Stanley Gill

Peptide-urea interactions as observed in diketopiperazine-urea cocrystal

In order to develop a more complete understanding of urea induced protein denaturation we have investigated the crystal structure of urea with the cyclic dipeptide diketopiperazine. This structure, determined to an R factor of 8.1%, shows extensive hydrogen bonding between urea and the peptide groups of diketopiperazine. These studies support a model where hydrogen bonding plays an important contribution in urea-induced protein denaturation. In the companion paper we present thermodynamic data for urea-peptide interactions in aqueous solution that further support this model.

The solubilities of five cyclic dipeptides in water and in aqueous urea at 298.15 K: a quantitative model for the denaturation of proteins in aqueous urea solutions

The solubilities of cyclo(L-alanylglycine), cyclo(L-alanyl-L-alanine), cyclo(glycyl-L-leucine), cyclo(L-valyl-L-valine) and cyclo(glycyl-L-phenylalanine) were determined in water and in aqueous urea solutions up to concentrations of 9 molar urea at 298.15 K. The solubilities of all cyclic dipeptides increase with increasing urea concentration. A simple equilibrium model, taking into account the activity of urea and that of water, fits the solubility data yielding apparent equilibrium constants describing the interactions occurring between urea and the peptide groups plus the alkyl groups that are next to these peptide groups. The apparent equilibrium constants were converted to Gibbs energy parameters for each amino acid residue which were then used to make a quantitative estimate of the contribution of urea to the denaturation of proteins.

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.