biological functions such as vision and photosynthesis. Even though cis-trans
isomerisation of the retinal chromophore is known as the molecular basis of
human vision, the electronic and structural dynamics underlying this process and
the subsequent coupling to the protein environment by charge transfer are still
ambiguous. The same is true for the molecular basis of photosynthesis: for instance,
femtosecond absorption spectroscopy was recently employed to show that structural changes in the light harvesting complex II play a role in the photo protection
processes [1]. Such photophysical studies are typically done in the liquid phase,
because this is where most complex biomolecular systems perform. Even genuine
quantum processes such as propagation of a coherent superposition of electronicvibrational eigenstates over long distances have been spectroscopically observed in
light harvesting complexes under biological conditions [2, 3].
Over the last years, however, there has also been enormous interest in the
physics of gas-phase biomolecular systems. De Vries and Hobza [4] give three
major motivations for spectroscopic gas-phase studies, namely i) the importance to
distinguish intrinsic molecular properties from effects of the chemical environment;
ii) the possibility to investigate fundamental interactions and microsolvation; and
iii) the need of gas-phase data to test quantum chemical calculations. Most
experiments have focused on the infrared (IR) [5], visible and ultraviolet (UV)
ranges. Only a few recent studies have been performed in the near vacuum ultraviolet (VUV) [6]. In the following, the focus will be on even higher photon energies,
i.e. the entire VUV range and soft X-rays. In that spectral range, experimental
studies in the condensed phase additionally suffer from radiation damage and poor
transparency.
Until very recently, spectroscopy with energetic photons was limited to biomolecular systems in the condensed phase with the exception of comparably small
systems (nucleobases, amino acids etc.) for which gas-phase studies have been
performed. Many amino acids are relatively easy to investigate, as these molecules
can be brought into the gas-phase by mere evaporation. A number of experimental
VUV and soft X-ray spectroscopy studies have been performed and electronic
transitions were assigned to the various spectral features, and used for instance to
investigate conformational effects [7–9]. Only a few specific small peptides such as
glycyl-glycine can be evaporated without thermal decomposition [10]. However,
there are also some classes of small peptides, which possess unusually high stability
due to e.g. ring structure [11] that can be evaporated.
Most larger peptides and proteins and a number of amino acids disintegrate upon
evaporation. Alternative techniques based on desorption of neutral molecules, e.g.
matrix assisted laser desorption ionisation (MALDI [12]) or laser induced acoustic
desorption (LIAD [13]) have already been successfully applied. A particularly
powerful method for production of pure targets of gas-phase complex molecular
ions is electrospray ionisation (ESI [14]). The use of ionic rather than neutral
molecules has the benefit of allowing for straightforward manipulation.
Radiofrequency (RF) techniques can thus be used for mass selection, transport
and accumulation of the ions in traps.
210
T. Schlatho ¨ lter and R. Hoekstra
isomerisation of the retinal chromophore is known as the molecular basis of
human vision, the electronic and structural dynamics underlying this process and
the subsequent coupling to the protein environment by charge transfer are still
ambiguous. The same is true for the molecular basis of photosynthesis: for instance,
femtosecond absorption spectroscopy was recently employed to show that structural changes in the light harvesting complex II play a role in the photo protection
processes [1]. Such photophysical studies are typically done in the liquid phase,
because this is where most complex biomolecular systems perform. Even genuine
quantum processes such as propagation of a coherent superposition of electronicvibrational eigenstates over long distances have been spectroscopically observed in
light harvesting complexes under biological conditions [2, 3].
Over the last years, however, there has also been enormous interest in the
physics of gas-phase biomolecular systems. De Vries and Hobza [4] give three
major motivations for spectroscopic gas-phase studies, namely i) the importance to
distinguish intrinsic molecular properties from effects of the chemical environment;
ii) the possibility to investigate fundamental interactions and microsolvation; and
iii) the need of gas-phase data to test quantum chemical calculations. Most
experiments have focused on the infrared (IR) [5], visible and ultraviolet (UV)
ranges. Only a few recent studies have been performed in the near vacuum ultraviolet (VUV) [6]. In the following, the focus will be on even higher photon energies,
i.e. the entire VUV range and soft X-rays. In that spectral range, experimental
studies in the condensed phase additionally suffer from radiation damage and poor
transparency.
Until very recently, spectroscopy with energetic photons was limited to biomolecular systems in the condensed phase with the exception of comparably small
systems (nucleobases, amino acids etc.) for which gas-phase studies have been
performed. Many amino acids are relatively easy to investigate, as these molecules
can be brought into the gas-phase by mere evaporation. A number of experimental
VUV and soft X-ray spectroscopy studies have been performed and electronic
transitions were assigned to the various spectral features, and used for instance to
investigate conformational effects [7–9]. Only a few specific small peptides such as
glycyl-glycine can be evaporated without thermal decomposition [10]. However,
there are also some classes of small peptides, which possess unusually high stability
due to e.g. ring structure [11] that can be evaporated.
Most larger peptides and proteins and a number of amino acids disintegrate upon
evaporation. Alternative techniques based on desorption of neutral molecules, e.g.
matrix assisted laser desorption ionisation (MALDI [12]) or laser induced acoustic
desorption (LIAD [13]) have already been successfully applied. A particularly
powerful method for production of pure targets of gas-phase complex molecular
ions is electrospray ionisation (ESI [14]). The use of ionic rather than neutral
molecules has the benefit of allowing for straightforward manipulation.
Radiofrequency (RF) techniques can thus be used for mass selection, transport
and accumulation of the ions in traps.
210
T. Schlatho ¨ lter and R. Hoekstra