Inorganic Radicals - Free-Radical-Induced DNA Damage and Its Repair - page 85

(17)

The phosphate radical, PO 4 • 2

− , is related to SO 4 • − . It may be similarly generated
photolytically or radiolytically from peroxodiphosphate (Maruthamuthu and
Neta 1977, 1978; Maruthamuthu 1980; Kumar and Adinarayana 2000). Its reduc-
tion potential is lower than that of SO 4 • − , that is, the latter reacts with phosphate,
although the rate of reaction is only slow (with HPO 4 2 − k = 1.2 10 6 dm 3 mol − 1

×

s − 1 , with H 2 PO 4 − k < 7 10 4 dm 3 mol − 1 s − 1 ; Maruthamuthu and Neta 1978). Its

reactions are of some interest in the context of DNA free-radical chemistry, since
in DNA this type of radical may be formed upon oxidation of the phosphate
groups, for example, by ionizing radiation (direct effect) or photoionization at
short wavelengths.
The H 2 PO 4 • radical has p K a values of 5.7 and 8.9, and the oxidation power
decreases in the order SO 4 • − > H 2 PO 4 • > HPO 4 • − > PO 4 • 2 − (Maruthamuthu and
Neta 1978). The H 2 PO 4 • radicals abstract H-atoms at slightly higher rates than
SO 4 • − , and in their addition reactions they are similarly electrophilic (

×

ρ = − 1.8)

as the SO 4 • − radical (Maruthamuthu and Neta 1977).
When generating the di(pseudo)halide radical anions radiolytically, one has
to keep in mind t hat t he ha lide ions do not react w it h • H, but HN 3 /N 3 − does. Orig-
inally, it has been suggested that N 3 • and H 2 are formed (Alfassi et al. 1986), but it
was later shown that it reacts according to reaction (18) (Deeble et al. 1990).

• H + N 3 − + H +

→ • NH 2 + N 2 (18)

The SeO 3 • − , a Se(V) species with a high redox potential (cf. Table 5.2) can be pro-
duced radiolytically from Se(VI) upon reduction by e aq − [reaction (19)] and from
Se(IV) by • OH [reaction (20); Kläning and Sehested 1986].

SeO 4 2 − + e aq − SeO 3 • −

→ (19)

SeO 3 2 − + • OH SeO 3 • − + OH −

→ (20)

The SeO 3 • − radical has been used with advantage to oxidize DNA specifically at
G sites (Martin and Anderson 1998; Milligan et al. 2002).
The strongly oxidizing • NO 3 radical (E 0 = 2.0 V vs. SCE in acetonitrile) can be
generated photolytically in acetonitrile [reaction (21)].

(NH 4 ) 2 Ce(NO 3 ) 6 + h ν → • NO 3 + (NH 4 ) 2 Ce(NO 3 ) 5 (21)

It has been used to study the oxidative cleavage of Thy dimers (Krüger and Wille
2001; Chap. 10.14).
All these radicals have oxidizing properties, but there are also some inor-
ganic radicals which have reducing properties (E < 0 V; cf. Table 5.2).
For example, H-abstraction from formate [reaction (22)], addition of • OH to
CO [reaction (23)] or the reaction of e aq − with CO 2 [reaction (24)] yields the (re-
ducing) CO 2 • − radical. The p K a value of its conjugate acid • CO 2 H [equilibrium
(25)] continues to be in dispute. Using different approaches to determine its p K a ,