Urea-diketopiperazine interactions: a model for urea induced denaturation of proteins.

The solubility of diketopiperazine (DKP) in aqueous urea (U) solutions with molalities ranging from 0 to 16 mol kg-1 (corresponding to urea activities ranging from 0 to 10 mol kg-1) has been measured as a function of the urea activity at 298.15 K. In accordance with a previous study the solubility of diketopiperazine increases with increasing urea activity but drops sharply at a urea activity of 5.7 +/- 0.2 mol kg-1. This drop in solubility can be attributed to the formation of a DKP.U2 cocrystal. The solubility data were fitted to a simple model based on the stoichiometry of the DKP.U2 to yield an intrinsic equilibrium constant kappa describing the interactions occurring between a urea molecule and a peptide group of diketopiperazine in aqueous solution, its value being kappa = 0.0447 +/- 0.0007 kg mol-1. When the activity of water is taken into account, kappa has a lower value of 0.0398 +/- 0.0007 kg mol-1.