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The features of fluorescence emission in a dye-doped dense multiple scattered medium under pulsed laser pumping are considered in terms of confined excitation in small zones associated with laser speckles occurring in a pumped medium. The results of numerical modeling of the fluorescence emission kinetics are compared to the experimental data obtained using the rhodamine 6G-doped layers of the densely packed TiO2 (anatase) particles pumped at 532 nm by 10 ns laser pulses. The intensity of pump radiation during the action of laser pulses was varied from 1·105 W/cm2 to 5·107 W/cm2. In the recovery of the ratios of stimulated to a spontaneous emission, the spectra of the stimulated component were fitted using the spectral function derived by R. Dicke. In the framework of the considered concept, saturation of the ratio of the stimulated to a spontaneous emission and linear growth of an integrated fluorescence output with a practically unchangeable half-width of the emission spectra at high pump intensities are interpreted.

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We have studied the interaction between polycyclic aromatic hydrocarbons (pyrene and anthracene) with human serum albumin (HSA) and human blood plasma. We have shown that the increase in the fluorescence intensity and the decrease in the polarity index of pyrene on going from an aqueous solution to a pH 7.4 buffer solution of HSA suggests that polycyclic aromatic hydrocarbons are localized in the hydrophobic microphase of the proteins. The increase in the fluorescence intensity for anthracene and pyrene, and also the decrease in the polarity index of pyrene on going from HSA to blood plasma is connected with the fact that polycyclic aromatic hydrocarbons can bind both to plasma proteins and to plasma lipids. When sodium dodecyl sulfate (SDS) is added to the blood plasma in a concentration greater than the critical micelle concentration, we observe an increase in the fluorescence intensity and the polarity index of pyrene. We hypothesize that this is connected with localization of pyrene near the interface between the hydrophobic and hydrophilic phases of the protein-SDS system. We have established that SDS leads to a change in the structure of blood plasma proteins and promotes escape of polycyclic aromatic hydrocarbons from the protein globules.Key words: human blood plasma, human serum albumin, polycyclic aromatic hydrocarbons, sodium dodecyl sulfate, fluorescence.Introduction. Luminescent probes are widely used in studying structural changes in proteins, lipoproteins, and also in studying the properties of biological membranes [1,2]. The properties of nonpolar microregions in microheterogeneous media have been studied using luminescent probes: polycyclic aromatic hydrocarbons, which due to hydrophobic interactions tend to escape from the polar aqueous medium or macrophase of the solution into the nonpolar microphase, i.e., into the interior volume of the surfactant micelles, which is followed from the changes in the vibrational structure of the fluorescence spectra of the probes [3]. The use of the fluorescence of pyrene, which belongs to the class of polycyclic aromatic hydrocarbons, is well known [4-6] for determination of changes in the physicochemical properties of biological systems and model lipid membranes. Analysis of the changes in the vibrational structure of the fluorescence spectra of pyrene [3-5] also makes it possible to obtain information about the polarity of the microenvironment of the luminescent probe when its position changes in different microheterogeneous media. In [6], using data from analysis of the vibronic structure of the monomer luminescence spectrum of pyrene in a model membrane of phosphatidylcholine liposomes, it was hypothesized that the pyrene molecules can not only move laterally along the surface of the membrane, but also can undergo transmembrane motion. Thus the use of a method based on analysis of changes in the vibronic structure of the fluorescence spectra of pyrene molecules is promising for studying processes connected with a change in where polycyclic aromatic hydroca...