Table II.
Parameters Characterizing the Thermal Unfolding of RNase Sa 7S and 8S Variants in 30 mM MOPS, pH 7.0
| Variant | ΔHma (kcal/mol) | ΔSmb (cal/mol/K) | ΔCpc (kcal /mol/K) | Tmd (°C) | ΔTme (°C) | ΔΔG° (60°C)f (kcal/mol) |
|---|---|---|---|---|---|---|
| 7S | 105 | 309 | 1.68 | 66.6 | — | — |
| 7S (I92D) | 73 | 237 | 1.41 | 34.0 | –32.6 | –9.6 |
| 7S (D79A) | 102 | 293 | 1.15 | 75.3 | 8.7 | 2.2 |
| 8S | 97 | 277 | 1.07 | 77.2 | — | — |
| 8S (I92D) | 84 | 261 | 1.41 | 50.2 | –26.9 | –7.1 |
| 8S (I70D) | 78 | 247 | 1.08 | 41.5 | –35.6 | –9.5 |
| 8S (I92A) | 102 | 303 | 1.33 | 63.9 | –13.3 | –3.2 |
| 8S (Y80A) | 87 | 263 | 1.29 | 58.0 | –19.2 | –4.8 |
a
Enthalpy change at Tm. The error is ±5 kcal/mol.
b
ΔSm = ΔHm/Tm.
c
Determined by Kirchoff analysis. The errors are ±0.05 kcal/mol/K.
d
Midpoint of the thermal unfolding curve. The error is ±0.3°C.
e
ΔTm = Tm (variant) – Tm of 7S or 8S.
f
ΔΔG° = ΔG° (variant) – ΔG° of 7S or 8S, calculated as described in Ref.22 using a reference temperature of 60°C. The error is ±0.3 kcal/mol. Negative values indicate decreases in stability.