Nanosecond relaxation processes in liposomes

Abstract

The fluorescence characteristics of aromatic molecules are often strongly influenced by their immediate environment. For this reason, fluorescence probes have found wide application for monitoring changes occurring in biological or model membrane systems (Azzi, 1975; Radda, 1975; Badley, 1976). The majority of the reported studies have made use of steady-state fluorescence measurements. These reflect a time average of any excited-state interactions that occur on the nanosecond time scale. Fluorescence decay measurements, on the other hand, provide direct kinetic information regarding these interactions. Direct measurements are possible in a time interval limited on the short side by the time resolution of the instrumental technique and on the long side by the decay time of the fluorophore. As a consequence, the time-dependent characteristics of the fluorescent probe environment are better defined. The parameters characterizing the excited state behavior of 2-anilinonaphthalene (2-AN) in fluid solutions such as cyclohexane, ethanol, and mixtures thereof, and in a viscous solvent (glycerol) have previously been reported (DeToma and Brand, 1977). They were found to be strongly influenced by the presence of relatively small amounts of polar molecules in a nonpolar solvent. This particular property makes 2-AN a valuable fluorescent probe for a heterogeneous polar-apolar liposomic environment. In addition, the information obtained in the model solvent systems mentioned above constitutes a firm basis for interpreting the results obtained with liposomes. In the present communication we describe the results obtained by measuring the fluorescence decay, time-resolved emission spectra (TRES), and the decay of the emission anisotropy (DEA) of 2-AN adsorbed to dimyristoyllecithin (DML) single bilayer liposomes. It will be shown that the rotational motion of 2-AN as well as its excited-state interactions occur on the nanosecond time scale both above and below the gel-liquid transition temperature of these vesicles. The nanosecond relaxation processes found to occur with the liposomes will be compared to those observed with 2-AN in homogeneous solvents.