Effects of urea and guanidine hydrochloride on peptide and nonpolar groups

Abstract

The free energy transfer of several N-acetyl(glycine)n ethyl esters (n = 1-3) and side chain derivatives (Ala, Val, Nva, Leu, Nle and Phe) from water to urea and guanidine hydrochloride solutions was determined from the solubility and distribution coefficients of these compounds between aqueous and nonaqueous phases. These uncharged model peptides, unlike the amino acids used for a similar study, avoid complication due to charge effects for the transfer process. The compounds with an increase in the number of glycyl groups show additivity of the group free energy toward the transfer from water to urea solution but not to guanidine hydrochloride solution. The derivatives with a side chain show that the principle of group additivity does not hold true for the aliphatic side chains for the transfer to either urea or guanidine hydrochloride solutions. In fact, the free energy of transfer of the side chains, i.e., aliphatic ones, is energetically unfavorable in moderately high denaturant concentration. Phenylalanyl, the only aromatic side chain studied here, showed a favorable free energy of transfer to the denaturant solutions. In addition, the values of the favorable free energy obtained in this study are much smaller than the values obtained from the study of the amino acids. The transfer of the glycyl group to the denaturant solutions is exothermic whereas the transfer of the side chains is endothermic in nature. The interaction of these solutes with urea and guanidine hydrochloride solutions was explained on the basis of the hypothesis of Roseman and Jencks (1975), which states that the driving force for the favorable interaction effect of cosolvents with solutes arises from a more favorable sum of the free energies of cavity formation and nonpolar interaction in the presence of cosolvents (denaturants). With polar solutes, the cosolvent must also be able to form H-bonds to make the overall free energy of transfer favorable.