photo-dissociation processes is in the picosecond range, the intersystem crossing in
the protonated PAAAs is no longer competitive.
9.8
Small Peptides
These kinds of studies (spectroscopy, fs pump/probe, coincidence and ab-initio
calculations) have been pursued on small peptides containing an aromatic residue.
In the spectroscopic work, spectra have been obtained up to 20 amino acids, and the
spectra are still very clean and well resolved [60]. This means that the number of
conformers formed in a cold trap is quite small, and that these peptides fold into a
very limited number of conformers. One alternative, however, is that only a small
part of the population can dissociate after optical excitation. These large molecules
contain so many degrees of freedom that they cannot fragment during the experimental time window (ms) if the process is statistical in the ground state, so that the
detection scheme requires some non-statistical fragmentations.
It has been shown that for some pseudo-random peptide sequences containing
either Tyr or Trp, some peptides were fragmenting through C α –C β bond rupture
while others did not [61]. This rupture seems to be much more efficient for Tyr than
for Trp.
So the non-statistical fragmentation schemes have clearly to be understood.
Some pieces of information have emerged from studies on small peptides (di- and
tripeptide), and basically the scheme is similar to the one presented for the PAAAs.
The excited lifetime for Trp-containing peptides is shorter (100 fs) than for the Tyr
(ps) ones [53, 62]. The fragmentation pathway could be interpreted in the same
manner as for PAAAs. The localisation of the electron on the peptide chain triggers
some specific fragmentations. In a very simple approximation, for Trp-containing
peptides, the ππ* state is heavily mixed with excited states localised on the peptide
chain and the excitation may extend along the chain. In Tyr-containing peptides, the
energy gap between the ππ* state and other excited states is larger, the transfer of
excitation is more difficult, and it requires some rearrangements such as proton
transfer between the amide group and the carbonyl group in order to induce the
fragmentation which will stay localised near the initial excitation and then lead
more efficiently to C α –C β rupture.
Conclusions
The excited-state dynamics of protonated aromatic amino acids is very complex.
Some understanding of the phenomena involved in the fragmentation processes
seems to have been reached. It has required a huge effort from high quality
spectroscopy giving information on the structural landscape, dynamical studies
on more than 10 orders of magnitude from femtoseconds to seconds (for
comparison 10
10 is the age of the universe in years). Quite interestingly,
dynamical events are occurring on the whole temporal scale. Coincidence
experiments have given the number and sequence of neutral fragments and the
fragmentation times for each ion. Last, but not least, high level ab-initio
176
C. Dedonder et al.
the protonated PAAAs is no longer competitive.
9.8
Small Peptides
These kinds of studies (spectroscopy, fs pump/probe, coincidence and ab-initio
calculations) have been pursued on small peptides containing an aromatic residue.
In the spectroscopic work, spectra have been obtained up to 20 amino acids, and the
spectra are still very clean and well resolved [60]. This means that the number of
conformers formed in a cold trap is quite small, and that these peptides fold into a
very limited number of conformers. One alternative, however, is that only a small
part of the population can dissociate after optical excitation. These large molecules
contain so many degrees of freedom that they cannot fragment during the experimental time window (ms) if the process is statistical in the ground state, so that the
detection scheme requires some non-statistical fragmentations.
It has been shown that for some pseudo-random peptide sequences containing
either Tyr or Trp, some peptides were fragmenting through C α –C β bond rupture
while others did not [61]. This rupture seems to be much more efficient for Tyr than
for Trp.
So the non-statistical fragmentation schemes have clearly to be understood.
Some pieces of information have emerged from studies on small peptides (di- and
tripeptide), and basically the scheme is similar to the one presented for the PAAAs.
The excited lifetime for Trp-containing peptides is shorter (100 fs) than for the Tyr
(ps) ones [53, 62]. The fragmentation pathway could be interpreted in the same
manner as for PAAAs. The localisation of the electron on the peptide chain triggers
some specific fragmentations. In a very simple approximation, for Trp-containing
peptides, the ππ* state is heavily mixed with excited states localised on the peptide
chain and the excitation may extend along the chain. In Tyr-containing peptides, the
energy gap between the ππ* state and other excited states is larger, the transfer of
excitation is more difficult, and it requires some rearrangements such as proton
transfer between the amide group and the carbonyl group in order to induce the
fragmentation which will stay localised near the initial excitation and then lead
more efficiently to C α –C β rupture.
Conclusions
The excited-state dynamics of protonated aromatic amino acids is very complex.
Some understanding of the phenomena involved in the fragmentation processes
seems to have been reached. It has required a huge effort from high quality
spectroscopy giving information on the structural landscape, dynamical studies
on more than 10 orders of magnitude from femtoseconds to seconds (for
comparison 10
10 is the age of the universe in years). Quite interestingly,
dynamical events are occurring on the whole temporal scale. Coincidence
experiments have given the number and sequence of neutral fragments and the
fragmentation times for each ion. Last, but not least, high level ab-initio
176
C. Dedonder et al.