HIGH-VALENT Cr, Mn, AND Fe SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 334

(a)
(b)
Figure 6.31. Proposed mechanism for the reaction of [Fe IV (O)(N4Py)] 2+ with Ac-Cys-
NHtBu (a) Ac-Met-NHtBu (b) (adapted from Abouelatta et al. [334] with the permis-
sion of the American Chemical Society).
orders of magnitude faster than the control reaction (Table 6.5). Second-order
rate constants were determined as 1.9 × 10 2 /M/s and 3.2 × 10 −1 /M/s for Cys and
Met, respectively. The reactions of Cys and Met with the ferryl species showed
different KIE values (Fig. 6.31). The reactions performed under D 2 O/CD 3 CN
solvent mixtures gave deuterium KIE ( k H / k D ) values of 4.3 and 1.0 for Cys
and Met, respectively (Fig. 6.31). The results suggest a rate-limiting electron
transfer, followed by a proton transfer in the reaction mechanism of the ferryl
species with Ac-Cys-NHtBu (Fig. 6.31a). A concerted HAT was ruled out,
which would have displayed a much larger KIE than 4.3. The electron transfer
step is supported by the high E 1/2 value [345, 346] for the Fe III/IV couple of
[Fe IV (O)(N4Py)] 2+ and the reducing capability of thiols. The ET-PT steps
formed the thiyl radical and the [Fe III (OH)(N4Py)] 2+ species. The Fe III species
slowly decomposed to [Fe II (CH 3 CN)(N4Py)] 2+ . Formation of disulfide 1 was