Figure 1

Förster resonance energy transfer (FRET). (a) A molecule labelled with donor and acceptor chromophores (in this case a stiff polyproline peptide with Alexa Fluors 488 and 594 [86]). The donor fluorophore (green) can be excited specifically (blue). It can then either emit a fluorescence photon itself or transfer its excitation energy to the acceptor (red). (b) The process can be depicted in terms of a Jablonsky diagram illustrating the transitions between ground and excited states of donor (D) and acceptor (A). The rate of energy transfer, k_F, depends on the distance, r, between donor and acceptor, resulting in the characteristic distance dependence of the transfer efficiency E (c). At the Förster distance, R₀, E = 0.5. For currently available FRET pairs suitable for single molecule spectroscopy, R₀ is in the range of 4 to 8 nm.