Unfolding Thermodynamics of Trp-Cage, a 20 Residue Miniprotein, Studied by Differential Scanning Calorimetry and Circular Dichroism Spectroscopy

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Abstract
Small proteins provide convenient models for computational studies of protein folding and stability, which are usually compared with experimental data. Until recently, the unfolding of Trp-cage was considered to be a two-state process. However, no direct experimental evidence for this has been presented, and in some cases, the contrary has been suggested. To elucidate a detailed unfolding mechanism, we studied the thermodynamics of unfolding of Trp-cage by differential scanning calorimetry (DSC) and circular dichroism (CD) spectroscopy. The observation that at low temperatures only ∼90−95% of Trp-cage exists in the native conformation presented an analytical challenge. Nevertheless, it was found that the DSC and CD data can be fitted simultaneously to the same set of thermodynamic parameters. The major uncertainty in such a global fit is the heat capacity change upon unfolding, ΔCp. This can be circumvented by obtaining ΔCp directly from the difference between heat capacity functions of the native and unfolded states. Using such an analysis it is shown that Trp-cage unfolding can be represented by a two-state model with the following thermodynamic parameters: Tm = 43.9 ± 0.8 °C, ΔH(Tm) = 56 ± 2 kJ/mol, ΔCp = 0.3 ± 0.1 kJ/(mol·K). Using these thermodynamic parameters it is estimated that Trp-cage is marginally stable at 25 °C, ΔG(25 °C) = 3.2 ± 0.2 kJ/mol, which is only 30% more than the thermal fluctuation energy at this temperature.