HIGH-VALENT Cr, Mn, AND Fe SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 290

% reduction

E ' o (V)

0 20 40 60 80 100

-0.90

Oxalate

Pyruvate

-0.70

Formate

-0.42

Isocitrate

-0.38

Xanthine

-0.36

β -Hydroxybuturate

-0.35

-0.33 Malate

Cysteine

-0.32

Glutathione

-0.23

- Penicillamine

-0.19 Lactate

-0.08 Glycolate

-0.03 Succinate

+0.08 Ascorbate

- Methionine

- Trptophan

Figure 6.6. The extent of reaction of chromium with various compounds after 1 minute
in 1 M Tris-HCl, pH 7.4. Reactions were carried out at 25°C for all compounds except
methionine, tryptophan, glycolate, and xanthine, which were at 37°C. Initial
[Cr(VI)] = 2.7 × 10 −4 M and [compound] = 2.5 × 10 −2 M, except [xanthine] = 6.3 × 10 −3 M
(redrawn from Connett and Wetterhahn [87]).

which triggers the reduction of Cr(IV) via metastable Cr(V) intermediates
[103]. The intermediates of Cr(V) and Cr(IV) as mono- and bisglutathionato
complexes with or without a coordinated peroxo moiety were characterized.
Recently, the reductions of [(salen)Cr V =O] + (salen = bis(salicylidene)ethyl-
enediamine) complexes by methionine and substituted methionine in aqueous
acetonitrile have been conducted [104]. The rate law was determined to be
first order with respect to each reactant. The reaction involved the transfer of
oxygen from the complex to methionine to form methionine sulfoxide and a
stable µ-oxo dimer of Cr. Kinetics of the reactions of Cr VI and Cr V complexes
with dialkyl sulfides showed the rate constants for the reduction of Cr V were
two orders of magnitude higher than that of Cr VI [105].