The paradox between m values and ΔCp's 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, Delta Cp, Delta N, Delta S, and Delta G quantities were determined. Because both Delta 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 Delta Cp, ratio for the two proteins. This is not the case, for it is found that the in value of RTCAM is 1.5 times that of RNase T1, while the denaturation Delta Cp's for the two proteins are identical. The paradox of why the two parameters, m and Delta Cp, an not equivalent in their behavior is of importance in the interpretations of their respective molecular-level meanings. It is found that the measured denaturation Delta Cp's are consistent with Delta Cp's calculated on the basis of empirical relationships between the change in surface area on denaturation (Delta ASA), and that the measured m value of RNase T1 agrees with m calculated from empirical data relating m to Delta 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.