Interactions of myoglobin with urea and some alkylureas. I. Solvation in urea and alkylurea solutions.

The interactions of myoglobin with urea, methyl-, N,N'-dimethyl- and ethylurea in aqueous solutions were studied by density measurements. From the densities at constant chemical potential and constant molality, the partial specific volumes of myoglobin in these solutions as well as the extent of preferential binding of urea and alkylurea to myoglobin were determined. It has been found that water and not the denaturant is preferentially bound in urea solutions and alkylurea solutions up to 4 M so that the Gibbs free energy of myoglobin, i.e., its chemical potential in a denaturant solution, is larger than in water. This behavior of myoglobin is different from that of other globular proteins for which preferential binding of urea has been found. It appears that preferential hydration of myoglobin is due to its high content of ionic groups.