Mechanism of hydration of urea and guanidium ion: A model study of denaturation of proteins

Abstract

The hydration of urea and guanidium ion have been studied by quantum chemical and molecular mechanics methods. The hydration of urea takes place only through the amino groups and not through the carbonyl group. One mole of urea decreases the free concentration of water by seven moles and one mole of guanidium ion by twelve moles. The interaction energies (ΔE's) for the hydration of urea and guanidium ion in both the primary and secondary spheres are more than that of water-water interactions. Also, the interaction energy of urea and guanidium ion with water is more than the interaction energy of urea and guanidium ion with the potential hydrogen bonding sites in the peptide groups of the model pentapeptides. The "collagen type structure" (with φi=-30°, ψi=120°) has been taken as model for functional domain which is stabilized by interaction with water through free potential hydrogen bonding sites. It is shown that these denaturants, reduce the free concentration/activity of water effectively and remove the water molecules from the protein surface. The computational results also demonstrate why lower concentrations of guanidium ion is sufficient to bring about the denaturation in comparison to urea. The interaction studies also reveal that there is no complex formation between the peptides groups and denaturant molecules. The cause of protein aggregation in the presence of denaturants and hence, the loss of biological activity is also discussed.