Förster Resonance Energy Transfer (FRET)

Distance-dependent nonradiative energy transfer between donor and acceptor chromophores acting as a spectroscopic nanoruler.

Specifications & Metrics

type: Biophysical & Biosensor
readout: Acceptor-to-donor fluorescence emission ratio or donor fluorescence lifetime decay
measures: Protein–protein proximity (1–10 nm), conformational changes, and second-messenger biosensor dynamics
format: Live-cell fluorescence microscopy, flow cytometry, or homogeneous time-resolved fluorescence (HTRF) microplates
pioneer: Theodor Förster (theoretical description of dipole–dipole energy transfer)

Named after German physicist Theodor Förster, FRET transfers excitation energy nonradiatively from an excited donor chromophore to a nearby acceptor chromophore through long-range dipole–dipole coupling.
Because energy-transfer efficiency scales inversely with the sixth power of the inter-chromophore distance (1/r⁶), FRET is exquisitely sensitive to distance changes within the 1 to 10 nanometer range typical of protein complexes.
Intramolecular FRET biosensors sandwich a ligand-binding or phosphorylation domain between cyan and yellow fluorescent proteins (CFP/YFP) to report real-time cAMP, PKA, or kinase activity in living cells.