Evidence for residual structure in acid- and heat-denatured proteins.

A number of very careful studies have been made in recent years of the thermal transitions which small globular proteins undergo at low pH. These transitions reflect the destruction of the ordered conformation of the native protein, and the products of the transition have the properties of highly disordered polypeptide chains. The principal objective of such studies has, however, been to determine the thermodynamic parameters of the transition, and relatively little effort has been made to characterize t,he products of the transitions precisely. Excellent examples of such studies are the studies of the thermal unfolding of ribonuclease, as reported by Hermans and Scheraga (l), Scott, and Scheraga (2), and by Brandts (3); the thermal unfolding of lysozyme, reported by Sophianopoulos and Weiss (4); and the thermal unfolding of chymotrypsinogen, as reported by Brandts and Lumry (5) and by Brandts (6, 7). Recent papers from this laboratory (8-11) on the other hand, have been primarily concerned with characterization of the product of denaturation. The studies have been confined, however, to proteins which have been denatured at room temperature by the addition of guanidine hydrochloride. The results have shown that this denaturation process leads to the formation of random coils, devoid of long-range, noncovalent structure. This conclusion appears to apply not only to proteins which have no disulfide cross-links, or in which disulfide bonds have been broken, but also to proteins which retain disulfide bonds. If disulfide bonds are present, the freedom of motion