Chemistry Reference
In-Depth Information
4.1
Some Basic Properties of H
•
and e
aq
−
Much of basic free-radical chemistry of DNA and its constituents have been
elucidated with the help of radiation techniques. This requires one to address
brief ly the properties of the H atom and the hydrated electron, e
aq
−
, which are
important intermediates in the radiolysis of water (Chap. 2.2).
The
•
OH radical, which is also generated under these conditions, may be con-
verted into
•
H by reacting it with excess H
2
(Christensen and Sehested 1983).
This may require a special pressure cell (Christensen and Sehested 1980). The
scavenging of
•
OH with
t
BuOH (
k
= 6
10
8
dm
3
mol
−
1
s
−
1
) is often the more con-
venient approach. Under adequate conditions, this leaves H
•
largely untouched,
since its rate of reaction with
t
BuOH is low (
k
= 1.7
×
10
5
dm
3
mol
−
1
s
−
1
; Buxton et
×
10
6
dm
3
mol
−
1
s
−
1
; Wojnárovits et al. 2004).
H
•
is the conjugate acid of e
aq
−
[p
K
a
(H
•
) = 9.1; reactions (1),
k
= 2.2
al. 1988; recently revised at 1.15
×
10
7
dm
3
×
mol
−
1
s
−
1
and (2),
k
= 2.3
10
10
dm
3
mol
−
1
s
−
1
(Buxton et al. 1988), for the ther-
modynamic properties of this system, see Hickel and Sehested (1985)]. Thus, in
pure water, the lifetime of e
aq
−
is quite long (Hart et al. 1966), even long enough
to monitor its presence spectrophotometrically under steady-state
60
Co-
×
γ
-radi-
olysis conditions (Gordon and Hart 1964).
(1)
e
aq
−
+ H
+
→
H
•
(2)
Reaction (1) is best described as a proton transfer from the weak acid H
•
to the
strong base OH
−
(Han and Bartels 1992). For the rapid conversion of e
aq
−
into H
•
in neutral solution (i.e., at low H
+
concentration), phosphate buffer may be used
[reaction (3);
k
= 1.1
10
7
dm
3
mol
−
1
s
−
1
(Grabner et al. 1973)]. The rate constant
depends somewhat on the phosphate concentration, and at 1 mol dm
−
3
phos-
phate (pH
×
10
7
dm
3
mol
−
1
s
−
1
(Ye and Schuler
∼
5.7) the reported value is 1.85
×
1986).
e
aq
−
+ H
2
PO
4
−
H
•
+ HPO
4
2
−
→
(3)
The hydrated electron is characterized by its strong absorption at 720 nm (
ε
=
10
4
dm
3
mol
−
1
cm
−
1
(Hug 1981); the majority of the oscillator strength is
derived from optical transitions from the equilibrated
s
state to the
p
-like ex-
cited state (cf. Kimura et al. 1994; Assel et al. 2000). The 720-nm absorption is
used for the determination of its reaction rate constants by pulse radiolysis (for
the dynamics of solvation see, e.g., Silva et al. 1998; for its energetics see, e.g.,
Zhan et al. 2003). H
•
only absorbs in the UV (Hug 1981), and rate constants have
largely been determined by EPR (Neta et al. 1971; Neta and Schuler 1972; Mezyk
and Bartels 1995) and competition techniques (for a compilation, see Buxton et
al. 1988). In many aspects, H
•
and e
aq
−
behave very similarly, which made their
distinction and the identification of e
aq
−
difficult (for early reviews, see Hart
1964; Eiben 1970; Hart and Anbar 1970), and final proof of the existence of the
1.9
×
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