Fluorescence from Gas-Phase
Biomolecular Ions
6
Steen Brøndsted Nielsen
Abstract
This chapter deals with measurements of fluorescence from electronically
excited biomolecular ions where there are no interactions with an external
environment. Biomolecules with no natural fluorophores are labelled with a
dye for such experiments. First, some of the advantages, but also difficulties,
of fluorescence spectroscopy compared to absorption spectroscopy are
discussed. Extensive work has been done on the isolated dyes in characterising
them with respect to their dispersed fluorescence spectra, excited-state lifetimes,
and gas-phase Stokes shifts. After a brief introduction, results from experiments
on dye-derivatised biomolecular ions that provide important information on
folding/unfolding processes and local structural changes are presented.
Examples included here are a model DNA duplex, the Trp-cage protein,
polyproline peptides, and the cytochrome c heme protein. The chapter ends
with a discussion on the oxyluciferin anion, the molecule responsible for light
emission from fireflies where the electronic transition has charge-transfer
character.
6.1
Introduction
While action spectroscopy provides highly relevant information on the level separation between the electronic ground and excited states of an isolated
biochromophore ion, similar information can be obtained from light-emission
experiments. If the excited state lives long enough, the ion may undergo a geometry
change on the excited state potential energy surface. The energy of the emitted
photon will then reflect the energy separation between the ground and excited states
but with the optimum geometry for the latter. In an environment where mobile
S.B. Nielsen (*)
Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark
e-mail: sbn@phys.au.dk
S. Brøndsted Nielsen and J.A. Wyer (eds.), Photophysics of Ionic Biochromophores,
Physical Chemistry in Action, DOI 10.1007/978-3-642-40190-9_6,
# Springer-Verlag Berlin Heidelberg 2013
105
Biomolecular Ions
6
Steen Brøndsted Nielsen
Abstract
This chapter deals with measurements of fluorescence from electronically
excited biomolecular ions where there are no interactions with an external
environment. Biomolecules with no natural fluorophores are labelled with a
dye for such experiments. First, some of the advantages, but also difficulties,
of fluorescence spectroscopy compared to absorption spectroscopy are
discussed. Extensive work has been done on the isolated dyes in characterising
them with respect to their dispersed fluorescence spectra, excited-state lifetimes,
and gas-phase Stokes shifts. After a brief introduction, results from experiments
on dye-derivatised biomolecular ions that provide important information on
folding/unfolding processes and local structural changes are presented.
Examples included here are a model DNA duplex, the Trp-cage protein,
polyproline peptides, and the cytochrome c heme protein. The chapter ends
with a discussion on the oxyluciferin anion, the molecule responsible for light
emission from fireflies where the electronic transition has charge-transfer
character.
6.1
Introduction
While action spectroscopy provides highly relevant information on the level separation between the electronic ground and excited states of an isolated
biochromophore ion, similar information can be obtained from light-emission
experiments. If the excited state lives long enough, the ion may undergo a geometry
change on the excited state potential energy surface. The energy of the emitted
photon will then reflect the energy separation between the ground and excited states
but with the optimum geometry for the latter. In an environment where mobile
S.B. Nielsen (*)
Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark
e-mail: sbn@phys.au.dk
S. Brøndsted Nielsen and J.A. Wyer (eds.), Photophysics of Ionic Biochromophores,
Physical Chemistry in Action, DOI 10.1007/978-3-642-40190-9_6,
# Springer-Verlag Berlin Heidelberg 2013
105