The unfolding enthalpy of the pH 4 molten globule of apomyoglobin measuredby isothermal titration calorimetry

The unfolding enthalpy of the pH 4 molten globule from sperm whale apomyogl obin has been measured by isothermal titration calorimetry, using titration to acid pH. The unfolding enthalpy is close to zero at 20 degrees C, in co ntrast both to the positive values expected for peptide helices and the neg ative values reported for holomyoglobin and native apomyoglobin. At 20 degr ees C, the hydrophobic interaction should make only a small contribution to the unfolding enthalpy according to the liquid hydrocarbon model. Our resu lt indicates that some factor present in the unfolding enthalpies of native proteins makes the unfolding enthalpy of the pH 4 molten globule less posi tive than expected from data for peptide helices.