1 Department of Physics and Astronomy, Science and Technology, Aarhus University2 Department of Physics and Astronomy, Science and Technology, Aarhus University
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.