Acid-induced folding of proteins

The addition of HCl, at low ionic strength, to the native state of apomyoglobin, beta-lactamase, and cytochrome c caused these proteins to adopt an essentially fully unfolded conformation in the vicinity of pH 2. However, contrary to expectation, the addition of further acid resulted in refolding to a compact conformation with the properties of a molten globule. The major factor responsible for the refolding is believed to be the binding of the anion, which minimizes the intramolecular charge repulsion that initially brought about the unfolding.