Optical activity of cystine-containing proteins. II. Circular dichroism spectra of pancreatic ribonuclease A, ribonuclease S, and ribonuclease S-protein.

The circular dichroism spectra of pancreatic ribonuclease A, RNase S, and RNase S-protein have been measured in the wave length interval 198 to 300 mp under various conditions. The circular dichroism spectra of RNase A and S are very similar, possibly identical, over this entire wave length interval at neutral pH. That of RNase S-protein is substantially different. It is shown that the portion of the RNase A spectrum in the peptide-absorbing region may be fitted by assuming that 11.5% of the residues are in an a-helical conformation and 33% in a -conformation. Two of the three characteristic bands of the a-helix (poly-a,L-glutamic acid) and both of the characteristic bands of the p-structure (poly-L-lysine) were not adjusted in any way. However, the shortest wave length a-helical band (192 my) and the random coil band at 198 my were allowed to vary, in intensity, for best fit. In addition, it was necessary to include a positive band at 226 my. Arguments justifying this inclusion are given and depend largely on the presence of a small band observed in native RNase A at or near 240 my. The latter is believed to result from overlap of the (assumed) positive band near 226 my and the much more intense negative bands due to peptide transitions. In RNase Sprotein, the far ultraviolet bands are considerably diminished in intensity, but the maximum near 240 my is greatly intensified and shifted by 3 to 4 m/ to shorter wave lengths. It appears likely that the anomalous positive circular dichroism near 240 mjy is generated, or contributed to, by aromatic residues, some of which may be titratable tyrosines. However, titration, acetylation, and nitration results failed to establish this conclusively. A larger, negative, band at 275 my is shown, in agreement with others, to arise predominantly from tyrosine residues. In this case, also, modification experiments did not allow us to decide relative