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

Urea-induced denaturations of RNase T1 and reduced and carboxyamidated RNas e T1 (RTCAM) as a function of temperature were analyzed using the linear ex trapolation 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 denaturat ion, the prediction is that the ratio of m values for RNase T1 and RTCAM sh ould 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, w hile the denaturation Delta Cp's for the two proteins are identical. The pa radox of why the two parameters, m and Delta Cp, an not equivalent in their behavior is of importance in the interpretations of their respective molec ular-level meanings. It is found that the measured denaturation Delta Cp's are consistent with Delta Cp's calculated on the basis of empirical relatio nships between the change in surface area on denaturation (Delta ASA), and that the measured m value of RNase T1 agrees with m calculated from empiric al data relating m to Delta ASA. However, the measured m of RTCAM is so muc h out of line with its calculated m as to call into question the validity o f always equating m with surface area newly exposed on denaturation.