Interactions of alkylureas with l‐valine, l‐valyl‐l‐valine, l‐leucine, and l‐leucyl‐l‐leucine

Abstract

The solubilities of l‐valine, l‐leucine, l‐valyl‐l‐valine, and l‐leucyl‐l‐leucine have been determined in methyl‐, N,N′‐dimethyl‐, and ethylurea solutions. Gibbs free energies of transfer of both amino acids and dipeptides as well as the peptide unit from water to alkylurea solutions have been ascertained. Enthalpies of transfer for the same compounds and the peptide unit have been determined from calorimetric measurements. The additivity principle for the contribution of the peptide unit and the side chains is obeyed neither for Gibbs free energy nor for the enthalpy of transfer. The most characteristic feature of Gibbs free energy data are the negative values for the transfer of the peptide unit to urea solutions and the positive values for the transfer to alkylurea solutions. Moreover, Gibbs free energy of transfer of the l‐valyl and l‐leucyl side chains is negative in alkylurea and urea solutions. The enthalpy of transfer values for urea solutions are generally negative, whereas the values for alkylurea solutions are positive. The enthalpy values for the l‐valyl and l‐leucyl side chains are positive as well. However, the enthalpy values for the peptide unit are all negative.