Hatano Y., Katsumura Y., Mozumder A. (Eds.) Charged Particle and Photon Interactions with Matter - Recent Advances, Applications, and Interfaces

Подождите немного. Документ загружается.

520 Charged Particle and Photon Interactions with Matter

In the core, energy is deposited by direct interactions with the ion (Section 19.1.1), and the density

of energy deposition is much higher than in the penumbra. Anionic and cationic base radicals form

very close to each other and columbic forces drive substantial recombination of these radicals before

they are trapped at 77K. Neutral sugar radicals, however, are able to survive core recombinations

more easily and as a consequence, most of the trapped neutral sugar radicals are produced in the core.

With the presumption that most of the base radicals are produced in the penumbra, it is pos-

sible to use their yields to determine the partition of energy between the core and the penumbra.

For example, in the sample illustrated in Figure 19.3 and Table 19.5, one can conclude that 53% of

the ion energy (0.10/0.19) is deposited in the penumbra, and the remainder in the core. For a series

of experiments using argon ion irradiation, it was concluded that for ve samples with hydration

9≥Γ≥ 16 D

O/nucleotide and LET ≤ 650keV/μm, the percentage energy deposited in the penumbra

ranged

from 49% to 56%, or roughly 50%.

The

large absolute Gs for neutral sugar radical production with ion-beams are not simply

explained by the partition of energy. A further hypothesis regarding the effects of track structure

on yields is that excited state reactions in the core are partially responsible for the relatively high

yield of sugar radicals (Becker et al., 2003). The high density of energy deposition in the core could

clearly lead to excited states. For those excited states that are created on ion radicals, excited state

chemical reaction channels likely exist. A striking conrmation of this hypothesis was obtained in

a series of papers in which the direct excitation of G

•+

in DNA and in model compounds and of A

•+

model compounds leads to sugar radicals (Becker et al., 2007; Becker and Sevilla, 2008; Kumar

and

Sevilla, 2008a). This work is described in detail in Section 19.5.

Further

LET effects with regard to the irradiation of hydrated DNA are observed in the yields of

the prompt strand-break radicals

ROPO

i−

and

dephos

′

. The yield of these radicals with argon and

oxygen ion-beams is ca. 10 times higher than that found with γ-irradiation (Tables 19.1 and 19.5).

The pathway for formation of these radicals originates with LEE, so a question arises regarding what

factor is causing a approximate 10-fold increase in the number of such radicals. The likely answer

is that an electronic and/or vibrational excited state is involved in strand-break formation. Thus,

LEE capture induces excited anionic states that are one source of

ROPO

i−

and

dephos

′

. A second

source can arise from excited states of existing anionic radicals. For example, the higher density of

energy deposition in the core facilitates formation of excited states in proximity to existing radicals.

DFT calculations also indicate that LEE capture by DNA can be modeled by calculations of excited

states of the anion radicals. Thus excited states are very much involved in LEE capture by DNA and

clearly lead to subsequent strand breaks (Kumar and Sevilla, 2007, 2008b, 2009b).

Because a strand break that is part of a multiply damaged site (damage cluster) is likely to be

less repairable than an isolated strand break (vide infra), the spatial clustering of radicals is highly

δ-ray

18+

ca. 50% of energy

in core

ca. 50% of energy

in penumbra

–

core

= ca. 6.8 nm

S = sugar radicals, ROPO

•

–

•

, C(N3)H

•

, T

•

–

•

Figure 19.4 Schematic depiction of chemical track structure from Ar ion beam irradiation of hydrated DNA.

PhysicochemicalMechanisms of Radiation-Induced DNA Damage 521

relevant to understanding damage pathways. A pulsed electron double resonance (PELDOR) study

of argon ion irradiated hydrated DNA found that clusters of radicals with a local radical concen-

tration higher than the overall radical concentration do exist (Bowman et al., 2005). In this study,

the observing magnetic eld was set outside the range of the base radical ESR spectra, but on the

neutral sugar radical spectrum, and a strong pulse was applied. The pulse causes transitions in most

of the radicals present. The effect of the pulse on the neutral sugar radicals is then observed. It was

concluded

that clusters of radicals do exist, as shown in Table 19.6.

What

is most remarkable is that in the 1.7 kGy dose sample, although the average radical concen-

tration is 0.20μmol/cm

, the cluster radical concentration is 18.8μmol/cm

. This is conrmation of

the existence of signicant multiply damaged sites in Ar-irradiated DNA. Since the observing mag-

netic eld detects effects on the neutral sugar radicals, and the experimental technique used detects

mainly a local concentration of radicals, the spherical cluster radius observed is actually a measure

of the core radius (Figure 19.4). In addition, because the core is largely populated with neutral sugar

radicals, it is likely that more than one of the radicals in a cluster will lead to a strand break. Since

two strand breaks within 10–20 base pairs (3.4–6.8nm) on opposite strands is generally regarded to

constitute a double strand break, if two breaks on opposite strands occur within a cluster, a double

strand

break will almost certainly result.

The

track parameters reported, that is, the core radius, the radical concentration in the core, and

the energy partition between the core and penumbra all agreed well with track-structure calcula-

tions

(Magee and Chatterjee, 1987; Chatterjee and Holley, 1993; Chatterjee, 2006).

19.4.3 Strand breakS and daMage cluSterS froM heavy-ion direct effectS

Very little work has been published (also see Chapter 21 for further information) regarding strand

breaks from ion-beam irradiation using experimental protocols that assure direct-type effects

are being measured. Using hydrated (Γ = 35 H

O/nucleotide) pUC18 prepared in a tris buffer,

Urushibara et al. (2008) determined the yield of ssb, dsb, and damage clusters in pUC18 DNA using

α-radiations of varying LET at 5.6°C. Using α-radiation, it was found that the yield of frank ssb

decreased as LET increased (Table 19.7) and that heat treatment did not change the yield of ssb.

Contrary to the behavior of ssb, the yield of dsb increased to a maximum at an LET of 141keV/μm,

and then dropped at 148keV/μm.

Determination of the yield of newly formed ssb after enzymatic treatment with Nth and Fpg to

reveal isolated base damage indicated the presence of such signicant damage at the lower LETs

used, but the yield of these sites decreased markedly as the LET increased. In addition, measure-

ment of newly formed dsb after treatment with Nth and Fpg, to reveal damage clusters, indicated the

presence of signicant yields of such clusters, which also appeared to decrease as LET increased.

This latter decrease was interpreted as possibly being an artifact caused by the failure of the

BER glycolases to function properly, rather than as an actual decrease in base lesions. However, as

indicated earlier (vide infra), there actually is a substantial decrease in initial trapped base radical

yields as LET increases, caused by the partition of energy in a core and a penumbra, and the effects

table 19.6

multiply

amaged

ites

in a

r-irradiated

dna

track parameter

value for doses

1.7–50 kgy

Radicals

in cluster 17.7 ± 0.7

Cluster

radius (spherical cluster assumed) 6.8

Radical

concentration in cluster

18.8

μmol/cm

Radical concentration for whole sample (dose dependent)

0.20μmol/cm

522 Charged Particle and Photon Interactions with Matter

thereof. It may be the case that the loss of enzyme sensitive sites is real and due to track-structure

effects. Earlier experimental evidence from the same group using γ-irradiation showed that the

yields of ssb and dsb do not increase for Γ ≥ 15, suggesting that the indirect effect is suppressed in

the

samples (Yokoya et al., 2002).

earlier experiments by the same team, α-particle irradiated DNA was investigated at three

hydration levels, from dry DNA to Γ = 35 H

O/nucleotide (Yokoya et al., 2003). The dsb yield

increased from the rst to the second hydration level, and then appeared to plateau for fully hydrated

DNA. At all three hydration levels used, the yield of dsb was two to three times higher than that

found for γ-irradiated samples, indicating clusters of damage originated in the ion track core. In

a manner consistent with the work cited above, fewer Nth and Fpg enzyme sensitive sites were

detected than one might expect. As discussed earlier, it may be the case that the lack of enzyme

sensitive sites is actually due to fewer base lesions being formed rather than the inability of the BER

enzymes

to properly catalyze strand cleavage.

19.4.4 daMage by Soft and ultraSoft x-rayS

Extensive literature exists regarding the damage caused by soft and ultrasoft x-rays. For these radia-

tions, the photoelectric effect induces inner shell ionizations of light atoms, and damage is caused

both by the original ionization of the target atom as well as by the resulting auger and secondary

electrons as well. These beams have LETs higher than that of γ-rays or hard x-rays. A discussion

of these effects is outside the scope of this chapter, but recent reviews (Yokoya et al., 2006) as well

Chapter 21 in this book can be consulted.

19.5 exCited states oF ion-radiCals as dna damage preCursors

19.5.1 Sugar–phoSphate daMage via excited StateS of dna baSe ion-radicalS

Early ESR studies using γ-irradiated hydrated salmon sperm DNA (Wang et al., 1994) showed

that the yield of neutral sugar radicals increased with radiation dose, in apparent direct cor-

relation with the loss of existing one-electron oxidized guanine radicals (Wang et al., 1994).

This observation suggested that, in these γ-irradiated hydrated DNA samples, a portion of the

table 19.7

yields

of s

trand

reaks