We also found that at lower DTNB concentrations, where more
than 1 TNB molecule is liberated per sulfide (indicating a multistep
process), the kinetic traces retained their exponential character
(Fig. S1). In addition, the calculated pseudo-first-order rate constants
(kobs) at 320 nm and at 412 nm (representing the decay of
DTNB and the formation of TNB) agree within the experimental error
at all concentrations (for a representative case see Fig. S1). The kobs
vs DTNB plot was found to be linear with the fitted line passing
through the origin, showing that the reaction is first order for DTNB.
At first sight these are controversial because the noted change in
stoichiometry should also be associated with a marked change in
the kinetic pattern. As it will be shown later, the lack of such complication
is most likely due to a unique set of kinetic parameters
which yields a simple kinetic behavior. It is concluded that the rate
determining step in the model is reaction 2 which is followed by a
much faster release of the second TNB. In other words, sulfide reacts
much faster with RSSH than with RSSR (DTNB). In addition, the nondetectable
intercept in Fig. 2C suggests that the equilibrium in
reaction 2 is practically irreversible. At pH = 7.4, the apparent secondorder
rate constant of k2
app = 881 ± 15M−1s−1 for reaction 2 can thus
be obtained from the slope of the line on Fig. 2C.