The structural stability of an amino acid fragment containing the oligomerization domain (residues 303-366) of the tumor suppressor p53 has been studied using high-precision differential scanning calorimetry (DSC) and circular dichroism spectroscopy (CD). Previous NMR solution structural determinations have revealed that the fragment forms a symmetric 29.8 kDa tetramer composed of a dimer of dimers (p53tet) [Lee, W., Harvey, T. S., Yin, Y., Yau, P., Litchfield, D., & Arrowsmith, C. H. (1994) Nature Struct. Biol. 1, 877-890]. Thermal unfolding of the tetramer is reversible and can be described as a two-state transition in which the folded tetramer is converted directly to unfolded monomers (N4<==>4U). According to the DSC and CD data, the population of intermediate species consisting of folded monomers or dimers is insignificant, indicating that isolated dimeric or monomeric structures have a much lower stability than the dimer and do not become populated during thermal denaturation under the conditions studied. The transition temperature of unfolding is found to be highly dependent on protein concentration and to follow the expected behavior for a tetramer that dissociates upon unfolding. Experiments conducted at pH 4.0 in 25 mM sodium acetate at a tetramer concentration of 145.8 microM have a transition temperature (Tm) of 75.3 degrees C while at 0.5 microM the value drops to 39.2 degrees C. The enthalpy change of unfolding at 60 degrees C is 26 kcal (mol of monomer)-1 with a heat capacity change of 387 cal (K.mol of monomer)-1. The stability of p53tet is dependent on pH and salt concentration.(ABSTRACT TRUNCATED AT 250 WORDS)