The paradox between m values and δ Cp ' for denaturation of ribonuclease T1 with disulfide bonds intact and broken

Urea‐induced denaturations of RNase T1 and reduced and carboxyamidated RNase T1 (RTCAM) as a function of temperature were analyzed using the linear extrapolation method, and denaturation m values, Δ Cp , Δ H , Δ S , and Δ G quantities were determined. Because both Δ Cp and m values are believed to reflect the protein surface area newly exposed on denaturation, the prediction is that the ratio of m values for RNase T1 and RTCAM should equal the Δ Cp ratio for the two proteins. This is not the case, for it is found that the m value of RTCAM is 1.5 times that of RNase T1, while the denaturation Δ Cp ' for the two proteins are identical. The paradox of why the two parameters, m and Δ Cp , are not equivalent in their behavior is of importance in the interpretations of their respective molecular‐level meanings. It is found that the measured denaturation Δ Cp ' are consistent with Δ Cp ' calculated on the basis of empirical relationships between the change in surface area on denaturation (Δ ASA ), and that the measured m value of RNase T1 agrees with m calculated from empirical data relating m to Δ ASA . However, the measured m of RTCAM is so much out of line with its calculated m as to call into question the validity of always equating m with surface area newly exposed on denaturation.