Chemistry Reference
In-Depth Information
10
2
dm
3
mol
−1
s
−1
), in agreement what has been found for the reaction of HO
2
•
with DTT (120 dm
3
mol
−1
s
−1
; Lal et al. 1997).
Strand breakage is reported to follow largely first-order kinetics (t
½
3 s; i.e.
similar to that observed in the absence of O
2
; monitored by conductance changes
in a pulse radiolysis experiment), but at least for high-molecular-weight poly(U)
there is a dose-per-pulse related component which points a second-order contri-
bution (Schulte-Frohlinde et al. 1986b). Although there is some inf luence of pH
on the observed rate, it is not as pronounced as in the absence of O
2
. A pH-inde-
pendent first-order process would be compatible with an H-abstraction from the
C
(2
≈
)-position with subsequent very rapid (i.e. O
2
-independent) strand breakage
[reaction (9)]. The same process has also been studied by time-resolved laser
light-scattering, where two contributions were noticed, a fast one (t
½
′
≤
50 µs,
1.6 s
−1
, 70%, with a even slower component of about
10%; Jones and O'Neill 1990). In agreement with the conductivity measurements,
the kinetics of the slow process are of first order (no effect of dose rate).
20%) and a slow one (
k
≈
11. 2 . 3
Poly(C)
Poly(C) behaves similarly to poly(U) in its
•
OH-induced reactions as indicated
by the high strand breakage yields (Müller 1983) and concomitant release of mo-
nophosphate residues (Table 11.6). Moreover, spontaneous
G
(Cyt) = 1.2
10
-7
×
10
-7
mol J
−1
after heating for 2 h at 95 °C are found in N
2
O-saturated
solutions (Hildenbrand et al. 1993). The kinetics of strand breakage (7.9 s
−
1
; Jones
and O'Neill 1991) and also the yields are also very similar to those obtained with
poly(U); the reaction is, however, somewhat faster. Mechanistically, the reac-
tions are considered to be analogous to those observed with poly(U) (see above).
In agreement with this, oxidation of the cytosine-
•
OH-adducts by 1,4-benzo-
quinone leads to a reduction of strand breakage by
and 2.3
×
60% (Bamatraf et al. 1998).
Interestingly, the nitroxyl radical formed upon the addition of nitroarenes to the
reducing
C
(5)-
•
OH-adducts also gives rise to strand breakage (Bamatraf et al.
1998), and it has been concluded that these radicals can also abstract an H-atom
from the sugar moiety. In the presence of O
2
, the kinetics of strand breakage
resemble those of poly(U) (Jones and O'Neill 1990).
∼
11. 2 . 4
Poly(A)
In poly(A), the strand breakage yield is only a quarter of that observed with
poly(U) or poly(C) (Table 11.2). Pulse radiolysis experiments using laser light-
scattering for detection revealed two processes with t
½
500 ms
(Washino and Schnabel 1982; Washino et al. 1984). The slower one was quenched
by cysteamine, wherefrom the rate of strand break has been calculated at 1.7 s
−
1
and that of the reaction of cysteamine with the precursor radicals at 3.4
∼
120 µs and
∼
10
6
dm
3
mol
-1
s
-1
. The low yields of strand breaks is also ref lected in the comparatively
low yields of base release
G
(Ade)
immediate
= 0.55
×
10
-7
mol J
−1
(Hildenbrand et al.
×
10
-7
mol J
−1
(Fuciarelli et al. 1987),
G
(Ade)
after heating
= 1.0
10
-7
mol
1993), 0.28
×
×
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