the enthalpy of denaturation by microcalorimetry.

Denaturation of the protein phycocyanin in urea solution was investigated by microcalorimetry, ultraviolet and visible spectroscopy, circular dichroism and sedimentation equilibrium. The results consistently demonstrated that in the presence of 7 M urea this protein is completely denatured. By assumings a two-state mechanism, an apparent free energy of unfolding at zero denaturant concentration, (formula: see text) was found to be 4.4 kcal/mole at pH 6.0 and 25 degrees C. By microcalorimetry the enthalpy of denaturation of phycocyanin app was found to be -230 kcal/mole at 25 degrees C. The relatively large negative enthalpy change results from protein unfolding and changes in protein solvation.