Solvation of human serum albumin in aqueous alkylurea solutions

Solvation of human serum albumin in aqueous solutions of urea, methyl-, N,N′ -dimethyl-, and ethylurea was studied by density measurements. From the densities at constant molality and constant chemical potential, the preferential solvation parameters of human serum albumin were determined. In all solutions the denaturant is preferentially bound. From preferential solvation data, Gibbs free energies of transfer from water to alkylurea solutions were calculated. Since the enthalpies of transfer were determined previously the entropies of transfer would also be obtained. Based on these and model compound data an attempt is made to identify various interactions involved in solvation. The total binding of denaturant to protein could also be calculated from preferential solvation data. The following major binding sites have been identified: ionic groups on the surface of protein molecules, peptide groups and aromatic side-chains. The correlation between the total number of sites and the number of urea molecules bound is satisfactory.     

Interactions of alkylureas with l-valine,l-valyl-l-valine,l-leucine,and l-leucyl-l-leucine

The solubilities of l -valine, l -leucine, l -valyl-l -valine, and l -leucyl-l -leucine have been determined in methyl-, N,N′-dimethyl-, and ethylurea solutions. Gibbs free energies of transfer of both amino acids and dipeptides as well as the peptide unit from water to alkylurea solutions have been ascertained. Enthalpies of transfer for the same compounds and the peptide unit have been determined from calorimetric measurements. The additivity principle for the contribution of the peptide unit and the side chains is obeyed neither for Gibbs free energy nor for the enthalpy of transfer. The most characteristic feature of Gibbs free energy data are the negative values for the transfer of the peptide unit to urea solutions and the positive values for the transfer to alkylurea solutions. Moreover, Gibbs free energy of transfer of the l -valyl and l -leucyl side chains is negative in alkylurea and urea solutions. The enthalpy of transfer values for urea solutions are generally negative, whereas the values for alkylurea solutions are positive. The enthalpy values for the l -valyl and l -leucyl side chains are positive as well. However, the enthalpy values for the peptide unit are all negative.