132 C. B. AKFINSEN AND R. R. REDFIELD 



VISCOSITY STUDIES ON NATIVE AND OXIDIZED RIBONUCLEASE 



o. 



8 



10 12 



14 



16 



18 



20 22 24 26 



mg/ml 



Fig. 3. Viscosity studies on native and oxidized ribonuclease. 



carboxyl groups present in other side chains along the protein structure. Indeed, 

 evidence for the latter type of hydrogen bond in ribonuclease has been presented 

 by several workers, including Shugar (1952), Harrington and Schellman (1956), 

 and Tanford and collaborators (1955), all of whose studies suggest the presence 

 of two or three such hydrogen-bonded tyrosines out of the six tyrosine residues 

 present in the molecule. In passing, one should also mention the careful studies 

 of Linderstr0m-Lang and Hvidt (1955) on the hydrogen exchange of peptide 

 bond linked hydrogens, in which they have found that the rate of exchange of a 

 large percentage of these exchangeable hydrogen atoms becomes almost in- 

 stantaneous in guanidine chloride or urea containing solutions. 



One can fix, approximately, the positions of the various half-cystine residues 

 along the peptide chain of ribonuclease by an examination of the partially re- 

 constructed molecule shown in the first figure (and particularly in the molecule 

 as derived from the data of Hirs, ]\Ioore and Stein). By the utilization of the 

 spacing between such cystine residues and by studies which we are now carrying 

 out on the problem of assignment of half-cystine sequences to specific disulfide 

 bridges, one may construct a fanciful diagram which accounts in principal, at 

 least, for the viscosity behavior summarized in the last figure. It should be 

 emphasized that the specific cross-linking of half-cystine residues shown in this 



