The peptide–urea interaction. Excess enthalpies of ternary aqueous solution of sarcosylsarcosine diketopiperazine and alkylureas at 298.15 K

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Abstract
The excess enthalpies of ternary aqueous solutions of sarcosylsarcosine diketopiperazine and alkylureas have been determined and compared with the properties of other diketopiperazine–urea systems at 298.15 K. A group-contribution analysis shows that cyclic dipeptides are characterized by more favourable peptide–peptide and peptide–urea interactions than the linear oligopeptides. The other contributions, and in particular the cross-interactions between peptides and alkyl chains, seem to assume values equal to those of non-cyclic solutes. The particular properties of sarcosylsarcosine diketopiperazine aqueous solutions are discussed and an unexpected analogy with monosaccharide aqueous solutions is found.