Stabilization by urea during thermal unfolding-mediated aggregation of recombinant human interleukin-1 receptor (type...

The protein denaturing properties of urea are well-known and still the subject of debate. It has been noted that in some cases where urea concentrations are relatively low stabilization is afforded against aggregation. An explanation for this unusual effect has seemingly remained elusive. Evidence is offered to propose urea stabilization is related to its influence on the solvation property of the protein molecules when in contact with an unfolded hydrophobic surface that tends to increase the entropy of the local aqueous solvent. This property of urea is expected to lower the entropic driving force of unfolded-mediated aggregation despite the increase in enthalpy. The data presented from toluene transfer experiments into 2 M urea + 0.1 M sodium phosphate solutions showed that the solvation free energy change was negative up to ∼75 °C. The associated ΔΔH was positive, leading to the conclusion that entropy drives the solvation process within the temperature domain from ∼20° to 75 °C. Using thermodynamic parameters from the toluene solvation experiments, it was possible to accurately determine the T(m) shift of recombinant human interleukin-1 receptor type II (rhuIL-1R(II)). Heating experiments above the apparent T(m) in the same urea/phosphate solution support the thesis that urea inhibits the entropy-driven aggregation process of rhuIL-1R(II), adding yet another molecule to the list of low urea concentration stabilized molecules.