HIGH-VALENT Cr, Mn, AND Fe SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 285

4000

Cr(VI)

Cr(IV)

2000

Cr(V)

Cr(III)

0

300 400 500 600 700

wavelength (nm)

Figure 6.5. Typical uV-visible spectra of different oxidation states of Cr in 250 mM
ehbaH 2 /ehba/H buffer, pH 3.5. Main absorbing species (>90%) are Cr VI = − 4 ;

Cr(V) = [CrO(ehba) 2 ] − ; Cr(IV) = [CrO(ehbaH) 2 ] o ; Cr(III) = [Cr(ehbaH) 2 (OH 2 ) 2 ] +
(adapted from Codd et al. [44] with permission from Elsevier Inc.).

( ) [ HCrO ]

GSH at pH ≤ 6 yields Cr(VI)-thioester. Complexes between cysteine,
N-acetylcysteine, and γ-glutamylcysteine have also been characterized [43].
There have been several reports on the formation of Cr(V) complexes in the
reaction of Cr(VI) with reductants such as hydroxyl acids, sugars, diols, ascor-
bic acids, heterocycles, amino acids, peptides, and proteins. The chemistry and
their spectroscopic characterization have been reviewed [25, 44-46]. Cr(V)-
porphyrin and Cr(V)-nitride complexes have also been prepared [1, 47, 48].
There are only a few examples of Cr(IV) complexes. A relatively stable
Cr(IV)-2-hydroxy acid complex has been produced in aqueous solutions at
pH 2-4 [49, 50]. The spectra of Cr(VI/V/IV, III) species with the 2-ethyl-2-
hydroxobutanoato (ehba) ligand are shown in Figure 6.5 [44]. The Cr(IV)
complex showed absorbance in the wavelength range from 400 to 600 nm
(ε max = (2 - 4) × 10 3 /M/cm. The spectrum of Cr(IV) was useful in studying the
reactions of Cr(IV) with DNA and other molecules [44].

6.1.3 Reduction of Cr(VI/V/IV) by Substrates

Numerous studies have reviewed the reduction of Cr(VI) by inorganic and
organic substrates (S) [51-58]. Most of the work was conducted under strong
acidic conditions (pH ≤ 2). One-electron and two-electron transfer steps
have been proposed in the mechanisms of Cr(VI) reductions (reactions
6.21-6.23):