Dynamic fluorescence anisotropy imaging microscopy in the frequency domain (rFLIM)

We describe a novel variant of fluorescence lifetime imaging microscopy (FLIM), denoted anisotropy-FLIM or rFLIM, which enables the wide-field measurement of the anisotropy decay of fluorophores on a pixel-by-pixel basis. We adapted existing frequency-domain FLIM technology for rFLIM by introducing linear polarizers in the excitation and emission paths. The phase delay and intensity ratios (AC and DC) between the polarized components of the fluorescence signal are recorded, leading to estimations of rotational correlation times and limiting anisotropies. Theory is developed that allows all the parameters of the hindered rotator model to be extracted from measurements carried out at a single modulation frequency. Two-dimensional image detection with a sensitive CCD camera provides wide-field imaging of dynamic depolarization with parallel interrogation of different compartments of a complex biological structure such as a cell. The concepts and technique of rFLIM are illustrated with a fluorophore-solvent (fluorescein-glycerol) system as a model for isotropic rotational dynamics and with bacteria expressing enhanced green fluorescent protein (EGFP) exhibiting depolarization due to homotransfer of electronic excitation energy (emFRET). The frequency-domain formalism was extended to cover the phenomenon of emFRET and yielded data consistent with a concentration depolarization mechanism resulting from the high intracellular concentration of EGFP. These investigations establish rFLIM as a powerful tool for cellular imaging based on rotational dynamics and molecular proximity.