The pK(a), values of two peptides derived from the formin-binding protein 28 WW domain [Ac-Lys-Thr-Ala-Asp-Gly-Lys-Thr-NH(2) (D7), Ac-Tyr-Lys-Thr-Ala-Asp-Gly-Lys-Thr-Tyr-NH(2) (D9)] were determined by potentiometric titration in the temperature range from 25 to 60 degrees C, and their heat capacities were determined, by differential scanning calorimetry, in the temperature range from 10 to 90 degrees C. For both peptides, heat capacity has a maximum at t approximate to 50 degrees C, with height about 0.1 kcal/(mol x deg), suggesting that a modest unfolding transition occurs. The first two pK(a)'s are low at temperatures below 50 degrees C, suggesting that the two lysine residues are close to each other and the peptides have bent shapes at lower temperatures; this effect is greater for D7 compared with D9. With increasing temperature beyond 50 degrees C (i.e., that of the thermodynamic unfolding transition), pK(a1) and pK(a2) increase rapidly for D9, whereas their temperature variation is less significant for D7. This observation, and the fact that the enthalpies and entropies of the dissociation of the two first protons (determined from the temperature dependence of the respective PK(a)'s) decrease significantly near the transition temperature, suggest that the peptide undergoes a transition from a bent to an amorphous shape and that the presence of charged lysine residues stabilizes the folded state.