11.3 VUV Photofragmentation of Protonated Polypeptides
11.3.1 Leucine Enkephalin as a Model System
In the previous section, three photofragmentation mass spectra for leucine enkephalin were displayed (see Fig. 11.2). It is important to note, that at 8 eV, intensities
are about a factor of 15 smaller than for 15 and 20 eV. This is due to the fact that the
ionisation energy of the lowest energy conformer of leucine enkephalin amounts to
8.87 eV [24]. Dissociative photoexcitation rather than photoionisation manifests
itself in the mass spectrum. The strongest peaks are observed at m/z 278/279 and at
m/z 120 and can be assigned to b 3 or y 2 fragments and the F immonium ion,
respectively. For the rest, the observed peaks are mainly due to regular backbone
scission and internal fragments, as also observed by low excitation energy
techniques such as collision induced dissociation (CID [25]) or surface induced
dissociation (SID [26]) with intensities shifted to smaller fragments.
For photon energies exceeding the ionisation energy, the spectra are dominated
by fragments with m/z ¼ 80–240 amu (see spectra for 15 eV and 20 eV in
Fig. 11.2b). The immonium ions (see Fig. 11.2a) at 86 (L), 120 (F), 136 (Y) and
the common fragments of these groups (91 and 107) are strongest. This is at
variance from what is observed with CID or SID but resembles results obtained
by keV ion impact (KID [27]). At 20 eV the fragmentation pattern observed is
richer than at 15 eV. Of particular interest is a series of a, b and c fragments which
have lost the Y side chain.
From a series of mass spectra such as the ones displayed in Fig. 11.2b, it is
possible to obtain partial ion yields curves as a function of photon energy by peak
integration. For the four strongest (YGGFL+H)
+ fragmentation channels, the
results are displayed in Fig. 11.3e–h. All curves feature a broad peak with a typical
full width at half maximum of about 10 eV. Only for m/z ¼ 120 (F) is there an
absolute maximum at hν ¼ 15 eV, whereas the maximum is around hν ¼ 20 eV for
the remaining fragments. Some fragments feature a local maximum at 15 eV.
The general shape of the partial ion yields in Fig. 11.3 is very similar to what has
been observed for neutral gas phase amino acids by Jochims et al. [29, 30] for
neutral gas phase amino acids. In the amino acids data, clear inflections are
observable at each energy, where a deeper lying molecular orbital becomes accessible. The photofragment yield curves thus depend directly on the energetic ordering of the valence molecular orbitals, i.e., the molecular density of states.
Experimentally, the density of valence states manifests in photoelectron emission spectra, which are displayed in Fig. 11.3a–c for three of the amino acids
present in leucine enkephalin [7, 28]. The spectra are dominated by intense peaks
due to valence electrons at binding energies between 10 and 19 eV. For glycine
(Fig. 11.3a) the three lowest states are due to the N lone pair n N (HOMO), the
hydroxyl O lone pair n O and the bonding carbonyl orbital π CO . Only the aromatic
amino acids (here tyrosine and phenylalanine, Fig. 11.3b,c) have a slightly different
valence structure since the π orbitals on the phenyl (phenylalanine) and phenol
(tyrosine) rings have lower ionisation energies than the n N orbitals. Last but not
214
T. Schlatho ¨ lter and R. Hoekstra
11.3.1 Leucine Enkephalin as a Model System
In the previous section, three photofragmentation mass spectra for leucine enkephalin were displayed (see Fig. 11.2). It is important to note, that at 8 eV, intensities
are about a factor of 15 smaller than for 15 and 20 eV. This is due to the fact that the
ionisation energy of the lowest energy conformer of leucine enkephalin amounts to
8.87 eV [24]. Dissociative photoexcitation rather than photoionisation manifests
itself in the mass spectrum. The strongest peaks are observed at m/z 278/279 and at
m/z 120 and can be assigned to b 3 or y 2 fragments and the F immonium ion,
respectively. For the rest, the observed peaks are mainly due to regular backbone
scission and internal fragments, as also observed by low excitation energy
techniques such as collision induced dissociation (CID [25]) or surface induced
dissociation (SID [26]) with intensities shifted to smaller fragments.
For photon energies exceeding the ionisation energy, the spectra are dominated
by fragments with m/z ¼ 80–240 amu (see spectra for 15 eV and 20 eV in
Fig. 11.2b). The immonium ions (see Fig. 11.2a) at 86 (L), 120 (F), 136 (Y) and
the common fragments of these groups (91 and 107) are strongest. This is at
variance from what is observed with CID or SID but resembles results obtained
by keV ion impact (KID [27]). At 20 eV the fragmentation pattern observed is
richer than at 15 eV. Of particular interest is a series of a, b and c fragments which
have lost the Y side chain.
From a series of mass spectra such as the ones displayed in Fig. 11.2b, it is
possible to obtain partial ion yields curves as a function of photon energy by peak
integration. For the four strongest (YGGFL+H)
+ fragmentation channels, the
results are displayed in Fig. 11.3e–h. All curves feature a broad peak with a typical
full width at half maximum of about 10 eV. Only for m/z ¼ 120 (F) is there an
absolute maximum at hν ¼ 15 eV, whereas the maximum is around hν ¼ 20 eV for
the remaining fragments. Some fragments feature a local maximum at 15 eV.
The general shape of the partial ion yields in Fig. 11.3 is very similar to what has
been observed for neutral gas phase amino acids by Jochims et al. [29, 30] for
neutral gas phase amino acids. In the amino acids data, clear inflections are
observable at each energy, where a deeper lying molecular orbital becomes accessible. The photofragment yield curves thus depend directly on the energetic ordering of the valence molecular orbitals, i.e., the molecular density of states.
Experimentally, the density of valence states manifests in photoelectron emission spectra, which are displayed in Fig. 11.3a–c for three of the amino acids
present in leucine enkephalin [7, 28]. The spectra are dominated by intense peaks
due to valence electrons at binding energies between 10 and 19 eV. For glycine
(Fig. 11.3a) the three lowest states are due to the N lone pair n N (HOMO), the
hydroxyl O lone pair n O and the bonding carbonyl orbital π CO . Only the aromatic
amino acids (here tyrosine and phenylalanine, Fig. 11.3b,c) have a slightly different
valence structure since the π orbitals on the phenyl (phenylalanine) and phenol
(tyrosine) rings have lower ionisation energies than the n N orbitals. Last but not
214
T. Schlatho ¨ lter and R. Hoekstra