The phase transition between a compact denatured state and a random coil state in staphylococcal nuclease is first-order.

Three mutants of staphylococcal nuclease containing a tryptophan substitution have been examined in the full length (149 residues) protein and in a large fragment (residues 1 to 136). The large fragments are not in the native state and are a good model of the denatured state. However, these large fragments do show signs of residual structure that breaks down upon titration with guanidine hydrochloride. They share some similarities with what has become known as the molten globule state. The thermal unfolding of these mutant fragments was followed by tryptophan fluorescence. Tryptophan fluorescence was treated as an order parameter and analyzed to determine the order of the observed transition. The critical exponent of the order parameter as the transition temperature is approached is significantly higher than the value of 1/2 predicted by mean field theory for a second-order transition and is similar to that observed for the transition of the full length, wild-type, protein. This is strong evidence that the breakdown of this intermediate compact denatured state is a cooperative, first-order phenomenon.