demonstrates, however, that this fragment is produced in only one step.
A mechanism has been proposed to rationalise this observation: proton
transfer from the NH 3 group to the indole ring, followed by C α –C β bond
rupture, and in the ionic complex an hydrogen transfer from the amino
group of the glycine towards the nitrogen of the pyrrol group [32].
(d) m/z ¼ 205 ! m/z ¼ 204 + H ! m/z ¼ 130 + NH 2 CH 2 COOH
(very
slow)
(e) m/z ¼ 205 ! m/z ¼ 130 + NH 2 CH 2 COHOH (less than 10 ns, excited
state) [41].
m/z ¼ 130 corresponds to the C α –C β bond rupture. It is produced by two
mechanisms, fragmentation after H loss or through a direct fragmentation
in the excited state. The coincidence experiments have shown that this
fragmentation channel occurs on two time scales: a fast one (less than
10 ns) which is observed in tyrosine and tryptophan and not in tyramine
and tryptamine and a slow one (tens of μs or ms) observed for all four
species. On one hand, a direct barrier-less mechanism has been found
through ab-initio calculations: the first step is hydrogen transfer from the
amino group toward the carbonyl group and then a direct (barrier-less)
dissociation along the C α –C β bond in the excited state occurs [32, 41]. On
the other hand, the fragmentation after H loss leads to a very long time
dynamics: after H loss, the radical cations have a broad internal energy
distribution, and the cations having just enough energy to dissociate are
going to take a very long time (up to ms) to fragment as shown by the
experiments in the storage ring [30].
9.5
Spectroscopy in Cold Ion Traps
The PAAAs are very flexible molecules due to the rotational motions of the
protonated amino group and of the C–C single bonds, in particular the C α –C β
bond, which will give rise to low vibrational frequencies. The rotational barrier
around the C α –C β bond is only 0.4 eV [24] leading to vibrations as low as 30 cm
À1 .
Thus, precise spectroscopic information requires very cold molecules (less than
50 K) to avoid spectral congestion. The experimental method was pioneered by O.
Boyarkin and T.R. Rizzo in Lausanne [35, 36, 42] using a cold 22-pole ion trap
developed by D. Gerlich [43]. Since then, several experimental set-ups using this
technique have been built and a simplified version with lower performance using a
commercial Paul trap [44, 45] has been developed in particular by Wang et al. [43].
In brief, the experiment consists in sending ions into a trap filled with helium
buffer gas held at a few K. The ions are trapped for some time (ms) to ascertain
thermalisation before they are fragmented with a laser beam. All ions are then
extracted from the trap, a particular fragment ion is mass selected, and its signal
recorded as a function of the laser wavelength. If the fragmentation yield and
branching ratio are independent of the wavelength, the photo-fragmentation spectrum is similar to an absorption spectrum.
9 Excited-State Dynamics of Protonated Aromatic Amino Acids
163
A mechanism has been proposed to rationalise this observation: proton
transfer from the NH 3 group to the indole ring, followed by C α –C β bond
rupture, and in the ionic complex an hydrogen transfer from the amino
group of the glycine towards the nitrogen of the pyrrol group [32].
(d) m/z ¼ 205 ! m/z ¼ 204 + H ! m/z ¼ 130 + NH 2 CH 2 COOH
(very
slow)
(e) m/z ¼ 205 ! m/z ¼ 130 + NH 2 CH 2 COHOH (less than 10 ns, excited
state) [41].
m/z ¼ 130 corresponds to the C α –C β bond rupture. It is produced by two
mechanisms, fragmentation after H loss or through a direct fragmentation
in the excited state. The coincidence experiments have shown that this
fragmentation channel occurs on two time scales: a fast one (less than
10 ns) which is observed in tyrosine and tryptophan and not in tyramine
and tryptamine and a slow one (tens of μs or ms) observed for all four
species. On one hand, a direct barrier-less mechanism has been found
through ab-initio calculations: the first step is hydrogen transfer from the
amino group toward the carbonyl group and then a direct (barrier-less)
dissociation along the C α –C β bond in the excited state occurs [32, 41]. On
the other hand, the fragmentation after H loss leads to a very long time
dynamics: after H loss, the radical cations have a broad internal energy
distribution, and the cations having just enough energy to dissociate are
going to take a very long time (up to ms) to fragment as shown by the
experiments in the storage ring [30].
9.5
Spectroscopy in Cold Ion Traps
The PAAAs are very flexible molecules due to the rotational motions of the
protonated amino group and of the C–C single bonds, in particular the C α –C β
bond, which will give rise to low vibrational frequencies. The rotational barrier
around the C α –C β bond is only 0.4 eV [24] leading to vibrations as low as 30 cm
À1 .
Thus, precise spectroscopic information requires very cold molecules (less than
50 K) to avoid spectral congestion. The experimental method was pioneered by O.
Boyarkin and T.R. Rizzo in Lausanne [35, 36, 42] using a cold 22-pole ion trap
developed by D. Gerlich [43]. Since then, several experimental set-ups using this
technique have been built and a simplified version with lower performance using a
commercial Paul trap [44, 45] has been developed in particular by Wang et al. [43].
In brief, the experiment consists in sending ions into a trap filled with helium
buffer gas held at a few K. The ions are trapped for some time (ms) to ascertain
thermalisation before they are fragmented with a laser beam. All ions are then
extracted from the trap, a particular fragment ion is mass selected, and its signal
recorded as a function of the laser wavelength. If the fragmentation yield and
branching ratio are independent of the wavelength, the photo-fragmentation spectrum is similar to an absorption spectrum.
9 Excited-State Dynamics of Protonated Aromatic Amino Acids
163