Specific molecular properties of organic solvents determine the fluorescence depolarization of DPH and TMA-DPH in membranes.

In pig liver microsomes and protein-free egg PC liposomes the effects of organic solvent molecules on the fluorescence depolarization of 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-[4-(trimethylamino)phenyl]-6-phenyl-hexa-3,5-triene (TMA-DPH) were investigated. Aromaticity, alkyl chain length, and stereometry of the solvent molecules are shown to determine the changes of fluorescence depolarization. A concentration-dependent decrease in the fluorescence anisotropy is obtained with aromatic molecules but not with aliphatic molecules. The decrease correlates with the increasing side chain length of alkylbenzenes for both fluorophors and both membrane systems. Benzene in microsomes deviates characteristically from this trend. Measurements of TMA-DPH reveal smaller changes of anisotropy but yield the same qualitative results. Moreover, it is possible to distinguish the effects of the different stereometric properties of the three xylene isomers on the fluorescence anisotropy of DPH. According to their alkyl chain length and their specific stereometry, they exert the strongest fluidizing effect of all solvents investigated.