Principles of Fluorescence Spectroscopy

228 SOLVENT AND ENVIRONMENTAL EFFECTS
Figure 6.43. Emission spectra of 9-AC, ANS and NPN in aqueous
buffer (——), in the presence of calmodulin (- - -), and in the presence
of calmodulin plus calcium (– – –). Revised and reprinted with permission
from [97]. Copyright © 1980, American Chemical Society.
the dansyl probes have been used to study the binding of
organic molecules to cyclodextrins. 98 Cyclodextrins are
cyclic sugars. The interior surface of cyclodextrin is
hydrophobic, and can bind appropriately sized molecules.
Figure 6.44. Dansyl-labeled cyclodextrin. Also shown is a schematic
of the effects of 1-adamantanol (black balls) on the location of the
DNS group. Revised from [98].
Figure 6.45. Emission spectra of dansyl-cyclodextrin with increasing
concentrations of 1-adamantanol. Probe concentrations = 10 M.
Revised from [98].
Carbohydrates are not fluorescent. In order to obtain a signal
cyclodextrin was labeled with the dansyl group (Figure
6.44). The labeled cyclodextrin was titrated with 1-adamantanol,
which resulted in a decrease in fluorescence intensity
and a red shift in the emission spectra (Figure 6.45). In
the absence of 1-adamantanol the dansyl group binds in the
cyclodextrin cavity, resulting in a blue-shifted and
enhanced emission. Addition of 1-adamantanol displaces
the dansyl group, resulting in greater exposure to the aqueous
phase and a red shift in the emission spectra.
6.10. ADVANCED SOLVENT SENSITIVE PROBES
The increased understanding of solvent effects and formation
of ICT states has resulted in the development of additional
fluorophores that are highly sensitive to solvent
polarity. 100–102 These probes are based on 2,5-diphenyloxazole
(DPO), which is a well-known scintillator. DPO is soluble
mostly in organic solvents, where it displays a high
quantum yield. By itself, DPO would not be very sensitive
to solvent polarity. Solvent sensitivity was engineered into
DPO by the addition of electron donor and acceptor groups
(Figure 6.46). The top compound is DPO. In the lower
structures the DPO is modified to contain an electron donor,
acceptor, or both groups.
Absorption and emission spectra of these four DPO
derivatives are shown in Figures 6.47 and 6.48. In
methanol, DPO displays structured absorption and emission
spectra, with only a small Stokes shift. Addition of the sulfonic
acid group alone results in a modest red shift. A larger
red shift is observed upon addition of the dimethylamino