Fluorescent Environment-Sensitive Dyes as Reporters of Biomolecular Interactions - Progress in Molecular Biology and Translational Science - page 37

Target

- +

DYE - +

DYE

Protein

Protein

Figure 2.1 Monitoring biomolecular interactions by a solvatochromic fluorescent dye.

field, etc.), and specific H-bonding interactions. Here, we do not consider
pH- and ion-sensitive dyes as environment sensitive, as the response of these
dyes is associated with changes in their chemical structure: protonation/
deprotonation or formation of a complex with an ion.
Several types of environment-sensitive fluorophores are of particular
interest for biomolecular applications: molecular rotors and solvatochromic
fluorescent dyes. Molecular rotors are an interesting class of environment-
sensitive dyes, which change their emission intensity in response to the
change of the solvent viscosity. 9 These dyes feature high rotational flexibility
of their conjugated system so that they are poorly emissive in nonviscous
environments (such as water or organic solvents). In viscous environments,
such as biological membranes and biomacromolecules, their rotation mobil-
ity is restricted, which dramatically increases their fluorescence quantum
yield. Thus, these dyes can turn on their fluorescence in response to inter-
actions that rigidify their environment. 10 Though these dyes were largely
applied as probes in biological membranes, their application for detection
of biomolecular interactions is still poorly explored and therefore will not
be reviewed here. A much better established class comprises solvatochromic
fluorescent dyes. They exhibit shifts in their emission maxima and some-
times change in their fluorescence quantum yield as a function of polarity
and hydration of their environment. 11,12 In these dyes, the dipole

moment S 1

transition) because of an intramolecular charge transfer from the electron-
donor group to the electron-acceptor group ( Fig. 2.2 ). Polar solvents
relax efficiently the excited molecules to the S solv 1 state as a result of
polarization of the solvent dipoles around the fluorophore dipole. An
increase in the solvent polarity decreases the energy of the S sol 1 state,
resulting in a red shift in the emission spectra ( Fig. 2.2 ). In addition,
protic solvents

increases dramatically upon electronic excitation (S 0 !

(which contain hydrogen atoms bound to oxygen

(hydroxyl) or to nitrogen (amine, amide, etc.)) interact with the