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Effects of different salts (NaCl, MgC12, CaC12, GdmC1, NaBr, NaC104, NaH2PO4, Na2S04) on the stability of the ubiquitin molecule at pH 2.0 have been studied by differential scanning calorimetry, circular dichroism, and Tyr fluorescence spectroscopies. It is shown that all of the salts studied significantly increase the thermostability of the ubiquitin molecule, and that this stabilization can be interpreted in terms of anion binding. Estimated thermodynamic parameters of binding for C1-show that this binding is relatively weak (Kd = 0.15 M) and is characterized by a negative enthalpy of -15 kJ/mol per site. Particularly surprising was the observed stabilizing effect of GdmCl through the entire concentration range studied (0.01-2 M), however, to a lesser extent than stabilization by NaCl. This stabilizing effect of GdmCl appears to arise from the binding of C1-ions. Analysis of the observed changes in the stability of the ubiquitin molecule in the presence of GdmCl can be adequately described by combining the thermodynamic model of denaturant binding with C1-binding effects.

The change in heat capacity DC p for the folding of ribonuclease A was determined using differential scanning calorimetry and thermal denaturation curves. The methods gave equivalent results, DC p ϭ 1.15 6 0.08 kcal mol Ϫ1 K Ϫ1 . Estimates of the conformational stability of ribonuclease A based on these results from thermal unfolding are in good agreement with estimates from urea unfolding analyzed using the linear extrapolation method. ⌬C p ϭ d~⌬H !0d~T !1 ! to estimate DC p , and the results are shown in Figure 1. The resulting DC p values are given in Table 2. DC p values based on plots of DH vH and DH fit from the DSC data, a global fit of all of the DSC data, and on the difference between the posttransition C p~D ! and pretransition C p~N ! baselines of the DSC experiments are also given. The DC p value based on plots of DH cal vs. Keywords

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