Biomedical Engineering Reference
In-Depth Information
isobaric anions are present, mass peaks can be calibrated then with high precision,
which allows an exact determination of the composition of fragment anions formed
in DEA. Moreover, the combination of a magnetic sector and an electric sector
allows different scan techniques to study the decay of metastable anions in an
unimolecular decomposition process. One possibility is the so called mass analyzed
kinetic energy scan (MIKE) technique where a decay of a mass analyzed anion in the
field free region between the magnetic and electric sector is monitored by scanning
the electric sector field voltage. A decay product with mass m
2
formed in the decay
of a mass analyzed anion with m
1
(which passes the electric sector at V
1
) will only
pass the electric sector at the reduced voltage
V
1
. Moreover, this technique
can be also used to study collision induced decay of mass selected anions which can
be used for structure analysis. This is especially important to distinguish isomeric
anions, which may show a different collision induced fragmentation pattern.
In a third experimental setup we also studied clusters of biomolecules to elucidate
the response of larger neutral biomolecular complexes upon low energy electron ex-
posure. Since biomolecules have low vapour pressure, we embedded thereby simple
building blocks into cold helium droplets [
19
], where they immediately cool down
and form agglomerates, i.e. for example nucleobase pairs, hydrated biomolecules,
etc., can be formed. Although the low temperature of the droplets (0.38 K) [
20
]is
far from true biological conditions, these experiments allow to study the aspects
of inelastic electron scattering from biomolecular complexes on the molecular and
nanoscopic level. Moreover, they perfectly allow the study of effects on the decay
of biomolecular transient negative ions embedded in the droplets. The experimental
technique to generate such doped droplets is to form pure helium droplets in a free
jet expansion and let them pass a differentially pumped pick-up cell which is filled
with vapour of biomolecules. For this the biomolecular samples are heated in a small
oven connected to the pick-up cell. The average size of the cluster can be varied by
changing the temperature of the oven which will result in a different partial pressure
in the pick-up cell. After electron capture by the doped droplets, anions formed are
mass analyzed with a standard double focusing mass spectrometer.
.
m
2
=
m
1
/
2.3
DEA studies with building blocks of DNA/RNA
and proteins
Since the pioneering experiments by the Sanche group, where double and single
strand breaks in electron irradiated plasmid DNA films were observed, a major
impetus arose to study electron attachment to the DNA (and RNA) building blocks.
Amino acids are other important targets, when biological tissue is exposed to
ionizing radiation. DNA is wrapped in chromosomes in a packed way around
proteins. Thus also amino acids, the building blocks of proteins, are studied
intensively to elucidate the radiation damage. In the following the most important
aspects of DEA will be discussed first for nucleobases and then for amino acids.
