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Biophys J, September 2002, p. 1631-1649, Vol. 83, No. 3
Department of Molecular Biology, Max Planck Institute for Biophysical Chemistry, D-37077 Göttingen, Germany
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.
Biophys J, September 2002, p. 1631-1649, Vol. 83, No. 3
© 2002 by the Biophysical Society 0006-3495/02/09/1631/19 $2.00
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