Figure 1.

Dependence of kinetic constants on pK_a for the reaction of thiols with different reagents. (A–D) Dependence of the observed second order kinetic constants (α k_RS⁻) on pK_a at pH = 7.4 and pH = 5.0 for the reaction of 1-chloro-2,4-dinitrobenzene (CDNB) and 4-chloro-7-nitrobenzofurazan (NBD-Cl) with thiols with different pK_a (see Section 4 for details). Continuous lines of bell-shaped curves coming from Equation (6) are obtained from the best-fit of the linear regressions, as shown in the corresponding insets. (E,F) Dependence of the second order kinetic constants (α k_RS⁻) on pK_a for the reaction of several thiols with different pK_a with taurine chloramine and with different disulfides. The data were obtained from previous studies [9,10]. The experimental points of the linear regressions (insets) are the Mean ± S.D. of the three independent experiments. The red arrows mark the maximum values of the bell-shaped graphs. The pK_a of the unperturbed protein cysteine is labelled with the green arrows.