HIGH-VALENT Cr, Mn, AND Fe SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 291

6.1.4.8 Non-Sulfur-Containing Amino Acids and Peptides. The reduction
of Cr(VI) by Gly, Ala, and 2-amino-2-methylpropanoic acid did not occur
either in dark or light [106]. However, when reactions were performed in
methanol, Cr(V) complexes were formed. The mixture of Cr(VI)/glycine/
methanol formed a complex, which is similar to the Cr(V)-methanol complex.
The reaction performed under light increased the yield of the complex. There-
fore, light was needed for the reduction to proceed at an appreciable rate. The
Cr(VI)-methanol reaction in the presence of alanine resulted in the Cr(V)-
alanine complex under both dark and light conditions. Several spectroscopic
techniques were applied to characterize this complex as a Cr(V)-alanine
dimer.
The Cr(V)-peptide complexes without thiol residues have also been char-
acterized by the EPR technique [106, 107]. The reaction of Cr(VI) with metha-
nol yielded two Cr(V)-methanol intermediates. However, methanol and
glycine peptides (triglycine, tetraglycine, and pentaglycine) resulted in the
reduction of Cr(VI) to form Cr(V)-methanol and Cr(V)-peptide complexes.
However, the reaction of Cr(VI) and alanine peptides (trialanine, tetraalanine,
and pentaalanine) formed only Cr(V)-peptide complexes. Other Cr(V)-pep-
tide complexes containing the C-terminus of the proteins were also prepared
[108]. The peptides were N , N -dimethylurea derivatives of tripeptides, Aib 3 ,
AibLAlaAib-DMF, and AibSAlaAib-DMF (Aib-2-amino-2-methylpropanoic
acid; Ala, alanine; N , N -dimethylformamide [DMF]). Results suggest the pos-
sible formation of Cr(V) species from the Cr(III) peptides in vivo . However,
the biological activity of such complexes needs to be fully examined in further
work.
6.1.4.9 Proteins. The chemistry of Cr(VI/V/IV) with a model thiolato
complex [Zn(SR) 2 ] (RSH= O -ethyl-L-cysteine) was studied [109]. The thiolato
complex represents the tetrahedral (2 S ,2 N ) Zn(II) binding site in zinc-finger
proteins. The reaction occurs through sequences of two- and one-electron
transfer steps with the formation of Cr(V/IV) intermediates. An intramolecu-
lar formation of the disulfide bond was proposed. The reaction of Cr(VI) with
human saliva has been studied to model the formation of Cr species in the
human respiratory tract after inhaling Cr(VI). The EPR spectroscopy identi-
fied different Cr(V)-sialic (neuraminic) acid species in the reaction [110]. The
EPR characteristics of the Cr(V) species were similar to those obtained in the
mixtures of Cr(VI) and isolated components of salivary glycoproteins [110].
Cr(V) species may damage the cell through cleavage of DNA and/or oxidation
of other biomolecules. A mechanism of biological damage caused by such
Cr(V) species still requires further examination.
A number of studies on the interaction of Cr(VI) with thioredoxins (Trx1
and Trx2) and enzymes (peroxyredoxins, Prx1 and Prx3) have been performed
[9, 111, 112]. The oxidation of Prx1 and Prx3 by Cr(VI) was observed in which
a significant oxidation of thioredoxins also occurred [9]. Cytosolic Trx1 was
less susceptible to Cr(VI) than mitochondrial Trx2. Ascorbate did not alter the