Biomedical Engineering Reference
In-Depth Information
[
71
]. The incident electron current and irradiation time were adjusted to give an
exposure well within the linear regime of the dose response curves and an equal
number of electrons to each sample. The nomenclature of GCAT with the potential
sites of cleavage yielding non-modified fragments (i.e., 1 to 16) is shown in the right
of Fig.
1.3
.
The reaction of LEE with the tetramers led to the release of all four non-modified
nucleobases with a bias for the release of nucleobases from terminal position.
The release of nucleobases from tetramers was ascribed to
-glycosidic bond
cleavage via DEA from initial electron capture by the base, as previously shown
in the cleavage of thymidine to thymine in the condensed [
54
] and gas phase [
72
].
All major non-modified fragments were formed except for those corresponding to
breakage at positions 7, 10, 11 in Fig.
1.3
. Cleavage of the backbone gave fragments
with and without a terminal phosphate, but the yield of fragments with a phosphate
was much greater than that without a phosphate. This indicated that
LEE induce the
cleavage of phosphodiester bonds to give non-modified fragments with a terminal
phosphate rather than a terminal hydroxyl group
.
Based on this result and previous interpretations of SB in DNA [
24
], Zheng
et al. [
71
] postulated that rupture of the phosphodiester bond was initiated by the
formation of a dissociative transient anion on the phosphate group. There are two
possible pathways leading to cleavage of the phosphodiester bond: (1) scission of
the C-O bond corresponding to breaks at positions 5, 8, 9, 12, 13 and 16 in Fig.
1.3
and (2) cleavage of the P-O bond resulting in breaks at positions 6, 7, 10, 11, 14, 15
in Fig.
1.3
. However, 95% of the products from the HPLC analysis corresponded to
those resulting from breakage at positions 5, 8, 9, 12, 13 and 16.
Thus, Zheng et al.
concluded that cleavage of the phosphodiester bond primarily takes place via C-O
bond cleavage leading to the formation of a sugar radical and a terminal phosphate
anion
[
71
]. The cleavage of C-O and P-O bonds was previously reported in electron
spin resonance studies of argon ion and
N
irradiated hydrated DNA [
73
-
75
]. These
studies also showed that C-O bond cleavage was the dominant process.
In subsequent investigations, Zheng et al. measured the yields of the previously-
mentioned products as a function of electron impact energy on GCAT [
76
]. From
4 to 15 eV, scission of the backbone gave similar non-modified fragments to
those previously observed at 10 eV. This result indicated that
phosphodiester bond
cleavage involves cleavage of the C-O bond rather than the P-O bond over the
entire 4 to 15-eV range
. Many of yield functions of fragments exhibited a maximum
near 6 eV, a large peak at 10-12 eV followed by a dip at 14 eV. The maxima were
interpreted as due to the formation of transient anions (i.e., core-excited and core-
excited shape resonances) leading to fragmentation. These resonances dominated
bond dissociation. All four non-modified bases were released from the tetramer
within the 4-15 eV range, by cleavage of the
”
-glycosidic bond [
76
]. Above 14 eV,
the electron resonances did not dominate the yield functions, which were interpreted
to arise from fragmentation via direct electronic excitation of dissociative states.
Afterwards, Zheng et al. [
77
] verified experimentally the theoretical hypothesis
of electron transfer from a base to the phosphate group of DNA. According to
calculations [
78
,
79
] an electron captured by a base can transfer to the phosphate
N
