Thermal effects on the circular dichroism spectra of ribonuclease A and of ribonuclease S-protein.

The thermal denaturation of ribonuclease A and of S-protein in neutral aqueous solutions has been studied by means of their circular dichroism spectra. For RNase A, the positive circular dichroism extremum near 241 rnp shows a biphasic dependence upon temperature, a linear change between 15” and 50” being followed by a sharp one above 52”. The negative circular dichroism extremum at 277 rnp exhibits only a sharp change at the higher temperatures and appears unchanged between 10” and 50”. Upon cooling of the solution from 65”, enzymic activity is fully recovered, as is the initial magnitude of the 277 rnp extremum, but the ellipticity of the cooled solution at 241 rnp is lower. A positive circular dichroism extremum at 237 rnp is observed in spectra of Sprotein below 25”, as well as a negative circular dichroism extremum at 277 rnp; both of these extrema disappear upon heating. In contrast to RNase A, the thermal changes observed for S-protein are not reversible.