Two site-directed mutants of Escherichia coli thioredoxin (L78K and L78R) were designed to study the effect of placing a charged residue in the hydrophobic core of the protein. Both mutants retain catalytic activity in the assembly of phage M13. Thermal denaturation of both these mutant proteins at pH 7.0 shows a reduction of stability of approximately 4 kcal.mol-1 with respect to the oxidized wild-type form. The thermal denaturation of the protein fits a dimeric state model. A significant reduction in the change in heat capacity (delta Cp) on unfolding is observed compared to oxidized wild-type thioredoxin. We present data to indicate that this reduction in delta Cp is attributable to structural perturbations resulting in localized unfolding of the native protein and exposure to solvent of residues that are buried in the wild-type protein.