Interactions of proteins with solvent components in 8M urea.

The interaction between proteins and urea was investigated by determining the apparent
partial specific volumes of nine proteins in the molecular weight range 9000 to
41,000 in 8 M urea solution. The apparent partial specific volumes were determined under
both isomolal and isopotential conditions. The preferential interaction with solvent components,
calculated from the obtained values, varied between zero and 0.14 g of urea per
gram of protein. In none of the cases was the interaction preferential with water. The
total binding of denaturant to each protein was calculated and the correlation between
the total number of residues and the number of moles of urea bound per mole of protein
was obtained. Several models were tested for the observed and expected number of denaturant
molecules bound to the protein. A good correlation was obtained for the model
in which one urea molecule was bound to each pair of peptide units and one to each
aromatic side chain. There was no correlation whatever with protein hydrophobicity. The
changes in volume upon transferring the proteins from dilute salt solution to 8 M urea
were calculated; comparison with values reported in the literature for various denaturants
showed good agreement.