Ten years ago nothing was known about the excited states of these protonated
species isolated in the gas phase since they were not easily produced; the development of electrospray sources was a necessary step for their easy production.
Besides, these molecules are floppy so that an efficient cooling is required to record
clear spectroscopy. Warm molecules display congested spectra [19, 20], and thus
the spectroscopic information is not as good as that for cold species. In this chapter,
we will mainly focus on protonated Tyr (TyrH
+
) and Trp (TrpH
+
), which have been
more extensively studied because their oscillator strengths are larger than that of
protonated Phe (PheH
+ ).
9.2
Ab-initio Calculations
While many papers have dealt with the excited-state properties of neutral amino
acids [21–23], less has been done on the protonated species [19, 24, 25]. The recent
improvements of ab-initio methods and in particular the Resolution-of-the-Identity
[26] approximation in association with the Coupled Cluster method as implemented
in the Turbomole package [27] gives results which compare very well with
experiments. In particular, it is possible with this method to perform excited-state
optimisations followed by excited-state vibrational analysis. Although the systems
are quite complex, the excited states which play very important roles in the excitedstate dynamics of the PAAAs can be calculated quite accurately. We will see that
the fragmentation pattern can be understood by considering three excited states,
which are basically the same for the three PAAAs and are built on four molecular
orbitals (shown in Fig. 9.2 for TrpH
+
). The HOMO, which is a π orbital localised on
the indole aromatic part, and three unoccupied orbitals play a crucial role. First,
there is the π* orbital localised on the indole ring with which it is possible to build
the ππ* state responsible for the optical absorption and carrying the oscillator
strength (for Trp there are two π* orbitals responsible for the so-called L a and L b
states of indole) [2]. The second important orbital is a Rydberg-type orbital
localised on the protonated amino group. This orbital has σ* symmetry with respect
to the glycine plane (σ* ΝΗ3 ). The third one is more localised on the carbonyl group
and has π* type symmetry with respect to the glycine plane (π* CO ).
To a first approximation, the excited states are built by promoting one electron
from the HOMO to one of these three unoccupied orbitals. Note that we are
presenting here a schematic view and one should not forget that the reality is
more complex, and that the excited states are not built with just two orbitals.
Ab-initio calculations have shown that excitation to any one of these states triggers
the dynamics of a proton/hydrogen through a very small barrier.
• In the ππ* state, the dominant process is a transfer from the ammonium group
towards the indole/phenol aromatic part.
• In the πσ* NH3 state, the C–NH 3 part becomes hypervalent (as in the case of NH 4 )
and can very easily lose one H atom.
• In the ππ* CO state, the H atom of NH 3 which is hydrogen bonded to the carbonyl
group can be transferred nearly barrier-less to the CO.
9 Excited-State Dynamics of Protonated Aromatic Amino Acids
157
species isolated in the gas phase since they were not easily produced; the development of electrospray sources was a necessary step for their easy production.
Besides, these molecules are floppy so that an efficient cooling is required to record
clear spectroscopy. Warm molecules display congested spectra [19, 20], and thus
the spectroscopic information is not as good as that for cold species. In this chapter,
we will mainly focus on protonated Tyr (TyrH
+
) and Trp (TrpH
+
), which have been
more extensively studied because their oscillator strengths are larger than that of
protonated Phe (PheH
+ ).
9.2
Ab-initio Calculations
While many papers have dealt with the excited-state properties of neutral amino
acids [21–23], less has been done on the protonated species [19, 24, 25]. The recent
improvements of ab-initio methods and in particular the Resolution-of-the-Identity
[26] approximation in association with the Coupled Cluster method as implemented
in the Turbomole package [27] gives results which compare very well with
experiments. In particular, it is possible with this method to perform excited-state
optimisations followed by excited-state vibrational analysis. Although the systems
are quite complex, the excited states which play very important roles in the excitedstate dynamics of the PAAAs can be calculated quite accurately. We will see that
the fragmentation pattern can be understood by considering three excited states,
which are basically the same for the three PAAAs and are built on four molecular
orbitals (shown in Fig. 9.2 for TrpH
+
). The HOMO, which is a π orbital localised on
the indole aromatic part, and three unoccupied orbitals play a crucial role. First,
there is the π* orbital localised on the indole ring with which it is possible to build
the ππ* state responsible for the optical absorption and carrying the oscillator
strength (for Trp there are two π* orbitals responsible for the so-called L a and L b
states of indole) [2]. The second important orbital is a Rydberg-type orbital
localised on the protonated amino group. This orbital has σ* symmetry with respect
to the glycine plane (σ* ΝΗ3 ). The third one is more localised on the carbonyl group
and has π* type symmetry with respect to the glycine plane (π* CO ).
To a first approximation, the excited states are built by promoting one electron
from the HOMO to one of these three unoccupied orbitals. Note that we are
presenting here a schematic view and one should not forget that the reality is
more complex, and that the excited states are not built with just two orbitals.
Ab-initio calculations have shown that excitation to any one of these states triggers
the dynamics of a proton/hydrogen through a very small barrier.
• In the ππ* state, the dominant process is a transfer from the ammonium group
towards the indole/phenol aromatic part.
• In the πσ* NH3 state, the C–NH 3 part becomes hypervalent (as in the case of NH 4 )
and can very easily lose one H atom.
• In the ππ* CO state, the H atom of NH 3 which is hydrogen bonded to the carbonyl
group can be transferred nearly barrier-less to the CO.
9 Excited-State Dynamics of Protonated Aromatic Amino Acids
157