C O M M U N I C A T I O N S
Figure 2. Effect of sodium thiocyanate (NaSCN) on yield of alkali-labile
lesions in 7a upon exposure to 137Cs.
measure the small amounts of (background corrected) cleavage at
individual nucleotides other than 1.
Assuming that cleavage at 1 was attributable to direct ionization,
and that the indirect effect was responsible for alkali-labile lesions
at all other nucleotides, we estimated that the direct effect accounted
for at least two-thirds of the alkali-labile lesions produced in 7a
upon exposure to 137Cs. Support for mechanistically partitioning the
cleavage products in this manner was obtained by irradiating 7a in the
presence of the known OH• scavenger, sodium thiocyanate (Figure
2).18 Cleavage at 1 was at most modestly reduced due to competition
for the energy emitted by 137Cs. More importantly, cleavage at
nucleotides other than 1 was reduced to background levels. This
indicated that the indirect effect of γ-radiolysis was responsible
for the detected damage at these positions and not direct ionization
of the deoxyribose backbone or phosphate groups. From these data,
we conclude that alkali-labile lesions produced by the direct effect
of ionizing radiation on DNA are funneled to 1. Moreover, hole
formation/migration is the dominant component of the direct effect
of 137Cs that gives rise to alkali-labile lesions in solution.
Figure 1. Formation of alkali-labile lesions via hole trapping by 1. (A)
Photolysis (350 nm) of duplexes containing 1 with or without covalently
linked anthraquinone hole injector. (B) Use of 1 to detect hole migration
in 7a exposed to 137Cs. Lane 1, 150 Gy; Lane 2, 300 Gy; Lane 3, Fe•EDTA.
Lanes 1-3 were treated with piperidine.
Acknowledgment. We are grateful for support from the Nation-
al Institute of General Medical Sciences (GM-054996). We thank
Professor Gerald Meyer, Mr. James Gardner, Ms. Tamae Ito, and Dr.
Shanta Dhar for assistance with the cyclic voltammetry apparatus.
Table 1. Alkali-Labile Lesion Formation in 7a upon Exposure to
137Cs
trial
% 1 (#)a
% other (#)a,b
ratio 1:other
Supporting Information Available: Experimental procedures for
the synthesis/characterization of all molecules, and all other experiments.
HPLC, ESI-MS, phosphorimages from experiments described herein.
This material is available free of charge via the Internet at http://
pubs.acs.org.
1
2
3
9.1 ( 1.3 (9)
9.5 ( 0.8 (10)
7.9 ( 0.9 (10)
4.1 ( 1.3 (9)
4.1 ( 0.8 (10)
3.8 ( 1.2 (10)
2.2 ( 0.8
2.3 ( 0.5
2.1 ( 0.7
a # ) Number of replicates. b Other ) nucleotides other than 1.
References
piperidine labile sites were not detected following direct photolysis
(350 nm) of 7a. In contrast, photolysis of 7b produced an alkali-
labile lesion solely at the site of 1 in amounts proportional to the
irradiation time. Additional cleavage sites were not detected upon
treatment with base excision repair enzymes (Fpg, Endo III) that
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perturbed the duplex structure in such a way so as to favor its own
cleavage was dispelled by analyzing piperidine treatment of 7a
following exposure to OH• generated by Fe•EDTA (Figure 1b).
Overall, these data demonstrated that 1 was the sole repository of
damage following hole injection in 7b. Furthermore, the probe
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replicates were carried out in each experiment in order to accurately
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