into vibrational energy by internal vibrational redistribution (IVR), then only states
with binding energies exceeding ~13 eV can lead to appreciable dissociation. This
is the case for GF, m ¼ 107 (Fig. 11.3e,h) and many other fragments. For the
immonium ions with m ¼ 120 (F) and m ¼ 136 (Y), the situation is clearly
different. These fragments are not formed following simple IVR. The yields of
both immonium ions peak at hν %15 eV which for (F) even is the absolute
maximum in yield. The π orbitals from the aromatic side chains have mostly low
binding energies up to around 12 eV so a large fraction of the ionisation processes
in this photon energy range will certainly originate from the aromatic groups. Here,
fast dissociation via repulsive molecular states seems to be more efficient than
dissociation following IVR.
The dominating fragment well above the ionisation threshold is the Y sidechain
with m ¼ 107. This fragment has not been observed in conventional fragmentation
studies of leucine enkephaline but is dominant in dissociative photoionisation of
neutral gas-phase tyrosine [28]. Below the ionisation threshold, loss of the neutral
m ¼ 107 tyrosine side chain is evident from the observed peak M-107 at m ¼ 449.
In the case of ionisation at higher photon energies, the additional charge can give
rise to a charge separation process. Also here, the process underlying the dissociation cannot be IVR but probably involves a repulsive molecular state.
The dominating m ¼ 107 fragment ion is accompanied by a series of N-terminal
fragments. These fragments are formed by backbone scission of the remaining
peptide ((b 2 À 107)
+
, (c 2 À 107)
+, (b 3 À 107)
+
, (c 3 À 107)
+
, (a 4 À 107)
+
, and
(b 4 À 107)
+
)—a process which is not observed in conventional fragmentation
experiments. A tentative scheme for this process is shown in Fig. 11.4. In this scheme
the photoionisation from the tyrosine (Y) sidechain leads to a non-adiabatic scission
of the Y Cα-Cβ bond. The cationic sidechain appears as a dominant feature in the
mass spectrum. The remaining protonated peptide cation then undergoes IVR before
the excess excitation energy induces backbone scission according to the mobile
proton model [32]: Upon an increase of vibrational excitation energy, the proton is
mobilised and samples various sites in the molecule. Eventually, a CID-like fragmentation pattern of the system lacking the sidechain is observed. In the mass
spectrum, the ratio between the a 4 -107 and b 4 -107 peaks is about 1 which hints at
an excitation energy of about 4.8 eV [26]. The fast loss of the charged tyrosine side
chain after 20 eV photoabsorption is thus an efficient mechanism to cool the
remaining peptide. It is conceivable, that such loss processes facilitate survival of
functional peptide substructures after absorption of very energetic photons.
11.3.2 Peptide Length Effects and Cross Sections
For a systematic investigation of VUV photoionisation as well as charge and energy
migration, it is beneficial to look into a slightly simpler type of peptide. Synthetic
systems of the type (YG n F+H)
+ (for a sketch, see Fig. 11.5) still feature the two
aromatic amino acids tyrosine (Y) and phenylalanine (F), but now both are in
terminal positions. In between the termini, a variable number of (sidechain-less)
216
T. Schlatho ¨ lter and R. Hoekstra
with binding energies exceeding ~13 eV can lead to appreciable dissociation. This
is the case for GF, m ¼ 107 (Fig. 11.3e,h) and many other fragments. For the
immonium ions with m ¼ 120 (F) and m ¼ 136 (Y), the situation is clearly
different. These fragments are not formed following simple IVR. The yields of
both immonium ions peak at hν %15 eV which for (F) even is the absolute
maximum in yield. The π orbitals from the aromatic side chains have mostly low
binding energies up to around 12 eV so a large fraction of the ionisation processes
in this photon energy range will certainly originate from the aromatic groups. Here,
fast dissociation via repulsive molecular states seems to be more efficient than
dissociation following IVR.
The dominating fragment well above the ionisation threshold is the Y sidechain
with m ¼ 107. This fragment has not been observed in conventional fragmentation
studies of leucine enkephaline but is dominant in dissociative photoionisation of
neutral gas-phase tyrosine [28]. Below the ionisation threshold, loss of the neutral
m ¼ 107 tyrosine side chain is evident from the observed peak M-107 at m ¼ 449.
In the case of ionisation at higher photon energies, the additional charge can give
rise to a charge separation process. Also here, the process underlying the dissociation cannot be IVR but probably involves a repulsive molecular state.
The dominating m ¼ 107 fragment ion is accompanied by a series of N-terminal
fragments. These fragments are formed by backbone scission of the remaining
peptide ((b 2 À 107)
+
, (c 2 À 107)
+, (b 3 À 107)
+
, (c 3 À 107)
+
, (a 4 À 107)
+
, and
(b 4 À 107)
+
)—a process which is not observed in conventional fragmentation
experiments. A tentative scheme for this process is shown in Fig. 11.4. In this scheme
the photoionisation from the tyrosine (Y) sidechain leads to a non-adiabatic scission
of the Y Cα-Cβ bond. The cationic sidechain appears as a dominant feature in the
mass spectrum. The remaining protonated peptide cation then undergoes IVR before
the excess excitation energy induces backbone scission according to the mobile
proton model [32]: Upon an increase of vibrational excitation energy, the proton is
mobilised and samples various sites in the molecule. Eventually, a CID-like fragmentation pattern of the system lacking the sidechain is observed. In the mass
spectrum, the ratio between the a 4 -107 and b 4 -107 peaks is about 1 which hints at
an excitation energy of about 4.8 eV [26]. The fast loss of the charged tyrosine side
chain after 20 eV photoabsorption is thus an efficient mechanism to cool the
remaining peptide. It is conceivable, that such loss processes facilitate survival of
functional peptide substructures after absorption of very energetic photons.
11.3.2 Peptide Length Effects and Cross Sections
For a systematic investigation of VUV photoionisation as well as charge and energy
migration, it is beneficial to look into a slightly simpler type of peptide. Synthetic
systems of the type (YG n F+H)
+ (for a sketch, see Fig. 11.5) still feature the two
aromatic amino acids tyrosine (Y) and phenylalanine (F), but now both are in
terminal positions. In between the termini, a variable number of (sidechain-less)
216
T. Schlatho ¨ lter and R. Hoekstra