In simple terms, Fluorescence (or Förster) Resonance Energy Transfer, often abbreviated as FRET, is the energy exchange between a high-energy excited ‘donor’ molecule to an ‘acceptor’ molecule. The interaction is unique in that it is an energy transfer that does not involve an electron, and is only possible within very small (<10nm) distances. This distance limit has given rise to the use of FRET as a measure of nanoscale distances between biological structures, and has been called a “Spectroscopic Ruler” because of it. Using selected FRET pairs can enable the researcher to investigate many biomolecular interactions. Common choices include fluorophores or chromophores, depending on the desired method of analysis.
An example of a FRET pair is B-Phycoerythrin (donor) and Cy5 (acceptor).