Figure 1.

Chevron curves of wild-type barnase at 25°C and pH 6.3 (solid circles) and pH 7.23 in the presence of 0.1 M Na₂SO₄ (open circles). The solid curve is the best fit of the observed rate constants to a three-state model with Eqs. 1–3. It yields the following values: K_IU = 0.009; m_IU = 4.05 M⁻¹; k_IN = 11.9 s⁻¹; m_IN = 2.63 M⁻¹; k_NI = 8.6 × 10⁻⁶ s⁻¹; m_NI = 2.80 M⁻¹; m_NU (sum of m_IU, m_IN, and m_NI) = 9.49 M⁻¹; and m_IU/m_NU = 0.43. The dashed line illustrates the linear extrapolation of unfolding rate constant log k_u. It should be noted that the k_u in water, obtained with guanidinium chloride (GdmCl) and linear extrapolation, is 10 times smaller than the value obtained with urea (1).