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permits investigation of molecular interactions beyond the theoretical resolution limit of optical microscopy


FRET (Förster (Fluorescence) Resonance Energy Transfer) permits the investigation of molecular interactions beyond the theoretical resolution limit of optical microscopy. It is based on the direct transfer of energy from one molecule to another, which can only occur over very small distances (1-10nm). The following conditions must be satisfied for FRET:

  1. The donor probe should have a sufficiently long lifetime for energy transfer to occur
  2. Donor and acceptor molecules must be approximately 1-10 nm apart.
  3. The absorption spectrum of the acceptor fluorophore must overlap the fluorescence emission spectrum of the donor fluorophore (by approximately 30%).
  4. For energy transfer, the donor and acceptor dipole orientations must be approximately parallel.

Energy transfer is demonstrated by quenching of donor fluorescence together with a reduction in the fluorescence lifetime, and an increase in acceptor fluorescence emission. FRET is very sensitive to the distance between the fluorophores and can be used to estimate intermolecular distances. FLIM imaging can be used in association with FRET studies to identify and characterize energy transfer.

In confocal imaging, there are several methods for FRET studies. These include; filter-based, acceptor bleaching, spectral and FLIM-based methods. The filter-based method requires band-pass filters and specialized software to correct for cross-talk and bleed-through. Acceptor bleaching methods require laser power to bleach fluorophores. Spectral analysis allows spectral changes to be monitored, and FLIM enables quantitative FRET analysis.



Cells expressing CFP or the FRET pair show different emission spectra. Please note that the cells expressing the FRET construct (top panel) mainly show YFP emission under 408nm laser light excitation only. Control cells (with expression of CFP and YFP without a spacer) showed mainly CFP emission.

By targeting cellular constituents with fluorescent dyes, fluorescent antibodies and/or fluorescent proteins (GFP and related probes), almost any molecule can be labeled for FRET studies. FRET is a valuable tool for investigating and quantifying interactions in signal transduction, the formation of protein complexes, protein-protein interactions receptor function, membrane dynamics and molecular spatial relationships.


Almost any Nikon upright microscope or inverted epi-fluorescence microscope can be configured to carry out FRET studies. Widefield techniques can be compromized by fluorescence emissions originating from above and below the plane of focus. These can be eliminated using confocal imaging (A1R+ and C2si+). In confocal FRET applications, fluorophore pairs must be tailored to the available lasers. Spectral confocal imaging has particular advantages for FRET studies in that closely related spectra can be easily and cleanly separated, and quantitative FRET analysis is possible with spectral unmixing and ratio calculation.


For best possible FRET imaging, the Eclipse Ti-E can be configured with the A1R+ and C2si+ confocal system and LIMO (This accommodates a range of lasers and offers continuous, variable laser control through AOM technology).


FRET with Fluorescent Proteins
Interactive tutorial on compatible FRET probes

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