Forster energy transfer mechanism FRET

Forster (fluorescence) resonance energy transfer (FRET) is the non-radiative energy transfer mechanism between donor chromophore in its excited electronic state and an acceptor chromophore. FRET is extremely sensitive to small distances as the energy transfer is inversely proportional to the distance between the donor and acceptor chromophore to the sixth power, making it highly suitable for imaging and sensing in biomedical. 

FRET is a nonradiative process that is, the transfer takes place without the emission or absorption of a photon. And yet, the transition dipoles, which are central to the mechanism by which the ground and excited states are coupled, are conspicuously present in the expression for the rate of transfer. For instance, the fluorescence quantum yield and fluorescence spectrum of the donor and the absorption spectrum of the acceptor are part of the overlap integral in the Forster rate expression, Eq. (1.2). These spectroscopic transitions are usually associated with the emission and absorption of a photon. These dipole matrix elements in the quantum mechanical expression for the rate of FRET are the same matrix elements as found for the interaction of a propagating EM field with the chromophores. However, the origin of the EM perturbation driving the energy transfer and the spectroscopic transitions are quite different. The source of this interaction term... 

A detailed theory of energy transfer by the Coulombic mechanism was developed by Forster, so the process is often referred to as Forster resonance energy transfer (FRET). According to the Forster theory, the probability of Coulombic energy transfer falls off inversely with the sixth power of the distance between the donor and the acceptor. For... 

FRET intensity can be expressed in function of the distance of the two molecules as shown in eq. (1) assuming a dipole-dipole (Forster s) mechanism for the energy transfer. This gives the basis for calculating the distance of two molecules by measuring FRET intensity ... 

In the case of Forster resonance energy transfer (FRET) [13], i.e. energy transfer by the dipole-dipole mechanism, and for randomly oriented donor-acceptor pairs, the depolarization after one transfer step (ensemble average) is almost complete [27]. For this reason, fluorescence anisotropy is a good indicator of energy transfer between identical fluorophores, hence of relative distances. Existence of efficient FRET may therefore reflect an association process. 

Details on the mechanisms and theories of excitation energy transfer via dipole-dipole interaction (FRET Forster resonance energy transfer) and via exchange interaction (Dexter s mechanism) can be found in B. Valeur, Molecular Fluorescence. Principles and Applications, Wiley-VCH, Weinheim, 2002, chap. 4 and 9. 

Construction of BODIPY-based probes with different mechanisms (a) PET (photoinduced electron transfer), (b) ICT (intramolecular charge transfer) and (c) FRET (Forster resonance energy transfer) channels. 

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