experiments that in solution the kinetics is also governed by the shielding of charges by
solvent molecules and limited by diffusion.
Conformations of polyproline peptides with different lengths and charge states
were explored by Jockusch and co-workers [17] based on FRET. Two rhodamine
dyes were used as a FRET pair. The action and emission spectra of the donor (D)
and acceptor (A) are shown in Fig. 6.4. It is evident that the emission spectrum
Fig. 6.2 Normalised fluorescence intensity per ion versus temperature for (a) doubly and (b)
triply protonated Trp-cage protein ions labelled with a dye. The decrease in fluorescence sets in
earlier for the 3+ ion than for the 2+ ion. Reprinted with permission from [18]. Copyright (2005)
American Chemical Society
Fig. 6.3 Potential-energy curves for ground state DT, photoexcited DT (the dye is the absorber),
and D
À
T
+ charge-transfer states. The energy of the charge-transfer state depends on the local
electric field: The higher the field, the lower the energy. D dye (and not donor), T Trp. Reprinted
with permission from [23]. Copyright (2008) American Chemical Society
6 Fluorescence from Gas-Phase Biomolecular Ions
109