9.5.1 Spectroscopy
Spectroscopy of protonated aromatic amino acids has been done by Rizzo’s group
[18, 36] at very low temperatures (around 12 K) in the case of phenylalanine and
tyrosine and by Fuke’s group in Kobe at a slightly higher temperature [46, 47]. At
high temperatures, the spectrum becomes unresolved due to spectral congestion
[19, 45] but 50 K seems to be enough to get nice spectroscopic information (see
below). The S 0 –S 1 transition energy is very similar to that of the neutral molecule
(shift of 50 cm
À1 and 400 cm
À1 to the red for Phe and Tyr, respectively). This is a
good indication that the protonation site is not on the chromophore, which would
have perturbed the electronic transition much more [48].
9.5.1.1 Phenylalanine
The Phe spectrum was first recorded [18] detecting the fragment masses m/z ¼ 74
Æ 1 or 92 Æ 1 on a limited range of excitation energy, i.e., from the S 1 band origin
up to 800 cm
À1 above. The spectrum is nicely resolved, and two conformers with
different rotations around the C α –C β bond were identified. A similar experiment at
lower spectral resolution, over a larger energy range (developed in two set-ups in
Orsay and in Marseille) has shown that for low photon energies the fragments are
m/z ¼ 75 and 92 (this fragment corresponds to the C α –C β bond rupture after proton
transfer to the benzene ring), and that the fragmentation channel changes to the
m/z ¼ 120 fragment (CO + H 2 O loss) at higher energies as shown in Fig. 9.5. The
bands observed at this resolution are the 0–0 transition at ~266 nm and a vibronic
transition associated with deformation of the benzene ring (~262 nm). Vibrational
structures are also observed at higher energies when the m/z ¼ 120 fragment is
produced. The experimental results could be indicative of a conical intersection
between two different electronic states.
9.5.1.2 Tyrosine
A high resolution spectrum of protonated tyrosine was first obtained at a very low
temperature in a 22-pole trap [18]. Recently, a wider spectral range has been
recorded at lower spectral resolution and higher temperature around 50 K (Fig. 9.6).
At low energy the dominant dissociation channel produces the m/z ¼108 fragment which corresponds to C α –C β bond rupture after a proton transfer to the phenol
ring. The m/z ¼ 107 fragment, which corresponds to C α –C β rupture but without
proton transfer, is very weak at low energies but becomes quite important at high
energies (below 240 nm) when the H-loss channel giving the m/z ¼ 181 ion is
appearing.
Below 275 nm, the m/z ¼ 108 fragment decreases in abundance while the
m/z ¼ 136 and 147 fragments (not shown here) are growing: these two channels
are H 2 O + CO loss and NH 3 + H 2 O loss, respectively, which are the most important
ones in collision induced dissociation.
Below 240 nm, at the appearance of the H loss channel, the m/z ¼ 119 (NH 3 loss
from m/z ¼ 136) and the m/z ¼ 123 (NH 3 + CH 2 CO loss) fragments increase in
164
C. Dedonder et al.
Spectroscopy of protonated aromatic amino acids has been done by Rizzo’s group
[18, 36] at very low temperatures (around 12 K) in the case of phenylalanine and
tyrosine and by Fuke’s group in Kobe at a slightly higher temperature [46, 47]. At
high temperatures, the spectrum becomes unresolved due to spectral congestion
[19, 45] but 50 K seems to be enough to get nice spectroscopic information (see
below). The S 0 –S 1 transition energy is very similar to that of the neutral molecule
(shift of 50 cm
À1 and 400 cm
À1 to the red for Phe and Tyr, respectively). This is a
good indication that the protonation site is not on the chromophore, which would
have perturbed the electronic transition much more [48].
9.5.1.1 Phenylalanine
The Phe spectrum was first recorded [18] detecting the fragment masses m/z ¼ 74
Æ 1 or 92 Æ 1 on a limited range of excitation energy, i.e., from the S 1 band origin
up to 800 cm
À1 above. The spectrum is nicely resolved, and two conformers with
different rotations around the C α –C β bond were identified. A similar experiment at
lower spectral resolution, over a larger energy range (developed in two set-ups in
Orsay and in Marseille) has shown that for low photon energies the fragments are
m/z ¼ 75 and 92 (this fragment corresponds to the C α –C β bond rupture after proton
transfer to the benzene ring), and that the fragmentation channel changes to the
m/z ¼ 120 fragment (CO + H 2 O loss) at higher energies as shown in Fig. 9.5. The
bands observed at this resolution are the 0–0 transition at ~266 nm and a vibronic
transition associated with deformation of the benzene ring (~262 nm). Vibrational
structures are also observed at higher energies when the m/z ¼ 120 fragment is
produced. The experimental results could be indicative of a conical intersection
between two different electronic states.
9.5.1.2 Tyrosine
A high resolution spectrum of protonated tyrosine was first obtained at a very low
temperature in a 22-pole trap [18]. Recently, a wider spectral range has been
recorded at lower spectral resolution and higher temperature around 50 K (Fig. 9.6).
At low energy the dominant dissociation channel produces the m/z ¼108 fragment which corresponds to C α –C β bond rupture after a proton transfer to the phenol
ring. The m/z ¼ 107 fragment, which corresponds to C α –C β rupture but without
proton transfer, is very weak at low energies but becomes quite important at high
energies (below 240 nm) when the H-loss channel giving the m/z ¼ 181 ion is
appearing.
Below 275 nm, the m/z ¼ 108 fragment decreases in abundance while the
m/z ¼ 136 and 147 fragments (not shown here) are growing: these two channels
are H 2 O + CO loss and NH 3 + H 2 O loss, respectively, which are the most important
ones in collision induced dissociation.
Below 240 nm, at the appearance of the H loss channel, the m/z ¼ 119 (NH 3 loss
from m/z ¼ 136) and the m/z ¼ 123 (NH 3 + CH 2 CO loss) fragments increase in
164
C. Dedonder et al.