of many conformers in the source. Indeed the calculations show that the energy gaps
between the excited states are strongly dependent on the conformation of the ion, in
particular through the rotation around the C α –C β bond, thus the lifetime of each
conformer can be quite different [33] and some conformers have a longer lifetime.
The full interpretation of the pump-probe signals can be found in Gre ´goire et al. [33].
9.7
The General Picture
9.7.1 Tryptophan
From the ab-initio calculations and experimental data, one can make a good
connection between the excited state, i.e., on which orbital the excited electron is
localised, and the fragmentation pathway. The overall picture of the excited-state
dissociation pathways is presented in Fig. 9.12 and summarised in the following:
The ππ
*
CO state leads to a barrier-less H transfer from the NH 3
+ group towards
the carboxyl group. The concerted electron–proton transfer to the carboxyl weakens
the C α –C β bond and will lead to the fragment ion at mass m/z ¼ 130. After internal
conversion and since the proton is on the acidic group, this channel leads easily to
the loss of water and CO (m/z ¼ 159).
The πσ* state leads to H loss or internal conversion through a small barrier. The
H-loss channel leads to the formation of the radical cation at mass m/z ¼ 204 that
can further fragment into its secondary fragment ion at mass m/z ¼ 130. The
internal-conversion channel leads to ammonia loss (m/z ¼ 188) followed by secondary fragmentations (CO 2 or C 2 H 2 O loss leading to m/z ¼ 144 and 146,
respectively).
The ππ* state leads to a proton transfer from the NH 3
+ group towards the indole
ring. The fragment ion associated with this proton-transfer reaction is m/z ¼ 132.
This is the only channel insensitive to the probe laser with a pump/probe signal
completely flat. One might expect that the other fragmentation channels would
enter the competition, in particular in the mobile proton model framework [59]. The
absence of a pump/probe signal in this channel clearly indicates that this is not the
case, and that the kinetics of fragmentation is faster than the proton exchange and
total energy redistribution.
9.7.2 Tyrosine
The overall picture for the TyrH
+ dynamics, presented in Fig. 9.13, is basically the
same as for TrpH
+
, except that the ππ*–πσ* NH3 energy gap is larger in tyrosine and
is given in terms of the potential energy surface instead of electron localisation.
Experimentally, it has been seen that the C α –C β bond rupture is conformer dependent. At low energies the ionic fragment is not the m/z ¼ 107 fragment (C α –C β
direct rupture) as thought initially but m/z ¼ 108 (C α –C β bond breaks after proton
transfer from the amino group toward the phenol ring). This mechanism has been
9 Excited-State Dynamics of Protonated Aromatic Amino Acids
173
between the excited states are strongly dependent on the conformation of the ion, in
particular through the rotation around the C α –C β bond, thus the lifetime of each
conformer can be quite different [33] and some conformers have a longer lifetime.
The full interpretation of the pump-probe signals can be found in Gre ´goire et al. [33].
9.7
The General Picture
9.7.1 Tryptophan
From the ab-initio calculations and experimental data, one can make a good
connection between the excited state, i.e., on which orbital the excited electron is
localised, and the fragmentation pathway. The overall picture of the excited-state
dissociation pathways is presented in Fig. 9.12 and summarised in the following:
The ππ
*
CO state leads to a barrier-less H transfer from the NH 3
+ group towards
the carboxyl group. The concerted electron–proton transfer to the carboxyl weakens
the C α –C β bond and will lead to the fragment ion at mass m/z ¼ 130. After internal
conversion and since the proton is on the acidic group, this channel leads easily to
the loss of water and CO (m/z ¼ 159).
The πσ* state leads to H loss or internal conversion through a small barrier. The
H-loss channel leads to the formation of the radical cation at mass m/z ¼ 204 that
can further fragment into its secondary fragment ion at mass m/z ¼ 130. The
internal-conversion channel leads to ammonia loss (m/z ¼ 188) followed by secondary fragmentations (CO 2 or C 2 H 2 O loss leading to m/z ¼ 144 and 146,
respectively).
The ππ* state leads to a proton transfer from the NH 3
+ group towards the indole
ring. The fragment ion associated with this proton-transfer reaction is m/z ¼ 132.
This is the only channel insensitive to the probe laser with a pump/probe signal
completely flat. One might expect that the other fragmentation channels would
enter the competition, in particular in the mobile proton model framework [59]. The
absence of a pump/probe signal in this channel clearly indicates that this is not the
case, and that the kinetics of fragmentation is faster than the proton exchange and
total energy redistribution.
9.7.2 Tyrosine
The overall picture for the TyrH
+ dynamics, presented in Fig. 9.13, is basically the
same as for TrpH
+
, except that the ππ*–πσ* NH3 energy gap is larger in tyrosine and
is given in terms of the potential energy surface instead of electron localisation.
Experimentally, it has been seen that the C α –C β bond rupture is conformer dependent. At low energies the ionic fragment is not the m/z ¼ 107 fragment (C α –C β
direct rupture) as thought initially but m/z ¼ 108 (C α –C β bond breaks after proton
transfer from the amino group toward the phenol ring). This mechanism has been
9 Excited-State Dynamics of Protonated Aromatic Amino Acids
173