Biomedical Engineering Reference
In-Depth Information
the NaH
2
PO
4
film is a strong indication that in both cases the same transient anion
(i.e., the same
NaH
2
PO
4
state) is involved. The signal producing the broad OH
peak seen in curve A of Fig.
1.11
, is expected to arise from P-OH bond cleavage,
through the DEA pathway
Œ
Within DNA, scission of this bond would produce a SB [
71
] but no OH
would
desorbed from the film. Furthermore, with Na
C
as the counter ion no OH group is
present in the backbone. However, in the film of 40-base single strands of DNA,
used to produce curve B in Fig.
1.11
, the counter ion was H
C
[
70
]. When the Na
C
counter ion is replaced by H
C
, an OH group is formed and cleavage of the P-O
bond perpendicular to the chain can produce a OH
desorption signal below 10 eV
(i.e., curve B in Fig.
1.11
) that matches the one seen from curve A. Thus, the OH
signal producing the broad peak in curve B has been interpreted to arise from the
decay of the same transient anion, as in NaH
2
PO
4
, but into the pathway
OH
−
+
+
P
P-OH
P-OH
within the backbone of DNA. Similar comparisons with DNA were made with ESD
signals from films composed of the other basic subunits of DNA [
26
].
1.4.3
Short single DNA strands
We have seen in curve B of Fig.
1.11
the desorption of OH
from a short DNA
strand stimulated by LEE impact. To obtain more details on the mechanisms of
DNA damage, the products remaining in such films after LEE bombardment were
analyzed by Zheng et al. [
71
]. They analyzed by HPLC the degradation products
from the tetramer GCAT, its abasic forms and CGTA. These oligonucleotides, which
constitute the simplest form of DNA containing the four bases (G, C, A and T),
made the analysis of degradation products much easier than would be the case
for longer single strand and double stranded configurations. Samples, prepared by
spin coating inside tantalum cylinders, were irradiated by 10-eV electrons from the
diverging beam LEE irradiator mentioned in Sect.
1.3
. The HPLC analysis was first
focused on SB and detachment of non-modified subunits of the tetramers CGTA
and GCAT, which included monomeric components (nucleobases, nucleosides and
mononucleotides), and oligonucleotide fragments (dinucleotides and trinucleotides)
