Biomedical Engineering Reference
In-Depth Information
Fig. 2.5
Ion yield as a
function of the initial electron
energy of the dehydrogenated
parent anion
Glycine:
Alanine:
/
formed
upon DEA to glycine (solid
line) and alanine (dotted line)
(see also [
40
,
41
]). The
corresponding molecular
structures are shown on top.
Also indicated are thresholds
for vibrational excitation in
competition to DEA (see
text). The ion yield above
4.5 eV was multiplied with a
factor of ten
H
CH
3
.
M-H
H
O
H
O
H
2
N
OH
H
2
N
OH
3
5
4
Glycine
Alanine
(OH)
ν
(M-H)
-
x10
0.5
1.0
1.5
2.0
2.5
5
6
7
Electron energy (eV)
on the higher energy side) and a second peak at about 5.3 eV [
40
,
41
]. Early DEA
studies ascribed the structure to initial electron attachment to the
orbital of the
¢
COOH group which is coupled to the repulsive
orbital [
40
]. However,
recent calculations questioned this DEA mechanism and instead a direct electron
capture into the purely repulsive
.
O-H
/
¢
orbital was proposed, which is a very
broad resonance of more than 5 eV width [
47
]. The vibrational structure can be
assigned to the excitation of the
.
O-H
/
strechting modes (see Fig.
2.5
)in
competition to DEA. The observed dips were interpreted as cusps [
48
]; however,
they may also be interpreted as vibrational Feshbach resonances arising from dipole
bound states as suggested recently for glycine [
49
]. The latter may be possible
for some conformers of simple amino acids formed in the vaporisation by the
thermal heating (the energetically lowest structure has a subcritical dipole moment).
Experimentally no confirmation for or against this prediction was achieved so far;
however, site selectivity studies with methylated esters of amino acids confirmed
that the structure below 5 eV can be ascribed indeed to hydrogen loss from the
carboxyl group [
45
]. In contrast, hydrogen loss at the 5.3 eV resonance can be
ascribed to the amino group [
50
]. In the course of our DEA experiments with
the amino acid valine, we also investigated the unimolecular und collision induced
decomposition of
¤.
OH
/ D 3
-
5
/
on
.
M-H
s timescales [
50
]. Thereby it turned out that the
/
anions formed at about 1.2 eV and at about 5.3 eV show a different
fragmentation pattern which is another confirmation that the resonances have a
.
M-H
