REACTIVE INORGANIC OXY-SPECIES OF C, N, S, AND P - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 246

[377, 378, 380]. The electron transfer-decarboxylation of gly and Ala by SO •−
radicals lead to reducing α-amino radicals (Eqs. 5.65, 5.66) [377]:

XCYHCOO SO

−

+

•−

→ XCYHCOO SO

•

+

2 −

(5.65)

4 4

XCYHCOO

•

→ CO +

•

CYHX . (5.66)

2

The formation of radicals via hydrogen abstraction occurs in the oxidation
of Leu and Ser by SO •− radicals [350]. A kinetic study of the reaction of L-Arg
with SO •− radicals indicated the formation of C-centered radicals as the tran-
sient species [380]. However, generation of N-centered radicals was suggested
in the reaction of SO •− radicals with L-Lys [379]. Carbon- and nitrogen-
centered radicals have been suggested in the reaction of SO •− radicals with the
guanidine derivatives [381].
Because dipeptides Ala-Ala and gly-gly reacted much faster than either
gly or Ala, a different initial step involving the peptide bond in the mechanism
was suggested (Eq. 5.67):

+ − •−

H NCHXCONHCHYCOO SO
H NCHXCONHCHYCOO

+

3 4

.

(5.67)

+ − •+

2

−

→ [ ] + SO

3

The interaction of SO •− with the aromatic amino acid of the dipeptides,
Trp-gly, gly-Trp, gly-Tyr, and Tyr-gly is expected to occur [377]. Figure 5.19
shows the oxidation of Tyr-gly and gly-Tyr by SO •− radicals [377]. An adduct
was initially formed, which then yielded the radical cation of the aromatics
and the formation of the hydroxycyclohexadienyl radical. The elimination of
sulfite yields the phenoxy radicals, which was detected experimentally [377].
The phenoxyl radicals were also observed in the reactions of flavonoids with
SO •− ( k ≈ 10 9 /M/s) [382]. Recently, SO •− was suggested to be a new reagent for
protein footprinting due to its potential as an oxidant for proteins [351].
Examples include the formation of protein cross-linkage through the interac-
tion with SO •− and occurrence of the metal-catalyzed oxidation of proteins by
SO •− [383, 384]. The inactivation of enzymes, ribonucleases, and lysozymes has
also been reported [349].

5.4 PHOSPHOROUS SPECIES

5.4.1 Phosphate Radicals

Photochemical and/or thermal processes in natural and atmospheric water
resulted in reactive species such as • OH and SO −• radicals, which have a capa-
bility of generating secondary radicals by reacting with inorganic anions. Phos-
phate ions can react with reactive species to form phosphate radicals that have
unexpected consequences in overall chemical processes [385]. For example,