Apparently Opposing Effects of Temperature and Guanidinium Chloride in the Denaturation of Ribonuclease A

Sa？etak The thermal denaturation of ribonuclease A in the presence of guanidinium chloride (GdmCl) was studied by means of circular dichroism (CD). In the presence of GdmCl the transition temperatures decrease with increasing denaturant concentration. However, closer examination of the results obtained shows the following feature: the negative values of molar ellipticity decrease with temperature in the absence of the denaturant; after the addition of the denaturant ellipticity minima appear at temperatures which depend on the denaturant concentration. The higher the GdmCl concentration the lower the temperature of the minimum. In 4 molar (and higher) GdmCl the minimum does not appear and the negative molar ellipticity increases throughout the whole temperature range examined. After reduction of the disulfide bonds, similar behaviour is observed with the minimum at each denaturant concentration being shifted towards a lower temperature. Though there is no obvious explanation for this behaviour, it appears that at the temperatures above the minimum some secondary structure is regained owing to decreased protein denaturant interactions