REACTIVE SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 24

1.4

E ° (RS • , H + /RSH)

p K r1

1.3

p K r2

1.2

slope ~ -0.06 V/pH
between pH ~ 3-8

p K o1

1.1

p K o2

1.0

p K r3
p K r4

E m (RS • , H + /RSH)
at pH 7.4 0.9

E ° (RS • /RS - )

0.8

p K o3

0.7 0

2 4 6 8 10 12 14

pH

Figure 1.9. Expected form of the dependence upon pH of the midpoint electrode
potential of the thiyl radical/thiol couple E m (RS • , H + /RSH) (adapted from Madej and
Wardman [236] with the permission of Elsevier Inc.).

E m at pH 7 for the thiyl radical of hydrogen sulfide is similar to the value of
GS • at pH 7.4. This is an indication of the moderate oxidizing power of GS • .
However, GS • has a higher oxidizing power than do radicals of 8-oxo-7,8-
dihydro-2′-deoxyguanosine, urate, and ascorbate [238-240]. The order of E m
in Table 1.7 suggests GS • would oxidize 8-oxo-7,8-dihydro-2′-deoxyguanosine
but not guanosine [240]. The order also suggests NO • can oxidize GSH [238].
The role of thiyl radicals can be studied through analysis in biological
systems [241]. However, thiyl radicals are highly reactive with half-lives on the
order of microseconds; therefore, their direct detection is challenging in bio-
logical systems. Rate constants for hydrogen transfer reactions of the thiyl
radical of cysteamine with amino acids and peptides are up to 10 5 /M/s [242-
245]. The reactions of thiyl radicals from GSH, cysteine, and pencillamine with
NO • are in the range of (2-3) × 10 9 /M/s [246]. A recent analysis of thiyl radicals
suggests a spin trapping electron paramagnetic resonance technique using
5,5-dimethyl-1-pyrrolone N -oxide as a trapping agent is a promising approach
for the analysis of thiyl radicals in biosystems [241].
In addition to thiyl radicals, the reactions of protein radicals with ascorbate
have also been studied to determine if the loss of ascorbate in a living organ-
ism can be related to these reactions [247]. Protein radicals were generated on
tryptophan and tyrosine residues of insulin, β-lactoglobulin, pepsin, chymo-
trypsin, and bovine serum albumin ( k = 2.9-19) × 10 9 /M/s. The tryptophanyl