535.37The influence of structural flexibility on dipole moments, energy-level locations, and charge distributions in prodan and laurdan molecules was studied. A quantum-chemical calculation of isolated prodan and laurdan molecules in the fluorescent state geometry was conducted. Rate constants for radiative and non-radiative processes and fluorescence quantum yields for these probe molecules were calculated. Interaction centers of prodan and laurdan with a proton-donor solvent were estimated quantitatively. The possibility of using fluorescent probes for estimating the polarity of proton-donor and proton-acceptor solvents was shown.Keywords: fluorescent probe, prodan, laurdan, structural flexibility, proton-donor properties, fluorescence quantum yield.Introduction. Fluorescent probes such as prodan (6-propionyl-2-dimethylaminonaphthalene) and laurdan (6-dodecanoyl-2-dimethylaminonaphthalene) are widely used in biological and biophysical research [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. The fluorescence properties of prodan and laurdan have been described in the literature as simultaneously existing local-excited (LE) and twisted internal charge-transfer (TICT) states. However, the latter term should be used very arbitrarily because the state has been observed only at very low temperatures [2]. These states are strongly delocalized at temperatures near ambient and above [2]. It has been shown [5] that the positions of fluorescence maxima in phospholipids bilayers depend on the phase state of the phospholipids, i.e., 435 and 440 nm for the short-wavelength band in the gel phase; 515 and 490 nm, for prodan and laurdan in the liquid-crystalline phase. Two fluorescence bands (430 and 500 nm) were observed for laurdan in glycerin with λ ex = 380 nm at room temperature. The short-wavelength band at 430 nm disappeared as the temperature was raised to 370 K.The large shifts of prodan and laurdan fluorescence bands in solvents of different polarity have been explained by the significant change of dipole moment in the excited state as compared with the ground state. According to formulas for calculating common effects of solvents [16,17], changing the prodan dipole moment from 5.5 to 10.2 D causes a shift of the S 0 → S 1 -transition [18,19] in its fluorescence spectra of 770 cm -1 and 800 cm -1 on going to isopropanol and water, respectively, from a non-polar solvent [19,20]. The experimental shifts in fluorescence spectra of prodan were 3790 and 5520 cm -1 in isopropanol and water, i.e., the size of the dipole moment and its change from

535.37 V. Ya. Artyukhov, and B. V. Korolev The fluorescent probe molecule -laurdan (6-dodekanoil-2-dimetilamin naphthalene) -is investigated by experimental and quantum-chemical methods. The influence of the structure non-rigidity on the dipole moments, arrangement of energy levels, and distribution of charges in the laurdan molecule is studied. An optimized structure is obtained and analyzed by the molecular dynamics method. Several structures with different rotation angles of the dimethylamino group are investigated, for which energy, oscillator forces, and nature of electron states are calculated. Fluorescence spectra of the laurdan molecule in an inert solvent are interpreted. Centers of possible interaction of the molecule with a proton-donor solvent are established. Rate constants of radiative and nonradiative processes and quantum fluorescence yields of the examined probe are calculated.