Interactions of myoglobin with urea and some alkylureas. II. Calorimetric and circular dichroic studies.

The interactions of myoglobin with urea, methyl-, N,N'-dimethyl- and ethylurea were studied by means of calorimetry and circular dichroism (CD). The enthalpies of transfer from water to aqueous denaturant solutions are positive for the alkylureas and negative for urea. The difference is due to the presence of hydrophobic groups in the alkylureas. Gibbs free energies of transfer for urea solutions were obtained from preferential binding data determined previously. An attempt is made to interpret the values of the thermodynamic quantities in terms of various interactions between protein and denaturant. Analysis of the far-ultraviolet CD spectra reveals some differences in the denaturing activity of urea and the alkylureas, the latter being stronger denaturants than urea. Myoglobin displays relatively high stability towards these denaturants since concentrations above 5 M are needed for achieving major conformational changes.