based on the spectral width (cf., Heisenberg’s uncertainty principle), not relying on
femtosecond laser experiments. Examples on this exist and are given in Chaps. 7
and 9. The complications of environment and electronic decoupling at higher
temperatures can be introduced gradually.
We believe that future work benefitting from cold ion spectroscopy could go
beyond the single chromophore to multiple chromophore systems where the electronic coupling between nucleobases changes the electronic properties significantly. Interchromophore coupling is likely of importance in DNA photophysics
[34, 35] but is certainly so in photosynthesis where chlorophylls are linked together
electronically within light-harvesting proteins [36]. The simplicity of the gas-phase
models could provide extremely important results that could be used to test
advanced theoretical models such as the Frenkel exciton model. Freezing out
structural fluctuations would allow one to be more specific about the actual structure of the ions, and the electronic coupling may increase significantly if π-stacking
interactions determine the structures of the dominant isomers.
While fluorescence spectroscopy has been done on a few fluorophore ions in
vacuo at room temperature (work by Jockusch, Parks, Zenobi and their co-workers
[37–39]), similar experiments to those described above on very cold ions are to our
knowledge lacking. Indeed, cold ions may have larger fluorescence quantum yields
than warm ions as they most likely internally convert slower to the electronic
ground state. Again work on multiple chromophore systems would be interesting;
for example it would be worth to measure the light emission from long-lived
charge-transfer states that are believed to account for long deexcitation times of
DNA in aqueous solution [34, 35].
In conclusion, the field of spectroscopy of biomolecular ions isolated in vacuo is
a relatively new field, but nevertheless the contributions made have been many and
significant as is clearly evident from the chapters in this volume. Most importantly,
however, the field certainly holds great promise for the future.
References
1. Berg, J.M., Tymoczko, J.L., Stryer, L.: Biochemistry, 6th edn. W.H. Freeman, New York
(2007)
2. Brøndsted Nielsen, S., Lapierre, A., Andersen, J.U., Pedersen, U.V., Tomita, S., Andersen, L.H.:
Absorption spectrum of the green fluorescent protein chromophore anion in vacuo. Phys.
Rev. Lett. 87, 228102 (2001)
3. Forbes, M.W., Jockusch, R.A.: Deactivation pathways of an isolated green fluorescent protein
model chromophore studied by electronic action spectroscopy. J. Am. Chem. Soc. 131,
17038–17039 (2009)
4. Chingin, K., Balabin, R.M., Frankevich, V., Barylyuk, K., Nieckarz, R., Sagulenko, P., Zenobi,
R.: Absorption of the green fluorescent protein chromophore anion in the gas phase studied by
a combination of FTICR mass spectrometry with laser-induced photodissociation spectroscopy. Int. J. Mass Spectrom. 306, 241–245 (2011)
5. Zuev, D., Bravaya, K., Makarova, M.V., Krylov, A.I.: Effect of microhydration on the
electronic structure of the chromophores of the photoactive yellow and green fluorescent
proteins. J. Chem. Phys. 135, 194304 (2011)
2 Introduction and New Aspects
17
femtosecond laser experiments. Examples on this exist and are given in Chaps. 7
and 9. The complications of environment and electronic decoupling at higher
temperatures can be introduced gradually.
We believe that future work benefitting from cold ion spectroscopy could go
beyond the single chromophore to multiple chromophore systems where the electronic coupling between nucleobases changes the electronic properties significantly. Interchromophore coupling is likely of importance in DNA photophysics
[34, 35] but is certainly so in photosynthesis where chlorophylls are linked together
electronically within light-harvesting proteins [36]. The simplicity of the gas-phase
models could provide extremely important results that could be used to test
advanced theoretical models such as the Frenkel exciton model. Freezing out
structural fluctuations would allow one to be more specific about the actual structure of the ions, and the electronic coupling may increase significantly if π-stacking
interactions determine the structures of the dominant isomers.
While fluorescence spectroscopy has been done on a few fluorophore ions in
vacuo at room temperature (work by Jockusch, Parks, Zenobi and their co-workers
[37–39]), similar experiments to those described above on very cold ions are to our
knowledge lacking. Indeed, cold ions may have larger fluorescence quantum yields
than warm ions as they most likely internally convert slower to the electronic
ground state. Again work on multiple chromophore systems would be interesting;
for example it would be worth to measure the light emission from long-lived
charge-transfer states that are believed to account for long deexcitation times of
DNA in aqueous solution [34, 35].
In conclusion, the field of spectroscopy of biomolecular ions isolated in vacuo is
a relatively new field, but nevertheless the contributions made have been many and
significant as is clearly evident from the chapters in this volume. Most importantly,
however, the field certainly holds great promise for the future.
References
1. Berg, J.M., Tymoczko, J.L., Stryer, L.: Biochemistry, 6th edn. W.H. Freeman, New York
(2007)
2. Brøndsted Nielsen, S., Lapierre, A., Andersen, J.U., Pedersen, U.V., Tomita, S., Andersen, L.H.:
Absorption spectrum of the green fluorescent protein chromophore anion in vacuo. Phys.
Rev. Lett. 87, 228102 (2001)
3. Forbes, M.W., Jockusch, R.A.: Deactivation pathways of an isolated green fluorescent protein
model chromophore studied by electronic action spectroscopy. J. Am. Chem. Soc. 131,
17038–17039 (2009)
4. Chingin, K., Balabin, R.M., Frankevich, V., Barylyuk, K., Nieckarz, R., Sagulenko, P., Zenobi,
R.: Absorption of the green fluorescent protein chromophore anion in the gas phase studied by
a combination of FTICR mass spectrometry with laser-induced photodissociation spectroscopy. Int. J. Mass Spectrom. 306, 241–245 (2011)
5. Zuev, D., Bravaya, K., Makarova, M.V., Krylov, A.I.: Effect of microhydration on the
electronic structure of the chromophores of the photoactive yellow and green fluorescent
proteins. J. Chem. Phys. 135, 194304 (2011)
2 Introduction and New Aspects
17