A DESCRIPTION OF THE ELECTROSTATIC INTERACTIONS OF FLUOROPHORES WITH POLAR SOLVENTS BASED ON A GAUSSIAN DISTRIBUTION OF INTERACTION ENERGIES (DANCA, PRODAN)

The dipole moments of a fluorophore in the ground and excited states arise from partial charges localized on specific parts of the molecule. These charges are considered to be monopoles which interact independently with solvent dipoles. The orientation of a solvent dipole with respect to each monopole is given by the Langevin function, assuming a Gaussian distribution of interaction energies. A mathematical description of the solvent dependence of electronic spectra of fluorophores is developed which involves mainly parameters which characterize the electrostatic monopole dipole interaction.;The theory predicts the change of the fluorescence energy with temperature to exhibit three regimes, arising from the temperature dependence of the solvent rotational correlation time. At low temperatures, a discrepancy between the theory and the data is rationalized by the existence of associated forms of the solvent. The fluorescence of PRODAN was studied in mixtures of 1-butanol and iso-octane. In agreement with previous work, multimeric forms of the polar solvent are evident. The apparent volume of PRODAN, measured by fluorescence polarization, increases rapidly with decreasing temperature in 4.3% 1-butanol in iso-octane (mol:mol) but is constant in pure iso-octane or 1-butanol. This indicates electrostatic interactions forming larger polymolecular species at low temperatures.;Quantitative application of this theory to biological fluorescence is not possible on account of the anisotropic nature of biological molecules. DANCA binds to apomyoglobin, K(,d) = 1.2 x 10('-5) M, with a shift of 80 nm to the blue. From studies of the wavelength, temperature, and pressure dependence of the emission of the bound probe it is concluded that the heme pocket is a rigid, polar environment with one general binding site for DANCA. This site is made up of a heterogeneous population of specific binding sites.;Throughout this work two closely related fluorophores with parallel ground and excited state dipole moments were employed, 6-propionyl-2-(N,N-dimethyl)-aminonaphthalene (PRODAN), and 4-(2'-(N,N-dimethyl)-amino-6'-naphthoyl)-cyclohexane carboxylic acid (DANCA). The fluorescence of both of these is very sensitive to solvent polarity.