We have measured the fluorescence anisotropy of 1,6-diphenyl-l,3,5-hexatriene (DPH) as its fluorescence lifetime is decreased by oxygen quenching. Such studies were done on DPH dissolved in the isotropic solvent mineral oil and for DPH embedded in phospholipid vesicles of either dimyristoyl-L-a-phosphatic'vlcholine (DMPC) or dioleovl-ra-phosphatidylcholine (D . In order to obtain adeque oxygen had to be used. nificant changes in intens were reversible. To contr the systems under study formed with nitrogen, arg Under these last-menti01 and anisotropy were insig observed with oxygen qut of the fluorophore are dl radians/seconds and its li long compared with the f parameter provides a m fluorophore's environme Oxygen quenching of flu the fluorescence lifetimc steady-state fluorescence z R and r , . For D P H in peratures u e found that quenching-anisotropy me; >PC), each a t several temperatures. e quenching increased pressures of lxygen quenching resulted in sig.y and anisotropy, and these effects 11 for possible effects of pressure on equivalent experiments were pern, or helium forming the gas phase. :d conditions, changes in intensity iificant when compared with those iching. The depolarizing rotations scribed by its rotation rate ( R ) in iiting anisotropy at times which are Jorescence lifetime, r,. This latter asure of the degree to which the t hinders its rotational diffusion. rescence 3rovides a means to vary simulta :ous observation of the iisotropy allows quantitation of both mineral oil a t two different temhe values of R obtained from this urement agreed precisely with those ibtained from steady-state anisotropy measurements and with the values obtained from differential polarized phase fluorometry (Lakowicz, J. R., et al. (1979) Biochemisfry I8 Fluorescence anisotropy measurements have been useful in the investigation of protein--ligand interactions (Anderson & Weber, 1965; Weber & Daniel, 1966), protein-protein interactions (Levison et al.. 1970; Dandliker & de Saussure, 1970). and the phase transitions of lipid bilayers (Cogan et al.. 1973; Shinitzky et al.. 1971). This usefulness derives primarily from the similarity of the rates of fluorescence emission and the rates of rotational diffusion of proteins or the rates of fluorophore rotation in lipid bilayers. Diffusional rates are generally sensitive to temperature and solvent viscosity, whereas the rate of fluorescence emission can be insensitive to these factors: consequently one must frequently