Optical Rotatory Dispersion and Conformation of Various Globular Proteins.

The crucial test regarding the usefulness of the optical rotatory dtspersion method for the study of protein conformation was in confronting the results of this method w&the x-ray structural analysis of myoglobm. The high resolution x-ray structural analysis of Kendrew et al. on myoglobin crystals (1) and the optical rotation studies of Beychok and Blout (2) and Urnes, Imahori, and Doty (3) on the solutions of the protein have shown copclusively that about 70% of the polypeptide chain of myoglobin is in the a-helical conformation. On the basis of this fact, there is little doubt that also the majority of all those globular proteins which have a high Drude constant (X,) of 250 to 290 rnp, or a strongly negative Moffitt constant (b,) of about -150” or more, are a-helical in the straight segments of the chain. The problem of the conformation of the globular proteins which have a low X, of 180 to 220 rnp, or a b. near zero, however, is quite controversial. Two major views have been expressed and discussed in recent reviews (4, 5): (a) that these “nonhelical” proteins are completely disordered (or in some cases contain only short a-helical segments), and (b) that some order other than a-helix must be present in these macromolecules. Another interesting possibility has been considered by some authors, vix. that in the instances of b. = 0 both rightand left-handed helices might be present (6, 7). The purpose of the present paper is to report new results on a series of globular proteins having bo near zero, and to compare the results with those obtained by using several a-helical proteins. The effect of detergents on the proteins, as expressed in the change of bO, was investigated, since previous work has shown that detergents strongly affect the conformation of the nonhelical proteins (8-12). It was found in these previous studies that the effect depended on the size of the hydrophobic “tail” of the surface-active anion. In the present work, the experiments were conceived and planned keeping in mind the elucidation of the conformational transitions, and the writing was stimulated by recent publications of other authors, especially Tanford, De, and Taggart (13, 14), Meyer and Kauzmann (15), Wu and Scheraga (7), and Ghazanfar, Rickli, and Edsall (16). Each group of these authors was concerned only with one protein, and it was of interest to compare the behavior of a larger number of proteins toward the same agent. It was hoped to find out in this way how far reaching some of the conclusions are regarding the conformation of the “nonhelical” proteins and the forces