Values of the thermodynamic quantities Delta H degrees and Delta S degrees for the reactions "Asp-peptide reversible arrow Asu-peptide reversible arrow beta-Asp-peptide" in aqueous solution have been obtained for the model peptides Ac-Gly-X-Gly-Gly-NHMe and Ac-X-Gly-NHMe (X = Asp, beta-Asp, Asu; Asu = aminosuccinyl residue) from the temperature-dependence of equilibrium constants. The Delta H degrees and Delta S degrees values for the cyclization reactions of the carboxylic-acid form of the Asp and beta-Asp side chains of the dipeptides and tetrapeptides are positive and coincident within experimental error. Medium values are 34 kJ mol(-1) and 127 J K-1 mol(-1) for Delta H degrees and Delta S degrees, respectively. The molar enthalpies and molar entropies of the Asp-dipeptide and beta-Asp-dipeptide, and of the Asp-tetrapeptide and beta-Asp-tetrapeptide, did not exhibit significant differences.