DILUTION ENTHALPIES OF ALKANOLS IN CONCENTRATED AQUEOUS-SOLUTIONS OF UREA AT 25-DEGREES-C

Enthalpies of dilution of some aliphatic alcohols were determined at 2 5-degrees-C in aqueous 7M urea solutions by flow microcalorimetry . Th e excess enthalpies were expressed as power expansion series in molali ties referred to 1 kg of constant composition urea-water mixture. This urea-water mixture was utilized throughout as a mixed solvent. The va lues of the second enthalpic virial coefficients were all found to be positive and generally lower than the corresponding values in water. L arge differences were encountered, as in water, by comparing normal an d branched isomeric propanols and butanols. For one system it was poss ible to measure the third coefficients, which were also positive. The second enthalpic coefficients were found to increase with the molecula r weight of the alkanols. These facts suggest that in the presence of a large concentration of urea, the excess enthalpies are mainly determ ined by apolar interactions. This is surprising and potentially rich i n consequences for a better understanding of the interactions among am ino acid residues distantly situated in the primary sequences but topo logically near in the loops of globular proteins. An analysis, carried out using the Savage-Wood additivity group method, shows that the ent halpic contributions (that appear to play a crucial role in water in m aking the polar interaction to be favorable) become essentially unfavo rable in urea-water solvent. The hypothesis that the peptide-peptide i nteractions are prevented by the preferential solvation of urea is als o discussed.