Chemistry Reference
In-Depth Information
In addition to the O
2
elimination reaction (17) and the Mannich reaction (14),
phenoxyl/O
2
•
−
combination products can undergo a water elimination if the po-
sition where O
2
•
−
has added to carries an H atom [e.g., reactions (20 and (21);
d'Alessandro et al. 2000].
As mentioned above, these reactions are potentially of some heuristic impor-
tance also for DNA in so far as little is known about the fate of G
•
(notably its
bimolecular decay; Chap 10.2). Both G
•
and phenoxyl radicals share quite some
properties; they are aromatic with high spin density at oxygen, reasonably strong
oxidants, do not react with O
2
at an appreciable rate, but quite readily with O
2
•
−
.
Many antioxidants quoted as potential protective agents against free-radical-
induced DNA damage have more than one phenolic group. Their chemistry is,
therefore, also of some interest in the present context. The semiquinone radicals,
derived from hydroquinone by one-electron oxidation or from 1,4-benzoqui-
none by one-electron reduction, are in equilibrium with their parents (Roginsky
et al. 1999), and these equilibria play a role in the autoxidation of hydroquinone
(Eyer 1991; Roginsky and Barsukova 2000). Superoxide radials are intermediates
in these reactions.
7. 3
Nitrogen-Centered Radicals
In DNA free-radical chemistry,
N
-centered radicals are generated from some
nucleobases upon one-electron oxidation (followed by H
+
loss). They are also
considered as important intermediates in the purine free-radical chemistry. It
is, therefore, worthwhile to address very brief ly some of the chemistry of
N
-cen-
tered radicals that were encountered in amines and amino acids.
The reaction of
•
OH with trimethylamine in aqueous solution leads to the
formation of the alkylaminium radical cation [CH
3
N
•
+
], aminoalkyl radicals
[(CH
3
)
2
NCH
2
•
] and protonated aminoalkyl radicals [(CH
3
)
2
N(H
+
)CH
2
•
] which
have markedly different properties (Das and von Sonntag 1986; for a theoretical
study see Armstrong et al. 1992). The aminoalkyl radicals react rapidly with O
2
thereby giving rise to O
2
•
−
(Das et al. 1987).
Aminyl radicals formed in the reaction of
•
OH with amino acids (
•
NH-CR
2
-
CO
2
−
) are oxidizing radicals and readily react with phenols (Bonifacic et al.
2000a). In the presence of a proton donor, they are converted into the corre-
sponding radical cation [reaction (22)] which immediately decarboxylates [reac-
tion (23); see also Lü et al. (2001)]. It also may undergo
-fragmentation thereby
forming CO
2
•
−
[reaction (24); Bonifacic et al. 2000a; Hug et al. 2000a; Stefanic
β
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